Surface_Terrain Analysis

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Surface_Terrain Analysis Powered By Docstoc
					         3D and Surface/Terrain
                                                             Prepared by:

                                      George McLeod

                                                            With support from:

                                                                 NSF DUE-0903270

                                                             in partnership with:

Geospatial Technician Education Through Virginia’s Community Colleges (GTEVCC)
         Digital Terrain Models
• A digital terrain model is a model providing a
  representation of a terrain relief on the basis
  of a finite set of sampled data
• Terrain data refers to measures of elevation at
  a set of points V of the domain plus possibly a
  set E of non-crossing line segments with
  endpoints in V

              Data Sampling
• Regular

• Irregular
Sampling effects resolution
 Our three Primary terrain Models
• Digital Elevation Models (DEMS) – aka Regular
  Square Grids (RSGs)
• Triangulated Irregular Networks (TINS) – aka
  Polyhedral terrain models
• Contour Maps – aka “topo” maps
The Data…


(LIght Detection And Ranging)
         Introduction to the Data
• Terrain mapping
• Land surface is 3-D
• Elevation data or
  z-data is treated as a
  cell value or a point
  data attribute rather
  than as a coordinate.
• Digital Elevation Model (DEM)
  – Gridded array of elevation points obtained from a
    variety sampling methods
A constant function can be associated with each
square (i.e., a constant elevation value). This is called
a stepped model (it presents discontinuity steps
along the edges of the squares)

• The function defined on each square can also be
  a bilinear function interpolating all four elevation
  points corresponding to the vertices of the
• Triangulated Irregular Network (TIN)
  – Series of non-overlapping triangles
  – Elevation values are stored at nodes
  – Sources: DEMs, surveyed elevation points, contour
    lines, and breaklines
  – Breaklines are line features
    that represent changes of
    the land surface such as
    streams, shorelines, ridges,
    and roads
• Example of a TIN based on irregularly
  distributed data
Data for Terrain Mapping and Analysis
• Triangulated Irregular Network (TIN)
  – Not every point in DEM is used
  – Only points most important
  – VIP (Very Important Points) algorithm
  – Maximum z-tolerance algorithm
  – Delaunay triangulation: all nodes are connected to
    their nearest neighbor to form triangles which are
    as equi-angular as possible.
                        Tins vs. Grids

                        DEM                                TIN

• Needs larger storage capacity   • Needs smaller storage capacity

• Computationally difficult       • Computationally simpler

• Flexibility of data sources     • Fixed with a given cell size

• Can add points                  • Cannot add sample points

• Better display                  • Raster display

• More efficient                  • Less efficient
                  Contour Mapping
• Contouring is most common
  method for terrain mapping
• Contour lines connect points
  of equal elevation (isolines)
• Contour intervals represent the
  vertical distance between
  contour lines.
• Arrangement of contour lines
  reflect topography
            Digital Contour Maps
Contours are usually available as sequences of points

A line interpolating points of a contour can be obtained in different ways
Examples: polygonal chains, or lines described by higher order equations
Digital Contour Maps: properties
They are easily drawn on paper

They are very intuitive for humans

They are not good for complex
automated terrain analysis
                   Contour Profile Mapping
• Vertical profile shows changes in elevation along a line, such as a hiking
  trail, road or stream.
                 Cartographic Terrain Mapping
•   Hill shading is also known as a shaded relief or simply shading
•   Attempts to simulate how the terrain looks with the interaction between sunlight
    and surface features.
•   Helps viewers recognize the shape of land-form features on a map.
• Four factors control the visual effect of hill-
   – Sun’s azimuth is direction of incoming light (0 to
   – The sun’s altitude from horizon (0-90°)
   – Surface slope (0-90°)
   – Surface aspect (0 to 360°)
                          Hypsometric Tinting
•   Hypsometric tinting

     – Applies different color symbols to represent elevation or depth zones.
                   Methods of Analysis
•   Slope measures the rate of change of elevation at a surface location

•   Aspect is the directional measure of the slope (degrees- 4 or 8 directions)

•   Hillshade, refer to previous slides

•   Line of sight refers to the straight line visibility from an observer to a feature

•   Viewshed analysis refers to the areas of the land surface that are visible
         from an observation point or points.

•   Watershed analysis refers to an area that drains water and other substances
         to a common outlet.

•   Area and volume calculations
Connectivity Function Example:
     Viewshed Analysis

Image Source: Chrisman, Nicholas.(2002). 2nd Ed. Exploring Geographic Information Systems. p 198. fig. 8-14   .
Line of Sight Analysis

Setting a hypothetical light source and calculating the illumination values for each
cell in relation to neighboring cells. It can greatly enhance the visualization of a
surface for analysis or graphical display.

                                                        Azimuth 315°, altitude 45°
•   Viewshed identifies the cells in an input raster that can be seen from one
    or more observation points or lines.
•   It is useful for finding the visibility. For instance, finding a well-exposed
    places for communication towers

                                                            hillshaded DEM as background
Surface Area and Volume
Application: Environmental Impact Analysis

    3D landscape model impact on natural beauty
         Application - Flood Risk

3D height data changing water levels-danger areas
The 3rd Dimension: Height Analysis – combining
           several methods together

 •   Contours
 •   Hill shading
 •   Spot height symbols
 •   Cliff & slope symbols
 •   Viewpoint symbols
  3D Terrain Analysis: Summation
• GIS does not always provide exact answers to
  problems, but by identifying trends based on
  geography, GIS can reveal patterns that can
  help us make informed decisions.
• A GIS can improve decision-making; it cannot
  make decisions for us.

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