To find the highest slope within an area around a point, create a slope grid, assign NoData values to the grid outside the desired radius, and view the grid info.
- Create the slope grid file.
- Click Grids | Calculate | Calculus.
- In the Grid Calculus dialog, click in the Input Grid dropdown list and select the map layer created using your grid file, or click the Browse button to navigate to and select your grid file.
- Choose Terrain Modeling | Terrain Slope.
- Change the Output Grid name and location, if desired.
- Uncheck the box next to Add grid as layer to.
- Click OK and the grid is created. (Click OK in the Surfer dialog telling you the grid was created.)
- Now create a BLN defining the area you want to calculate the maximum slope in or draw a polygon in a base layer. Please see this article for instructions to create a BLN file. If you have Surfer 14 or newer, you can create the circular area around a point in a base layer using the Features | New Features | Buffer command as described in the article Can I add a buffer around my Surfer object (or a setback around a well)?.
- Click the Grids | Edit | Assign NoData command.
- In the Assign NoData to Grid dialog:
- Select a map created using your slope grid file from the Input Grid field, or click the Browse button to navigate to and select your grid file.
- Select the base map created using your BLN, SHP, or DXF file from the NoData Polygon Boundary field, or click the Browse button to navigate to and select your boundary file.
- Toggle the appropriate option for the direction of blanking (inside, outside, or mixed). You can read more about these options in Surfer's Help.
- In the Output Grid field, enter a name for the new grid.
- Uncheck Add grid as layer to.
- Click OK.
- Click the Grids | Info | Grid Info command.
- In the Open Grid dialog, select the NoData grid file created above and click Open.
- In the Grid Information report, scroll down to the Univariate Grid Statistics section. The Maximum is the maximum slope value in the specified portion of the slope grid.
For additional statistics about the slope in this radius, you can:
- Convert the GRD file to a DAT file (Grids | Edit | Convert).
- Open the DAT file in the worksheet (File | Open in Worksheet).
- Sort on Column C (Data | Data | Sort).
- Remove the rows with NoData values (1.70141e+038).
- Select the desired data cells.
- Click the Data | Data | Statistics command.
- Check the boxes next to the desired statistics.
- Click OK to calculate those statistics for the selected cells.
Updated September 4, 2020