Calculate Slope for a Site Suitability Model

I recently received a call from one of our Surfer users who was trying to create a site suitability model for a new manufacturing development. The user needed to find areas within the proposed site where the slope was under 10 degrees. The site suitability model required specific slope to be respected; the areas that were under the threshold criteria of 10% would be considered potential locations within the site to locate the new development.

A site suitability model can be easily developed in Surfer by creating a slope grid from a digital elevation model or DEM for the area, masking the slope grid to the site boundary, and creating a contour map that highlights the areas that meet the 10% or under criteria. Since this is such an interesting workflow, I thought it would be a great topic to blog about it so others in the Surfer community could benefit from seeing the approach.

Create a Slope Grid in Surfer
The first step in creating the suitability model is to determine the slope of the DEM of the site using Surfer. Surfer’s Grid | Calculus command can be used to do this easily by following these steps:

1. In Surfer, click Grid | Calculus.
2. In the Open Grid dialog, navigate to the GRD or DEM and click Open.
3. In the Grid Calculus dialog, expand the Terrain Modeling selection and select Terrain Slope.
4. Name the Output Grid File and click OK to create the slope grid.

Highlighting the 10% Slope Areas
Now that the slope grid has been created from the digital elevation model, it needs to be blanked by the proposed site boundary so that only the areas within the site boundary contain slope data. To blank the slope grid:

1. Click Grid | Blank.
2. In the Open Grid dialog, navigate to the digital elevation model and click Open.
3. In the Open dialog, navigate to the BLN file of the area of interest and click Open.
4. In the Save Grid As dialog, name the grid and click Save.

Now that the slope grid has been blanked to the potential site boundary, a contour map can be created that highlights the areas within the site boundary that have a slope of 10% or less. This can be done by adding slope contours at 0% slope, 5% slope, and 10% slope.

1. Click Map | New | Contour Map.
2. In the Open Grid dialog, navigate to the blanked slope grid and click Open.
3. In the Object Manager, click the Contours layer to select it.
4. In the Property Manager, click the Levels tab.
5. Change the Level Method to Advanced and click the Edit Levels button.
6. In the Levels for Map dialog, delete all of the levels except 0, 5, and 10.
7. Assign these levels an appropriate color and fill pattern and click OK.

We now have a map of the locations within the site boundary that are less than 10% slope. In the map above, areas that are green are the most suitable for locating the new development and are under 5% slope. Areas that are highlighted in yellow are between 5% and 10% slope, which is still suitable for locating the development.  All areas in red have a slope over 10% and are not good location candidates for the new development. The contours can also be exported from Surfer to be used in 3rd party mapping and CAD applications.