Skip

Spatial Analyst—Module 2 Slope Gradient Model

Spatial Analyst—Module 2: Slope Gradient Model

If you encounter problems with the file provided on this page, please contact Soil Survey at 406-587-6818.

The information on this page is also available in Acrobat Reader format. Once you have saved the file to a local drive, open Acrobat Reader then open the saved file on your local drive.

Module 2: Slope Gradient Model (PDF; 777 KB)

Objectives and Tasks

Create a Slope Map and Intersect with Soil Polygons
  1. Review Slope Gradient Model
  2. Run Slope Gradient Model
  3. View and Symbolize the Slope Gradient Layer
  4. Intersect Slope Shapefile and Soil Polygon Layer
  5. Export Soil\Slope Intersect Data Table
  6. Summarize Slope by Soil Map Units

For the Curvature Model to work properly, the directory structure listed below must exist on your computer.

c:\temp
c:\home\data\curvature
c:\home\data\spatial_analyst\shapefiles
c:\home\data\spatial_analyst\tables

Review Slope Gradient Model

How does the Slope Gradient Model Work?
  • The Slope Gradient Model creates a polygon shapefile of selected slope breaks derived from an elevation grid (DEM). Users are required to enter the DEM, slope ranges, and slope breaks.
  • To overcome the residual effect of the contour-line bias, two generalization procedures are run. The first generalization is run on the DEM using a 2-cell radius, circular neighborhood function. The second generalization is run on a temporary slope layer, using a 3-cell rectangular neighborhood function.
Histogram of DEM
  • Should be smooth; peaks are contour-line elevations
  • Contour-line bias may cause a “rice paddy” effect on a hillslope.
Slope Gradient Model Flowchart

Input DEM
Raster Layer
convert m 2 ft: Converts meters to feet
elevft
Focal Statistics: Generalization: 2-cell circular neighborhood filtering
circ3elev
Slope: Calculation:Rise/Run
slopecirc2
Focal Statistics (2): 3-cell rectangular neighborhood generalization
slopecirc3
Reclassify: Assign slope ranges and breaks
slope_class
Single Output Map Algebra: 0.5% added to slopes
SingleOutput2
Int: Converts each value of a raster to an integer
Slope Raster
Raster to Polygon: Converts slope raster to a shapefile
Slope Shapefile
Add Field: Adds “Slope” Field
slope.shp
Calculate Field: Calculates “Slope field from “Gridcode” field
Slope Shapefile Output

Run Slope Gradient Model

  1. Open your ArcMap Project “sa_training”
  2. If Toolbox is not open, click on the Toolbox Icon
  3. From ArcToolbox, click on Soils -- Spatial Analyst Tools then Slope/Curvature Models then Slope Gradient
In the Slope Gradient dialog box
  1. Set Raster Layer to a DEM; e.g., “bv83_clip”
  2. Click on the Add Entry button and set the Old and New Values in the Reclassification window
  3. Click OK to run the model
  4. Once the model is completed, click Close
  5. The syntax for entering the Old Values requires a space on either side of the dashed line (# - #).
  • Old Values = slope ranges
  • New Values = slope breaks

View and Symbolize Slope Gradient Layer

To add the layer created by the Slope Gradient Model
  1. Click on the Add Data icon
  2. Navigate to c:\home\data\spatial_analyst\shapefiles
  3. Add the “slope.shp” layer to the map
  4. In the Data Frame window, double click on “slope” to open Layer Properties box
  5. Click on the Symbology tab
In the Layers Properties window under the Symbology tab
  1. Click on Categories then Unique values
  2. Select “Slope” for the Value Field
  3. Uncheck the “all other values” box
  4. Click on the “Add All Values” button
  5. Right click in the Color Scheme choice list and uncheck “Graphic View”
Under Color Scheme
  1. Select “Slope”
  2. Click OK

Intersect Slope Shapefile and Soil Polygon Layer

In ArcToolbox, click on Soils -- Spatial Analyst then Tools then Soil Intersect then Slope-Mapunit Dissolve

This model intersects a slope shapefile with a soil polygon layer. The model then dissolves polygons based on inputs (“MUSYM” and “Slope”).

If the output from this model is used in an Excel worksheet, the dissolve process is needed to reduce the number of records (Excel can only have up to 65,536 rows).

Slope-Mapunit Dissolve

This model is used to intersect a slope shapefile with a soil polygon layer. The model creates a shapefile named "slope_mapunit_dissolve" in c:\home\data\spatial_analyst\shapefiles (In order for this model to work, you need to create the same directory structure on your computer.)

In the Slope-Mapunit Dissolve dialog box
  1. Click on the Input Features dropdown arrow
  2. Add a soil polygon layer and the “slope” shapefile
In the Dissolve Field window
  1. Check “Slope” and “MUSYM.” This will aggregate data based on slope and map symbol.
  2. Click OK to begin process.
  3. A shapefile named “slope_mapunit_dissolve” is created in c:\home\data\spatial_analyst\shapefiles.

Export Soil\Slope Intersect Data Table

  1. Click on the Add Data icon
  2. Add the layer created by the Slope-Mapunit Dissolve Model
  3. Navigate to c:\home\data\spatial_analyst\shapefiles and select “slope_mapunit_dissolve”
In the Data Frame window
  1. Right click on the “slope_mapunit_dissolve” layer
  2. Select Open Attribute Table
  3. Click on the Calculate Area/Acres icon

If the Acres field is empty or has zeros, click on the Options button and select Reload Cache

To export your table
  1. Click on the Options button and select Export
  2. Browse to the c:\home\data\spatial_analyst\tables directory and name the file “soil_slope”
  3. Save the file to a “dBase Table”
  4. Click Save
  5. In the menus that follow, select OK to export and No to add the table to ArcMap

Summarize Slope by Soil Map Units

In Microsoft Excel
  1. Click on File then Open
  2. In the window “Look in,” navigate to c:\home\data\spatial_analyst\tables
  3. In the window “Files of type,” select “dBase Files”
  4. Click on the “soil_slope” file and click on Open
To create a Pivot Table
  1. In the dropdown menu, select Pivot Table and PivotChart Report
  2. Click on Data
In the Pivot Table Wizard
  1. Check the radio button for Microsoft Office Excel list or database
  2. Check the radio button for Pivot Table
  3. Click on Finish
From the PivotTable Field List
  1. Drag and drop the “MUSYM” and “Slope” fields into the “Drop Row Fields Here” box (make sure the MUSYM field is left of the Slope field)
  2. Drag and drop the “Acres” field to the “Drop Data Items Here” box

The resulting table displays the number of acres by slope class, for each map unit and lists the total acres by map unit.

To calculate the percent composition for each slope class, by map unit
  1. double click on the Sum of Acres field
  2. Click on Options
  3. Under Show data as:, select “% of column”
  4. Click OK to finish

These values represent the percentage of total acres for a map unit slope class. For example, 1.91% of the total acres are in a 2% slope class in map unit 010A. Also, 010A accounts for 2.18% of the total acres.

To view slope class percentages for a single map unit
  1. Click on the down arrow next to MUSYM
  2. Uncheck the box next to (Show All)
  3. Next, place a check next to any single map unit and click OK
Results
  • 87.62% of 010A is in a 0 to 2% slope class.
  • To view results for other map units, click on the down arrow, uncheck 010A, and place a check next to a different map unit.