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Rasters and Surfaces

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'Raster is faster but vector is corrector' (Berry 1995) ... Open Layer Properties and select Symbology tab. Three symbology methods: Stretched ... – PowerPoint PPT presentation

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Title: Rasters and Surfaces


1
Lecture 6
  • Rasters and Surfaces

2
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

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

4
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
5
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
6
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.

7
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.

8
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

9
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

10
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
11
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.

12
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.

13
Joining multiple input rasters Mosaic
  • Merge adjacent tiles into one larger raster
    dataset
  • Works best for continuous data e.g elevation

14
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)

15
Using Rasters As Information
  • Stretch Symbolization pulls the values across a
    colour range.
  • No explicit colour-value relationship.
  • A variety of algorithms.

16
Using Rasters As Information
  • Classified Symbolization gives specific number
    ranges a specific colour.
  • No smooth transitions.

17
Using Rasters As Information
  • For multiband images, you can pick which colour
    is used for each band.

18
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.

19
Other effects
  • Effects Toolbar
  • Adjust
  • Contrast
  • Brightness
  • Transparency
  • But be careful if you have more than one raster

20
Spatial Analyst
Spatial Analyst package
ArcToolbox gt150 tools
GRID in ArcInfo. Most commands are now available
via ArcToolbox or Toolbar
21
Raster Calculator
Apply weights to rasters and use Map Algebra to
create new grids
22
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

23
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.

24
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.

25
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.

26
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?

27
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.

28
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.

29
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.
30
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
31
Using TINs As Information
Elevation
Aspect
  • TINs have three associated Attributes Slope,
    Elevation and Aspect.
  • You can shade on any of these.

Slope
32
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
33
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.

34
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.

35
ArcScene
  • Much of the 3D functionality of ArcMap stuff
    for doing real 3D with tilted landscapes,
    animation and export to 3D formats.

36
Extruding and Baseheight
  • Baseheight height above ground level to show
    the feature.
  • Extrusion height to extend into air.
  • Both set under a layers properties.

37
(No Transcript)
38
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
39
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.

40
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41
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)
42
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.

43
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.

44
Next Lecture
  • AML and Programming ArcInfo with Andy Evans
  • Mondays practical
  • Network analysis
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