Digital_Terrain_Analysis_saiful_2008 by 1KeLzN7

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									Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       Digital Terrain Mapping and
                                                                                 Analysis




                                                                          Dr. A.K.M. Saiful Islam
                                                                          Institute of Water and Flood Management (IWFM)
                                                                           Bangladesh University of Engineering and Technology (BUET)
                                                                                     Lecture Topic
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       This lecture will focus on
                                                                       • Geo-referencing
                                                                          –   Coordinate Systems
                                                                          –   Map Projections
                                                                          –   Coordinate Transform
                                                                          –   Map & Time Distance, Scale and accuracy
                                                                          –   Interpolation techniques
                                                                       • Digital Terrain Mapping and Analysis
                                                                          – DEM
                                                                          – DTM
                                                                                 Coordinate Systems
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                          Geospatial data should be geographically referenced ( called
                                                                          georeferenced or geocoded) in a common coordinate system.

                                                                        Plane Orthogonal Coordinates
                                                                         One of the most convenient way of locating points is to use
                                                                         plane orthogonal coordinates with x (horizontal) and y (vertical)
                                                                         axis.
                                                                        Polar Coordinates
                                                                         A polar coordinate system with the angle (q ) measured from
                                                                         the polar axis (x axis) and distance (r) from the pole is used in
                                                                         some cases.
                                                                        3D Orthogonal Coordinates
                                                                         Three dimensional (3D) orthogonal coordinates are also used
                                                                         to locate points with the plane coordinates (x, y) and height or
                                                                         depth (z).
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                Coordinates
                                                         Plane Orthogonal Cartesian
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                              Polar coordinates
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                3D Coordinate System
                                                                                 The Shape of the Earth
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • The shape of the Earth can be represented by an ellipsoid of
                                                                         rotation (or called a spheroid) with the lengths of the major semi-
                                                                         axis (a) and the minor semi-axis (b).
                                                                       • Two type of coordinate system use: (i) Geodetic, (ii) Geocentric
                                                                         coordinates
                                                                            Geodetic and Geocentric Latitude
                                             
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                            Geocentric Latitude – The acute angle measured perpendicular to
                                                                              the equatorial plane and a line joining the center of the earth and a
                                                                              point on the surface of the reference ellipsoid.
                                                                          Geodetic Latitude – The acute angle between the equator and a line
                                                                              drawn perpendicular to the tangent of the reference ellipsoid. Map
                                                                              coordinates are given as longitude and geodetic latitude.




                                                                                [Source : http://ccar.colorado.edu/ASEN5070/handouts/geodeticgeocentric.doc ]
                                                                                        Map Projection
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • A map projection is a process of transforming location on
                                                                         the curved surface of the Earth with the geodetic
                                                                         coordinates (j , l) to planar map coordinates (x, y).

                                                                       • More than 400 difference map projections have been
                                                                         proposed. The map projections are classified by the
                                                                         following parameters.

                                                                          –   projection plane: perspective, conical, cylindrical
                                                                          –   aspect: normal, transverse, oblique
                                                                          –   property: conformality, equivalence, equidistance
                                                                          –   size: inside, tangent, secant
                                                                                        Projection property
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       1. Conformality is the characteristic of true shape, wherein a projection
                                                                          preserves the shape of any small geographical area. This is
                                                                          accomplished by exact transformation of angles around points.
                                                                           – The property of conformality is important in maps which are used for
                                                                             analyzing, guiding, or recording motion, as in navigation.

                                                                       2. Equivalence is the characteristic of equal area. Preservation of
                                                                          equivalence involves an inexact transformation of angles around
                                                                          points and thus, is mutually exclusive with conformality except along
                                                                          one or two selected lines.
                                                                           – The property of equivalence is important in maps which are used for
                                                                             comparing density and distribution data, as in populations.

                                                                       3. Equidistance is the characteristic of true distance measuring. The
                                                                          scale of distance is constant over the entire map.
                                                                           – Equidistance is important in maps which are used for analyzing
                                                                             velocity, e.g. ocean currents. Typically, reference lines such as the
                                                                             equator or a meridian are chosen to have equidistance and are termed
                                                                             standard parallels or standard meridians.

