Lecture 6

1. Lecture 6 Rasters and Surfaces

2. Review of Vector GIS capabilities • Vector data models: shapefiles, coverages, geodatabases • Inputting data • Editing spatial data in ArcEdit and ArcMap • Data management • Outputting data • Displaying data and presentation • Spatial analysis tools • 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. • 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 or if data is misssing • To work with other Coverages/Feature Classes each needs registering and a coordinate system adding.

7. Modelling with Raster Data • Representational or descriptive models - A raster can attempt to represent objects in a landscape such as roads, water bodies as well as attributes and relationships • Process models – Simulate processes in landscape and may be used as predictive tool eg forest fires (add rasters on wind speed, direction, slope etc) • http://geomac.usgs.gov/# - Wildland Fire Support • Types of process models include: • Suitability or capability analysis – determine optimal location for entities e.g. schools, landfill site. • 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. Effects Toolbar Adjust Contrast Brightness Transparency But be careful if you have more than one raster Other effects

19. Spatial Analyst Spatial Analyst package ArcToolbox >150 tools GRID in ArcInfo. Most commands are now available via ArcToolbox or Toolbar

20. Working with Spatial Analyst Extension • Enable the extension and add the Spatial Analyst toolbar • Set the working directory • Where your results are written • Analysis coordinate system • By default, set to the same coordinate system as first input raster grid • Set the map extent • Bottom left and top right of your analysis results • Set the cell size • By default, set to the raster layer with the largest cell size • Temporary vs Permanent files • Results usually temporary – need to ‘Make Permanent’

21. Reclassification • Replacing certain cell values with new ones • E.g. change some values to NoData to create an analysis mask • Group values together • Reclassify to a common range so that input rasters with different scales can be compared.

22. Converting vector to raster • Some data layers are available as vector data sets but need to be converted to raster so that analyses can be carried out. • Select the cell size out your output raster • Will often look ‘blocky’ in comparison with vector original

23. Spatial Analyst tools A selection: • Local functions/cell statistics • Neighbourhood statistics • Zonal statistics • Surface tools • Cut and Fill • Viewshed • Aspect • Slope

24. 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.

25. 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.

26. 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.

27. 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?

28. 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.

29. Raster Calculator • Very powerful • Perform mathematical operations to combine multiple grids to produce an output or analysis • Simple arithmetic operators, Boolean, Trignometrical etc

30. 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.

31. Making TINs: Converting • 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.

32. 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

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

34. 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

35. 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.

36. 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.

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

38. 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.

40. 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.

42. Fly-throughs • You can use ArcScene to generate 3D scenes and fly-throughs. http://www.ordnancesurvey.co.uk/oswebsite/gisfiles/section2/movies/bennevis.mpa

43. 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.

44. 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).

45. Next Lecture • AML and Programming ArcInfo with Andy Evans Monday’s practical • Working with the Spatial Analyst toolbar.