Lecture 6

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```							        Lecture 6
Rasters and Surfaces
Review of Vector GIS capabilities
 Vector data models: coverages, geodatabases
 Inputting data
 Editing spatial data in ArcEdit and ArcMap
 Data management
 Outputting data
 Displaying data and presentation
 Spatial analysis
 Network Analysis
This Lecture

 Rasters / GRIDs
 Surfaces / TINs
 3D - fly-throughs
Raster GIS

 Until a few years ago raster GIS was completely
divorced from vector GIS
 ESRI has overcome this

“Raster is faster but vector is corrector” (Berry 1995)

But the accuracy of this statement depends on application
AND
We must remember that both vector and raster are an
approximation

Berry, J.K. (1995) GIS World 8(6):35-35
Raster Data
 Pixel by Pixel data forms. Each is a Z Value for a
particular x,y position in the file.
 Can look at lots of formats.
 Most usually held in ArcGIS in GRID format.
 This format allows for Raster analysis.

2500 2502 2504 2506
2549 2501 2504 2506
2548 2500 2505 2505
2548 2549 2505 2505
Cartesian matrix
Raster Data
 Raster cells can be any size –
 Affects how the features of Earth are represented
 number of cells may affect storage and processing

 Can hold height information as the values, or categories of e.g. land
use.

 Can be integers (usually categories/discrete data) or floating point
(continuous data)

 Cells allocated NoData if they are not of the correct number type

 Can be a single band, or a composite image of several bands, three of
which you show as red/green/blue.

 To work with other Coverages/Feature Classes each needs registering
Uses of Raster data
 Sort same problems as vectors
 A raster can pretend it is a surface – although
the surface can be lumpy (however small you
make the cell size)
 Simulations eg forest fires (add rasters on wind
speed, direction, slope etc)
 http://geomac.usgs.gov/# - Wildland Fire
Support
 Hydrologic modelling – simulating how water
flows over the surface of the Earth.
Making Rasters: Importing to ArcMap
 Just open TIFF or
JPEG image files.
 Import from
Coverages.
 Import from Digital
Elevation Models
(DEMs) and other
formats.
 ASCII format common
for exchanging data
Pyramids
 Pyramids are a way of storing
Rasters, so that the resolution
shown changes with the
viewing scale.
 I.e. When you see more of the
map spatially, you see less of
the detail. Detail you wouldn’t
have seen anyway because of
the screen resolution isn’t
shown.
 This speeds up drawing.
 When importing data, you’re
asked if you want to generate
this.
 If you later want to generate
them – toolbox – Data
Management > Raster
Making Rasters: Registering
 Register               Creates a World file (.**w)
<image>                containing the transformations
{coverage}             needed
command in Arc.

 Allows you to pick
places on an image
Feature Class or x,y
coordinates if the
latter absent.
 Interactive.
Making Rasters: Projection

 A Raster must also have a defined coordinate
system.
 You can define this in ArcCatalog.
 Right-click on the file and select its Properties.
 The process is then the same as setting up a
Feature Dataset Spatial Reference.

 NB: Remember not to move image files outside of
ArcCatalog once the registration and projection are
defined.
Using Rasters: Digitising
 You can use Rasters in the
same way as any other
dataset, though the editing is
limited.
 One use is in Heads-up
tracing” of photos to give
Vectors.
 E.g. Aerial Photos to Road
Arcs.
Joining multiple input rasters: Mosaic
one larger raster dataset
 Works best for
continuous data e.g
elevation
Symbolizing rasters

By default, rasters are drawn in shades of
grey
Open Layer Properties and select
Symbology tab
Three symbology methods:
Stretched
Classified
Unique Values (<512 unique cell values)
Using
Rasters: As
Information

 Stretch Symbolization pulls
the values across a colour
range.
 No explicit colour-value
relationship.
 A variety of algorithms.
Using
Rasters: As
Information

 Classified Symbolization
gives specific number
ranges a specific colour.
 No smooth transitions.
Using
Rasters: As
Information

For multiband images,
you can pick which
colour is used for each
band.
 You can also pick the
Using
bands in Tools >            Rasters: As
Options.
Information
 Each Symbolization dialog
allows you to pick the
background and NoData
colours.
 By default these are
transparent.
 With < 24 levels you can
symbolize by Uniques.
Other effects

 Effects Toolbar
Contrast
Brightness
Transparency
 But be careful if you
have more than one
raster
Spatial Analyst             Spatial Analyst package

ArcToolbox
>150 tools

GRID in ArcInfo. Most commands are now
available via ArcToolbox or Toolbar
Raster Calculator

Apply weights to rasters and use “Map Algebra” to create new grids
Using Rasters: 3D Raster Analysis
 3D datasets are known
as Raster Surfaces.
 Rasters can be used to
store Surfaces (i.e. each
pixel value is a height).
 Of course, “height” need
not be literal height – it
could be the amount of
some variable.
 There are a suite of
analysis tools for this e.g.
 Cut and Fill
 Viewshed
 Aspect
 Slope
Cut and Fill Tool
 Takes in a before and after raster.
 If the first raster has had material removed from some
areas, and shifted to other.
 Results:
Raster map of changes.
Table of volumes.
 Polygon feature class showing changed regions.
Aspect
 Slope direction or the compass direction a hill faces
 Flat areas having no downslope direction are given a
value of -1

 Why use the Aspect function?