                                                                          [ Source: http://www.forestry.umt.edu/academics/courses/FOR503/Part4.htm ]
                                                                             Perspective Projection
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Perspective projections are classified based on the
                                                                         projection center or viewpoint.
                                                                                 Conical Projection
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Conical projections are classified by the
                                                                         aspect as well as the cone size
                 Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




Conic (secant)
                                               Conic (tangent)
                                                                                 Conic projection
                                                                                Cylindrical Projections
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Cylindrical projections are classified as in case of conical
                                                                         projections. One of the most popular cylindrical projections is the
                                                                         Universal Transverse Mercator (UTM) with a transverse axis, secant
                                                                         cylinder and conformality (equal angle).
                                                                                        UTM Projection
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Universal Transverse
                                                                         Mercator (UTM) with a
                                                                         transverse axis, secant
                                                                         cylinder and conformality
                                                                         (equal angle).

                                                                       • UTM is commonly used for
                                                                         topographic maps of the
                                                                         world, devided into 60
                                                                         zones with a width of 6
                                                                         degree longitude.
                                                                       Coordinate Transformation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       Coordinate transformation is to transform a
                                                                       coordinate system (x, y) to another coordinate
                                                                       system (u, v). The transformation is needed in the
                                                                       following cases:

                                                                       – to transform different map projections of many GIS
                                                                         data sources to an unified map projection in a GIS
                                                                         database,

                                                                       – to adjust errors which occur at map digitization
                                                                         due to shrinkage or distortion of the map
                                                                         measured, and

                                                                       – to produce geo-coded image by so called
                                                                         geometric correction of remote sensing imagery
                                                                         with geometric errors and distortions.
                                                                               Reference for Coordinate
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                                   Transformation
                                                                       • Coordinate transformation is executed by a selected
                                                                         transformation model (or mathematical equation), with
                                                                         a set of reference points (or control points), that are
                                                                         selected as tic masks at the corner points, reseau or
                                                                         ground control points.
                                                                                              Major Transformation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       a)   Helmert
                                                                            Transformation
                                                                            scale, rotation and shift

                                                                       b)   Affine Transformation
                                                                            skew, scale of x and y,and
                                                                                shift


                                                                       c)   Pseudo Affine
                                                                            Transformation
                                                                            bi-linear distortion

                                                                       d)   Quadratic
                                                                            Transformation
                                                                            parabolic distortion


                                                                       e)   Perspective Projection
                                                                            rectification of aerial photo

                                                                       f)   Cubic Transformation
                                                                            cubic and distortion)
                                                                                                 Distance
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       •   Distance is one of the important elements in measuring spatial objects in
                                                                           GIS. Several different concepts of distance are defined as follows.

                                                                       •   Euclidean Distance
                                                                           Euclidean distance D is the defined as the distance measured along a
                                                                           straight line from point (x1, y1 ) to point (x2, y2 ) in Cartesian coordinate
                                                                           system . D2 = ( x1 - x2 )2 + ( y1- y2 )2

                                                                       •   Manhattan Distance
                                                                           Manhattan distance D is defined as the rectilinear rout measured along
                                                                           parallels to X and Y axes
                                                                       •   D = | x1 - x2| + | y1-y2|
                                                                       Distances (Contd..)
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        •   Great Circle Distance
                                                                            Great circle distance D is defined as
                                                                            distance along the great circle of the
                                                                            spherical Earth surface from a point (1,
                                                                            1; latitude and longitude) to another
                                                                            point (2, 2) where R is the radius of the
                                                                            Earth (R = 6370.3 km) on the assumption
                                                                            that the Earth is a sphere.

                                                                        •   Mahalanobis Distance
                                                                             Mahalanobis distance D is a normalized
                                                                            distance in the normal distribution from
                                                                            the center (X0) to a point (X) in case of n
                                                                            dimensional normal distribution.
                                                                            Mahalanobis distance is used in the
                                                                            maximum likelihood method for the
                                                                            classification of multi-spectral satellite
                                                                            images. where S: variance-covariance
                                                                            matrix
                                                                                   Distances (Contd..)
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        Time Distance

                                                                       • Time distance is defined as the time required to move from point B
                                                                         to point A by using specific transportation means.
                                                                       Scale, Accuracy and Resolution
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Scale of map refers to the ratio of distance on a map over the
                                                                         corresponding distance on the ground.

