Rasters and Surfaces

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Lecture 6

• Rasters and Surfaces

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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

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This Lecture

• Rasters / GRIDs
• Surfaces / TINs
• 3D - fly-throughs

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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
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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
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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 and a coordinate system adding.
  

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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.

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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

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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 wouldnt have
  seen anyway because of the screen resolution
  isnt shown.
• This speeds up drawing.
• When importing data, youre asked if you want to
  generate this.
• If you later want to generate them toolbox
  Data Management gt Raster

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Making Rasters Registering

• Register ltimagegt coverage command in Arc.
• Allows you to pick places on an image and link
  them to a Feature Class or x,y coordinates if the
  latter absent.
• Interactive.

Creates a World file (.w) containing the
transformations needed
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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.

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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 Digitizing, i.e.
  traditional tracing of photos to give Vectors.
• E.g. Aerial Photos to Road Arcs.

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Joining multiple input rasters Mosaic

• Merge adjacent tiles into one larger raster
  dataset
• Works best for continuous data e.g elevation

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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 (lt512 unique cell values)

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Using Rasters As Information

• Stretch Symbolization pulls the values across a
  colour range.
• No explicit colour-value relationship.
• A variety of algorithms.

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Using Rasters As Information

• Classified Symbolization gives specific number
  ranges a specific colour.
• No smooth transitions.

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Using Rasters As Information

• For multiband images, you can pick which colour
  is used for each band.

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Using Rasters As Information

• You can also pick the bands in Tools gt Options.
• Each Symbolization dialog allows you to pick the
  background and NoData colours.
• By default these are transparent.
• With lt 24 levels you can symbolize by Uniques.

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Other effects

• Effects Toolbar
• Adjust
• Contrast
• Brightness
• Transparency
• But be careful if you have more than one raster

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Spatial Analyst
Spatial Analyst package
ArcToolbox gt150 tools
GRID in ArcInfo. Most commands are now available
via ArcToolbox or Toolbar
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Raster Calculator
Apply weights to rasters and use Map Algebra to
create new grids
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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

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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.

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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.

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Slope

• Most frequently run on an elevation dataset
• Steeper slopes are shaded red on the output slope
  raster.
• The function can also be used with other types of
  continuous data, such as population, to identify
  sharp changes in value.

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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?

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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 lt16
  observers) a list of which observers can see
  where.
• Obviously a very computationally intensive
  process that results in large results files.

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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.

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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.
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Making TINs

• From GRID goes through putting lines between
  high points and testing the z difference against
  the raster, then adds more lines/points where
  needed.

GRID
Both
TIN
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Using TINs As Information
Elevation
Aspect

• TINs have three associated Attributes Slope,
  Elevation and Aspect.
• You can shade on any of these.

Slope
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Using TINs As Smooth Relief Shading

• Place your DEM GRID (or other data) above your
  Elevation shaded TIN.
• Use the Effects Toolbar Transparency Tool to set
  the GRID to 70 transparency.

DEM
TIN
Overlay
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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.

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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 isnt the case with the Contour
  Wizard - smoothing possible.

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ArcScene

• Much of the 3D functionality of ArcMap stuff
  for doing real 3D with tilted landscapes,
  animation and export to 3D formats.

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Extruding and Baseheight

• Baseheight height above ground level to show
  the feature.
• Extrusion height to extend into air.
• Both set under a layers properties.

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Using TINs Fly-throughs

• With Arc 3D Analyst you can use such overlays to
  generate 3D scenes and fly-throughs.

http//www.ordnancesurvey.co.uk/oswebsite/gisfiles
/section2/movies/bennevis.mpa
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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.

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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)
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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.

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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.

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Next Lecture

• AML and Programming ArcInfo with Andy Evans
• Mondays practical
• Network analysis