Find all north-facing slopes on a mountain as part of a
search for the best slopes for ski runs.
Calculate the solar illumination for each location in a
region as part of a study to determine the diversity of
life at each site.
Find all southerly slopes in a mountainous region to
identify locations where the snow is likely to melt first
as part of a study to identify those residential
locations likely to be hit by runoff first.
Identify areas of flat land to find an area for a plane to
land in an emergency.
Slope
 Most frequently run on an
elevation dataset
 Steeper slopes are
slope raster.
 The function can also be
used with other types of
continuous data, such as
population, to identify
sharp changes in value.
Viewshed Tool
 “What can be see from…?” sometimes called
Visibility calculations.
 Can we see this new dam from a pleasure spot?
 Can we see a road bend from the top of a hilly road?
 Can we monitor the whole of a political march using
this set of CCTV cameras?
 Where will be damaged by a nuclear flash at this
point?
Visibility Tool
 Allows you to enter…
1+ observer positions as a Point Coverage.
A vertical and horizontal view angle for each.
Offset (height above the surface)
See help for more details
 Results.
Either a raster or Polygon file containing areas that
can be seen from the observers without obstruction.
Tables containing either the number of observers that
can see a point (frequency) or (for <16 observers) a
list of which observers can see where.

 Obviously a very computationally intensive
process that results in large results files.
3D Vectors
 There are some operations that are much easier
with 3D Vector data.
 Triangulated Irregular Networks (TINs) store
Surfaces as 3D Vectors. Each line represents a
slope breakpoint.
 Note again that while the Z direction (“up”) is
usually height, it could be some other variable.
Making TINs: Importing
 Raster to Tin
Massive Job, Very
slow.

The height difference
within which a Vector for a
location must fall when
compared with the GRID.

Automatically given
height conversion factor
e.g. feet to meters.
Making TINs
 From GRID – goes through
putting lines between high
points and testing the z
difference against the
lines/points where needed.
GRID

Both                          TIN
Using TINs: As Information

Aspect                 Elevation

 TINs have three
associated Attributes:
Slope, Elevation and
Aspect.
Slope      any of these.
Using TINs: As Smooth Relief Shading
 Place your DEM GRID (or
other data) above your
 Use the Effects Toolbar
Transparency Tool to set
the GRID to 70%
DEM      transparency.

TIN                     Overlay
Using TINs: Volume Analysis

 TINs can be used with the Volume Surface Tool to
calculate the volume of a surface from some base z
value upwards.
Tools for Rasters and TINs

 The Contour Tool will turn Rasters or TINs into
contour Polygon Feature Classes.

 In addition, there are conversion tools to convert
Rasters and TINs into Polygon files, and vice
versa.
 This is one way to get from an image of e.g. a
forest, to forest boundaries.

 However, the conversions to Polygons tend to give
blocky results because of the square edges of the
pixels. This isn’t the case with the Contour Wizard
- smoothing possible.
ArcScene

 Much of the 3D
functionality of
ArcMap + stuff for
doing real 3D with
tilted landscapes,
animation and
export to 3D
formats.
Extruding and Baseheight
 Baseheight – height
above ground level to
show the feature.
 Extrusion – height to
extend into air.
 Both set under a
layer’s properties.
Using TINs: Fly-throughs
 With Arc 3D Analyst
you can use such
overlays to generate
3D scenes and fly-
throughs.

http://www.ordn
ancesurvey.co.
uk/oswebsite/gi
sfiles/section2/
movies/bennevi
s.mpa
Animation
 Show the Animation toolbar.
 Either push the Record button on the Play
toolbar,
 Or take keyframes and use the Animation
Manager to string them together.
 Push play to see them.
 Can export to AVI files.
Using TINS: VRML
   One format you can convert TINs into is VRML.
   Also export VRML from ArcScene.
   Virtual Reality Modelling Language.
   This is easier to generate and manipulate than fly-throughs.
   Comes in a text file that looks like HTML.
   Users can then walk through the landscape.

VRML (Virtual Reality Modeling
Language, pronounced vermal or by its
initials, originally known as the Virtual
Reality Markup Language) is a standard
file format for representing 3-
dimensional (3D) interactive vector
graphics, designed particularly with the
World Wide Web in mind.
(Wikipedia, accessed 2 November
2007)
Summary

 We can examine Raster data in a number of
formats, but to do analysis on it we really need to
import it as a GRID in arc.
 GRIDs can store Raster 3D information.
 TINs can store Vector 3D information.
 We can display by height / Z Value in classes or
continuously, but we can also display aspect and
slope data with TINs.
Summary

 3D Analyses include…
Volume calculations (Cut and Fill, Volume).
Viewshed calculations.
Contour and boundary calculations.

 We can output our data as fly-throughs (with 3D
Analyst) and VRML.
Next Lecture
 AML and Programming ArcInfo with Andy Evans

Monday’s practical
    Network analysis

```
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