                                                                       • The scale is represented as 1: M or 1/M, where M is called the scale
                                                                         denominator.

                                                                       • The larger the scale, the more the detail described by the map and
                                                                         with higher accuracy.

                                                                       • Accuracy is generally represented by standard deviation of errors,
                                                                         that is difference between measurements and the true value.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                             accuracy and resolution
                                                            Relationship between scale,
                                                                        Principle of Interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       Interpolation is the procedure of estimating
                                                                       the value of properties at unsampled points or
                                                                       areas using a limited number of sampled
                                                                       observations.
                                                                          Interpolation Techniques
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • 1. Pointwise interpolation
                                                                              • 1(a) Thiessen polygon
                                                                              • 1(b) Weighted Average
                                                                       • 2. Interpolation by curve fitting
                                                                          – 2.1 Exact interpolation
                                                                              • 2. 1(a). Nearest neighbor
                                                                              • 2. 1.(b) Linear interpolation
                                                                              • 2. 1(c) Cubic interpolation
                                                                          – 2.2 Approximate interpolation
                                                                              • 2.2(a) Moving Average
                                                                              • 2.2(b) B-spline
                                                                              • 2.2(c) Curve Fitting by Least Square Method
                                                                        3. Interpolation by surface fitting
                                                                          – 3.1 Regular grid
                                                                              • 3.1(a) Bilinear Interpolation
                                                                              • 3.1(b) Bicubic Interpolation
                                                                          – 3.2 Random points
                                                                              • 3.2(a) TIN
                                                                            1. Pointwise Interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        Pointwise interpolation is used in case the sampled
                                                                         points are not densely located with a limited influence
                                                                         or continuity in surrounding observations, for example
                                                                         climate observations such as rainfall and
                                                                         temperature, or ground water level measurements at
                                                                         wells.
                                                                              1(a) Thiessen Polygons
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        Thiessen polygons can be generated using distance
                                                                         operator which creates the polygon boundaries as the
                                                                         intersections of radial expansions from the observation
                                                                         points.

                                                                        This method is also known as Voronoi tessellation.
                                                                               1(b) Weighted Average
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        A window of circular shape with the radius of
                                                                         dmax is drawn at a point to be interpolated, so
                                                                         as to involve six to eight surrounding
                                                                         observed points.
                                                                       2. Interpolation by Curve Fitting
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        the principle of curve fitting respectively to
                                                                         interpolate the value at an unsampled point
                                                                         using surrounding sampled points.
                                                                                    2. Curve Fitting
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                         Curve fitting is an important type of interpolation in
                                                                         many applications of. Curve fitting is divided into two
                                                                         categories.

                                                                        2.1 exact interpolation : a fitted curve passes through all
                                                                         given points.

                                                                        2.2 approximate interpolation : a fitted curve does not
                                                                         always pass through all given points.
                                                                             2.1 Exact interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       There are three methods:
                                                                       2.1(a) nearest neighbor : the same value as that
                                                                         of the observation is given within the proximal
                                                                         distance
                                                                            2.1 Exact interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       2.1(b) linear interpolation: a piecewise
                                                                       linear function is applied between two
                                                                       adjacent points.
                                                                              2.1 Exact interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • 2.1(c) cubic interpolation : a third order
                                                                         polynomial is applied between two adjacent points
                                                                         under the condition that the first and second order
                                                                         differentials should be continuous.
                                                                       2.2. Approximate Interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       There are three methods;
                                                                        2.2(a) Moving Average: a window with a range of -d to
                                                                         +d is set to average the observation within the region
                                                                       2.2 Approximate Interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       2.2(b) B-Spline: a cubic curve is
                                                                       determined by using four adjacent
                                                                       observations
                                                                       2.2 Approximate Interpolation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       2.2(c) Curve Fitting by Least Square Method.
                                                                        Least square method (sometimes called regression model) is a
                                                                        statistical approach to estimate an expected value or function with the
                                                                        highest probability from the observations with random errors. The
                                                                        highest probability is replaced by minimizing the sum of square of
                                                                        residuals in the least square method.
                                                                                                                              Equation

