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Spatial Analyst—Module 1 Merging/Projecting DEMs

Spatial Analyst—Module 1: Merging/Projecting DEMs

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Module 1: Merging/Projecting DEMs (PDF; 687 KB)

Objectives and Tasks

  1. Enable the Spatial Analyst Extension
  2. Add Spatial Analyst Toolbar to the View
  3. Merge DEMs
  4. Project DEM Data
  5. Clip a DEM to a Defined Area
  6. Calculate Elevation Values by Map Unit

Enable Spatial Analyst Extension

  1. Open an ArcMap session
  2. Click on Tools then Extensions
  3. From the popup menu, place a check next to the Spatial Analyst extension

Add Spatial Analyst to the View

  1. Click on View then Toolbars
  2. Click on Spatial Analyst
  3. Place the Spatial Analyst Toolbar to desired location

Merge DEMs

  1. To start, click on the ArcCatalog icon
  2. Click on the Toolbox icon
  3. In the ArcToolbox window, click on Soils -- Spatial Analyst Tools then Merge Rasters then Mosaic To New Raster
In the Mosaic to New Raster dialog box
  1. For Input Rasters, navigate to c:\home\data\spatial_analyst\tmp and select “bing and victor” (hold down the ctrl key and select both)
    “bing and victor” are automatically added to the batch window
  2. For Output Location, navigate to c:\home\data\spatial_analyst\tmp
  3. Name the Raster dataset name with an extension; e.g., “bv27”
  4. Set the Pixel type to “32_BIT_FLOAT” (from metadata)
  5. Set the Mosaic Method to “MEAN”
  6. Accept the other default settings
  7. Click OK
  8. Close ArcCatalog after the merge process is completed

Project DEM Data

In the following example, a DEM with a projection of Nad_27_UTM_Zone_11 will be projected to Nad_83_UTM_Zone_11.
Although ArcMap has the capability to project on the fly, it occasionally projects layers inaccurately (this is a known “bug” with version 9). Because of this potential flaw, it is recommended to project DEMs before they are brought into an ArcMap project. It is critical that DEMs are projected accurately because many other layers are derived from its data, such as slope, aspect, and curvature.

  1. To start, click on the ArcToolbox icon
  2. From ArcToolbox, click on Soils -- Spatial Analyst Tools then Projection then Project Raster
In the Project Raster dialog box
  1. For Input raster navigate to c:\home\data\spatial_analyst\tmp and select “bv27”
  2. Place the Output raster in c:\home\data\spatial_analyst\dems and name the file; e.g., “bv83”
  3. Click on the icon next to the Output coordinate system
  4. Click on Select then Projected Coordinate Systems then UTM then Nad 83 then NAD 1983 UTM Zone 11N.prj
  5. For Resampling technique select “CUBIC” (for rasters: do not use “NEAREST” resampling technique), accept the default value for Output cell size, and click OK

Clip a DEM to a Defined Area

In this following exercise, the Extract by Mask Tool will be used to clip a DEM. Also, Environment Settings will be utilized to set the Current Workspace, Scratch Workspace, Output Extent, and Cell Size.

  1. Click on the Add Data icon
  2. Navigate to c:\home\data\spatial_analyst\shapefiles
  3. Add the “new_area” shapefile
  4. The “new_area” shapefile will be used to clip the “bv83” DEM.
  5. To change the “new_area” symbol
  6. Left click on the symbol in the Layers windows
  7. Select “Hollow,” change the Outline Width to “2,” and select “red” for the Outline Color
  8. Click OK

Calculate Elevation Values by Map Unit

To set ArcToolbox Environment Settings for Workspace, Output Extent, and Cell Size:
  1. Right click in an open area in the Toolbox window
  2. Select Environments
In the Environment Settings dialog box, click on the dropdown arrow next to General Settings
  1. Set the Current Workspace to C:\home\data\spatial_analyst\dems
  2. Set the Scratch Workspace to C:\home\data\spatial_analyst\dems
  3. Set the Output Extent to Same as Layer “new_area”
Click on the dropdown arrow next to Raster Analysis Settings
  1. Set the Cell Size to Same as Layer “bv83”
  2. Accept all other default values and click OK
From ArcToolbox, click on Soils -- Spatial Analyst Tools then Extraction then Extract by Mask
  1. In the Extract by Mask dialog box
  2. Set the Input raster to “bv83”
  3. Set the Input raster or feature mask data to “new_area”
  4. Name the Output raster to “bv83_clip”
  5. Click OK
In the Layers window, collapse all legends, turn off “bv83,” and move the “new_area” layer to the top.
Review of Environment Settings
Environment Settings control Spatial Analyst processes within the ArcToolbox window
  1. The Current Workspace setting directs ArcGIS where to look for input data and where to store output data
  2. The Scratch Workspace setting directs ArcGIS where to store temporary data
  3. The Output Extent setting controls the overall size of the study area
  4. The Mask setting controls which cells within an Output Extent will be considered (clipping function)
  5. The Cell Size setting controls resolution (the smaller the cell size, the finer the resolution)

Calculate Elevation Values by Map Unit

In this exercise, an elevation grid (“bv83_clip”) is used with a soil polygon layer (“soil_clip”) to produce a map unit elevation table. The output table includes elevation values for Min, Max, Mean, and Standard Deviation for each map unit.

To add the “soil_clip” layer
  1. Click on the Add Data icon
  2. Navigate to c:\home\data\spatial_analyst\shapefiles
  3. Add “soil_clip.shp”

Click on the Spatial Analyst Toolbar then select Zonal Statistics  to create a table with elevation values for each map unit.

In the Zonal Statistics dialog box
  1. For Zone dataset, select a soil polygon layer; e.g., “soil_clip”
  2. For Zone field, select “MUSYM”
  3. For Value raster, select a DEM; e.g., “bv83_clip”
  4. Uncheck Chart statistics
  5. For Output table, browse to c:\home\data\spatial_analyst\tables and name the table “mapunit_elev”
  6. Accept all other default values
  7. Click OK to complete the process

The resulting table has several columns with statistical information about individual map units, including Min, Max, Range, Median, Mean, and Standard Deviation.