Creating A Subset Data

Creating a Subset of Elevation Data The Spatial Analyst CD contains wonderful elevation data for all of North America, all of Africa, and all of Eurasia. Suppose you want only a small portion of that, like just the state of Texas. No sense using all of North America when you only want one state. Simple process: Just subset the data. Actually, you can create a subset of any gridded data set which can be displayed in ArcView with Spatial Analyst. The procedure is commonly of interest when working with elevation data for a large area, but would apply equally to other gridded data sets such as temperature, population density, or landcover. Once you've created this subset, you can also reclassify it and convert the reclassified version into shapes, which makes it easier to bring the data into views of a different projection, or into computers that do not have Spatial Analyst. This exercise will walk through the procedure for creating a subset of just Texas from the North America elevation data on the Spatial Analyst CD, then turning that subset into shapes.

To do this exercise, you need: • ArcView 3.0a or higher for Windows95 or WindowsNT or Unix, installed • Spatial Analyst for Windows95/NT or Unix, installed • North America elevation data from the Spatial Analyst CD • Detailed Texas polygon, from administrative boundaries from "ESRI Data & Maps volume 1," or from "ArcWorld Supplement," or from ArcUSA 1:2M • Plenty of "swap space" for creation of temporary grids Basic instructions: • ALL CLICKS ARE "SINGLE CLICK" UNLESS NOTED AS "DOUBLE CLICK." • "Make active" = focus the computer's attention on a theme by clicking one time directly on the name of a theme, so it looks "raised" in the list. • "Turn on/off" = click one time in the check box to the left of the theme name, making a check mark appear/disappear, so the theme displays or does not display in the map.

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PART 1: Setting Up the Procedure 1. Engage ArcView and load the Spatial Analyst extension. 2. Create a new view. 3. Add the North America elevation data from the Spatial Analyst CD (typically, d:\data\north_am\na_dem ). Be sure to set the "Data Source Type" as "Grid Data Source" in the Add Theme window. 4. Change the projection to match the data. Choose View/Properties, then Projection, then Custom, then select "Lambert Equal Area Azimuthal." Set the "Central Meridian" at "-100.00" and "Reference Latitude" at "45.00". (Be sure to click back and forth in these two boxes once or twice, to make sure the numbers get "registered."). Back in the view, turn on the theme NA_DEM. 5. Add a polygon theme containing a detailed boundary for Texas, using one of these three options: • Esridata\world\admin95\admin95.shp (this exercise uses this choice) • Aw_supp\shapes\world\admin.shp • Arcusa\arcusa_d\usa_2mg\st2m 6. Turn on the polygon theme and check to see that the polygons for North America match well with the theme NA_DEM. 7. Query the polygon theme to select all shapes which contain the name "Texas". Using the ESRIDATA example, the expression would be "Admin_name" = "Texas" 8. With polygon(s) selected, use Theme/Convert to Shapefile to create a new shapefile for just Texas. Name this shape TX_POLY. Add this new theme to the view. 9. Turn on TX_POLY and make sure it draws properly. If it does not, back up and try again, starting at Step#4. 10. Delete from the view the polygon theme that was the source for TX_POLY. Make active TX_POLY and zoom to the selected theme. 11. Save the project.

PART 2: Slicing Out the Desired Data 12. In the Analysis menu, set the Analysis Properties as follows: • Extent: Same as Tx_poly.shp • Analysis Cell Size: Same as Na_dem • Analysis Mask: No Mask Set 13. With TX_POLY.SHP as the active theme, choose Theme/Convert to Grid. Call this new theme TX_GRID. In the Conversion Field dialogue box that appears, choose something which will have the same value across the state, such as "Admin_name". When ArcView asks if you want to join attributes, select "No". When ArcView asks if you want to add the theme to the view, select "Yes." 14. Turn on the new grid theme TX_GRID. This will become the "cookie cutter" that will be used to subset the elevation data.