                                                                                                                               Y  ax  b
                                                                                                                               Slope

                                                                                                                               a
                                                                                                                                     ( Xi  X )(Y  Y )
                                                                                                                                       ( Xi  X ) 2




                                                                                                                               intercept

                                                                                                                               b  Y  aX
                                                                       3. Interpolation by Surface Fitting
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        the principle of surface fitting respectively to
                                                                         interpolate the value at an unsampled point
                                                                         using surrounding sampled points.
                                                                                    3. Surface Fitting
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Surface fitting is widely used for interpolation of points on
                                                                         continuous surfaces such as digital elevation model
                                                                         (DEM), geoid, climate model (rainfall, temperature,
                                                                         pressure etc.) and so on.

                                                                       • Surface fitting is classified into two categories:
                                                                          – 3.1 surface fitting for regular grid and
                                                                          – 3.2 surface fitting for random points.

                                                                       • 3.1 Surface Fitting for Regular Grid
                                                                         Following two methods are commonly used.
                                                                          3.1(a) Bilinear Interpolation
                                                                          3.1(b) Bicubic Interpolation
                                                                       3.1 Surface Fitting for Regular Grid
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                         3.1(a) Bilinear Interpolation
                                                                            Bilinear function is used to interpolate z using the
                                                                           following formula with respect to normalized
                                                                           coordinates (u, v) of the original coordinates (x, y)
                                                                       3.1Surface Fitting for Regular Grid
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        3.1(b) Bicubic Interpolation
                                                                         Third order polynomial is used to fit a continuous
                                                                           surface using 4 x 4 = 16 adjacent points.
                                                                         3.2 Surface Fitting for random
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                                     Points
                                                                       3.2. (a) Triangular network called as
                                                                       Triangulated Irregular Network (TIN) is applied
                                                                       Compare Interpolation methods
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Thiessen polygons are Used for service area analysis of public
                                                                         facilities such as hospitals. Originally proposed to estimate aerial
                                                                         averages precipitation in 1985.

                                                                       • Inverse Distance Weighted can be a good way to take a first look
                                                                         at an interpolated surface. However, there is no assessment of
                                                                         prediction errors. Accuracy depends on the selection of a power
                                                                         value and the neighborhood search strategy. A smaller (6) actually
                                                                         produce better estimations than a larger number (12).

                                                                       • Thin-plate Splines (applies to surface) are recommended for
                                                                         smooth, continuous surfaces such as elevation and water table. Also
                                                                         used for interpolating mean rainfall surface and land demand
                                                                         surface.
                                                                                                   Kriging
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       •   Kriging is a geostatistical method for spatial
                                                                           interpolation.
                                                                       •   It can assess the quality of prediction with estimated
                                                                           prediction errors.
                                                                       •   It uses statistical models that allow a variety of map
                                                                           outputs including predictions, prediction standard
                                                                           errors, probability, etc.

                                                                       •   Semivariogram can be fitted as:
                                                                           1. Ordinary Kriging models:
                                                                              Spherical, Circular, Exponential,
                                                                              Gaussian and Linear.
                                                                           1. Universal Kriging models:
                                                                              Linear with Linear drift, and
                                                                              Linear with Quadratic drift
                                                                         Semivariogram
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • The semivariogram functions
                                                                         quantifies the assumption that
                                                                         things nearby tend to be more
                                                                         similar than things that are
                                                                         farther apart. Semivariogram
                                                                         measures the strength of
                                                                         statistical correlation as a
                                                                         function of distance.

