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Lecture 6

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.

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Lecture 6

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  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 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

  10. Making Rasters: Registering Creates a World file (.**w) containing the transformations needed • Register <image> {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.

  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 (<512 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 > Options. • Each Symbolization dialog allows you to pick the background and NoDatacolours. • By default these are transparent. • With < 24 levels you can symbolize by Uniques.

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

  20. Spatial Analyst Spatial Analyst package ArcToolbox >150 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 <16 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 • TINs have three associated Attributes: Slope, Elevation and Aspect. • You can shade on any of these. Aspect Elevation 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 isn’t 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 layer’s properties.

  37. 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

  38. 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.

  39. 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)

  40. 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.

  41. 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.

  42. Next Lecture • AML and Programming ArcInfo with Andy Evans Monday’s practical • Network analysis

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