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15. In the Analysis menu, set the Analysis Properties as follows: • Extent: Same as Tx_poly.shp • Analysis Cell Size: Same as Na_dem • Analysis Mask: Tx_grid 16. Make active the NA_DEM theme. Choose the Analysis/Map Calculator item. In the lefthand box of the Map Calculator window, double-click the layer "Na_dem", which will copy this item into the box at the bottom of the window. (DO NOT use "Na_dem.Count"). Don't make any other changes to this window. Move the window so that you can see the status bar at the bottom of the ArcView application window. Click the "Evaluate" button at the bottom of the Map Calculator window. ArcView will now begin to cut out of the NA_DEM theme just the Texas portion, which may take a minute. You should see a blue "progress bar" inching across the ArcView status bar. When the process is complete, you should see a new theme in the view's contents, called Map Calculation 1. Close the Map Calculator window. 17. Turn on the new theme. It should draw directly on top of the existing map of Texas. 18. Save the project.

PART 3: Re-classifying data. 19. Map Calculation 1 has a new data range, from 0-2558 meters (or thereabouts). Engage the Legend Editor (double-click the theme) to edit the legend, as follows. 20. Change Classification to 13 classes, and type in the values as follows: • 0-25 • 25-50 • 50-100 • 100-150 • 150-200 • 200-300 • 300-400 • 400-600 • 600-800 • 800-1000 • 1000-1500 • 1500-2000 • 2000-2600 21. Change the colors as follows. • Set the 2000-2600 as deep brick red. • Set 0-25 as deep forest green. • Set 300-400 as bright yellow, and leave this item highlighted. • Click the "ramp colors" button (far right end of the Legend Editor buttons, above the choices for color schemes). With the 300-400 item "highlighted", ArcView will ramp from deep green to yellow, then yellow to brick red. 22. Apply the change.

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23. With Map Calculation 1 as the active theme, choose Analysis/Reclassify, and construct a new classification scheme, as follows. a) Click the Classify button and create a new set with 13 numbers. b) In the "Old Values" column, type in the values used in Step#20 above. Leave the "New Values" going from 1 through 13. c) Click "OK" and ArcView will create a new set of data called Reclass of Map Calculation 1. 23. Double-click the new theme and change its legend, as follows: a) Change the Legend Type to Graduated Color. b) Change the Classification Field to Value. c) Hit the Classify button and choose 13 classes, by equal interval. d) Change the items in the LABEL field (NOT in the "Value" field) using the 13 classes employed above. e) Change the colors using the green to yellow to red scheme above. f) Apply the changes. 24. Save the project.

PART 4. Creating Elevation Shapes 25. Using the Identify tool on any cell in the NA_DEM or Map Calculation 1 themes would give a value showing the elevation for that one cell. Doing a similar identify in the Reclass of Map Calculation 1 theme shows a value of from 1-13, meaning only a more general zone instead of a specific elevation. Now that there are discrete zones, each with a distinct elevation range, these zones can be converted into polygons. These polygons can then be brought into other views where a different projection is in place, or to computers where Spatial Analyst is not available. 26. Make Reclass of Map Calculation 1 the active theme. Choose Theme/Convert To Shapefile. Call the new theme TX_ELEV.SHP. ArcView will create the new theme, which will take a minute or two, and the status bar will run through three blue progress bars in succession. When complete, add the theme to the view. 27. Turn on the new theme, TX_ELEV.SHP. Double-click the new theme and change the legend, as follows. a) Change the Legend Type to Graduated Color. b) Change the Classification Field to Gridcode. c) Hit the Classify button and choose 13 classes by equal interval. d) Change the items in the LABEL field (not in the "Value" field) using the 13 classes employed above. e) Change the colors using the green to yellow to red scheme above. f) Change all the symbols so that they use no outline. g) Save the legend in the same directory where the shape is stored, and call the legend TX_ELEV.AVL. h) Apply the changes.

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28. Make the elevation data draw more quickly by creating a "Spatial Index", as follows: a) Open the theme table for TX_ELEV.SHP. b) Click one time on the title of the field called "Shape." c) Choose Field/Create Index. d) Close the theme table. 29. Test the result by copying this theme and pasting it into a new view with a different projection. The data should still be readable, as the elevation shapes just created are stored in decimal degree fashion. 30. Save the project.

In the future, you can engage the theme TX_ELEV.SHP, load the legend TX_ELEV.AVL, and it will appear just as you have saved it here.

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