                                                                       • Semivariance:
                                                                         Y(h) = ½ [(Z(xi) - Z(xj)]2

                                                                       • Covarience = Sill – Y(h)
                                                                       Data Structure for Continuous
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                              Surface Model
                                                                       In GIS, continuous surface such as terrain surface, meteorological
                                                                          observation (rain fall, temperature, pressure etc.) population
                                                                          density and so on should be modeled

                                                                       • Grid at regular intervals
                                                                          – Bi-linear surface with four points or bi-cubic surface with
                                                                            sixteen points is commonly used
                                                                       • Random points
                                                                          – Triangulated irregular network (TIN) is commonly used.
                                                                            Interpolation by weighted polynomials is also used.
                                                                       • Contour lines
                                                                          – Interpolation based on proportional distance between
                                                                            adjacent contours is used. TIN is also used.
                                                                       • Profile
                                                                          – Profiles are observed perpendicular to an alignment or a
                                                                            curve such as high ways. In case the alignment is a straight
                                                                            line, grid points will be interpolated. In case the alignment is
                                                                            a curve, TIN will be generated.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                Different Types of DEM
                                                                              Remote Sensing and GIS in Water Management © Dr. Saiful Islam, IWFM, BUET




                                                                                                          DEM
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • A DEM (digital elevation model) is digital
                                                                         representation of topographic surface with the
                                                                         elevation or ground height above any geodetic
                                                                         datum. Followings are widely used DEM in GIS:
                                                                                                 DTM
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                           A DTM (digital terrain model) is digital representation of
                                                                            terrain features including elevation, slope, aspect,
                                                                            drainage and other terrain attributes.

                                                                           Usually a DTM is derived from a DEM or elevation data.

                                                                           several terrain features including the following DTMs.
                                                                           1. Slope and Aspect
                                                                           2. Drainage network
                                                                           3. Catchment area
                                                                           4. Shading
                                                                           5. Shadow
                                                                           6. Slope stability
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam


                                                                       Examples of DTM
                                                                                 1. Slope and Aspect
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       (i) Slope
                                                                       • The steepest slope (s) and the direction from the
                                                                          east () can be computed from 3 x 3 matrix.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                Slope calculation
                                                                                 Slope calculation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        Slope is defined by a plane tangent to a
                                                                         topographic surface, as modelled by the
                                                                         DEM at a point (Burrough, 1986).

                                                                        Slope is classified as a vector; as such it
                                                                         has a quantity (gradient) and a direction
                                                                         (aspect).

                                                                        Slope gradient is defined as the maximum
                                                                         rate of change in altitude (tan )
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                Example: Slope from elevation data
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       (ii) Aspect
                                                                       • The aspect that is, the slope faced to azimuth is
                                                                          180° opposite to the direction of q
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       Figure 1. Slope components, note that slope gradient can be
                                                                       express in percent or in degrees
                                                                                  Aspect calculation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        Aspect identifies the steepest downslope
                                                                         direction from each cell to its neighbors. It can
                                                                         be thought of as slope direction or the compass
                                                                         direction a hill faces.
                                                                        It is measured clockwise in degrees from 0 (due
                                                                         north) to 360, (again due north, coming full
                                                                         circle). The value of each cell in an aspect
                                                                         dataset indicates the direction the cell's slope
                                                                         faces. Flat areas having no downslope direction
                                                                         are given a value of -1.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                         data
                                                            Example: aspect from the elevation
                                                                           2. Drainage Network and
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                                  Watershed
                                                                       • The lowest point out of the eight neighbors is
                                                                         compared with the height of the central point
                                                                         to determine the flow direction.
                                                                              Surface Specific points
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • + is assigned if the height of the central point is higher than
                                                                         the one of the eight neighbors and - if lower.
                                                                          – A peak can be detected if all the eight neighbors are lower.
                                                                          – A pit or sink is formed if all the eight neighbors are higher
                                                                          – A pass can be extracted if the + and - alternate around the central
                                                                            point with at least two complete cycle.
                                                                                Remote Sensing and GIS in Water Management © Dr. Saiful Islam, IWFM, BUET




                                                                              4. Shade and 5.Shadow
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Shade is defined as reduced reflection depending on
                                                                         the angle between the terrain surface and the incident
                                                                         light such as the sun.

                                                                       • Shadow is projected areas that the incident light cannot
                                                                         reach because of visual hindrance of objects on terrain
                                                                         relief
                                                                                       Hill Shading
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        The effect of hill shading on the assumption of an ideally
                                                                       diffused reflecting surface (called Lambertian surface)
                                                                       can be computed as follows:
                                                                           Relative shading = cos  = |nxsx + nysy+ nzsz |≤ 1.0
                                                                           where  : angle between incident light vector s and surface normal n
                                                                                        Altitude
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • The altitude is the slope or angle of the
                                                                         illumination source above the horizon. The
                                                                         units are in degrees, from 0 (on the
                                                                         horizon) to 90 degrees (overhead). The
                                                                         default is 45 degrees.
                                                                                       Azimuth
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • The azimuth is the angular direction of the
                                                                         sun, measured from north in clockwise
                                                                         degrees from 0 to 360. An azimuth of 90 is
                                                                         east. The default is 315 (NW).
                                                                       Hillshading from elevation data
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                        The hillshade
                                                                        below has an
                                                                        azimuth of 315
                                                                        and an altitude
                                                                        of 45 degrees.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                             maps of Glacier National Park
                                                            Examples: A slope and hillshade
                                                                             Using hill shading for display
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • By placing an elevation raster on top of a created
                                                                         hillshade, then making the elevation raster transparent,
                                                                         you can create realistic images of the landscape.

                                                                                                               Hillshade + elevation
                                                                       Generation of Contour Lines
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Contour lines are one of the terrain features which
                                                                         represent the relief of the terrain with the same
                                                                         height. There are two types of contour lines in
                                                                         visualizing GIS data:

                                                                       • Vector Line Drawing
                                                                         In case when the terrain points are given in grid, the
                                                                         simplest method is to divide the square cell into two
                                                                         triangles mechanically.

                                                                       • Raster Image
                                                                         Contour image with painted contour terraces, belts or
                                                                         lines instead of vector lines will be generated in raster
                                                                         form.
                                                                        Interpolation of Elevation from
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                                   Contours
                                                                       • Digital elevation model (DEM) is very often generated by
                                                                         measuring terrain points along contour lines using a digitizer.
                                                                         DEM with contour points should be provided with an algorithm
                                                                         interpolate elevation at arbitrary points. There are several
                                                                         interpolation methods as follows.

                                                                        Profile Method
                                                                         A profile passing through the point to be interpolated will be
                                                                         generated and linear or spline curve applied.
                                                                        Proportional Distance Method
                                                                         According to distance to two adjacent contour lines, the
                                                                         elevation is interpolated proportionally with respect to the
                                                                         distance ratio.
                                                                        Window Method
                                                                         A circular window is set up around a point to be interpolated and
                                                                         adjacent terrain points are used to interpolate the value using
                                                                         second order or third order polynomials.
                                                                        TIN Method
                                                                         TINs are generated using terrain points along contour lines.
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam


                                                                       Interpolation Methods
                                                                       Examples: A Digital Elevation Model and
                                                                       associated contour map of Glacier Nat'l
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                                        Park
                                                                        Automated Generation of DEM
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Automated generation of DEM is achieved by photogrammetric
                                                                         methods based on stereo aerial photography and satellite stereo
                                                                         imagery.
                                                                       • Parallax is defined as difference between left and right photographs or
                                                                         image coordinates. The higher the elevation is, the bigger the parallax
                                                                         is. If the parallax is constant, equal elevation or contour lines will be
                                                                         produced.
                                                                       Triangulated Irregular Network (TIN)
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                       • Triangulated irregular network or TIN is a DEM with a
                                                                         network of triangles at randomly located terrain points.


                                                                       Contouring of TIN is based on
                                                                       the following procedure:

                                                                       step 1: find the intersect of contour and a
                                                                       side.
                                                                       step 2: assign the "reference point" with
                                                                       the symbol r to the vertex above the contour
                                                                       height and the "sub-point" with the symbol
                                                                       s to the vertex below the contour height.
                                                                       step 3: shift over to the transversing to find
                                                                       the third vertex in the triangle by checking
                                                                       whether it is a reference point (r) or sub-
                                                                       point (s).
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                                                Example: TIN Creation
Remote Sensing and GIS in Water Management @ Dr. A.K.M. Saiful Islam




                                   Thank you

								
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