011] Global Mapper - Terrain Information Polygons: Calculate Elevation/Slope Stats

Taking advantage of a brief moment of free time, I’m posting a short guide.

While we often visualize contour or slope analysis through maps, today we’ll explore a method to obtain general terrain information as simple numerical data.

To follow along easily, you can download the sample file below.

[File Download]

• Terrain_Info.zip (This is a safe proprietary data file for Global Mapper.)

Getting Terrain Information for a Specific Area

I’ve prepared a simple file that contains three layers: Terrain (DEM), Boundary, and Road.

First, let's look up the terrain information for the Boundary area.

To extract terrain data, the terrain layer must be active. You cannot perform this function if the terrain layer is turned off or if you are working outside the terrain layer's bounds.

Step-by-Step Instructions:

1. Select the Boundary Polygon using the Digitizer Tool.

2. Right-click and navigate to Analysis/Measurement -> Calculate Elevation/Slope Stats for Selected Feature(s).

3. A window will appear asking whether you want to create new point features at the minimum and maximum elevations found within the area.

• If you select "Yes," coordinate points for the highest and lowest spots will be added to your map.

• If you select "No," only the statistics will be generated without adding points.

Once you make a selection, a window displaying the calculated information for the area will pop up.

Explanation of Key Statistics

Here is a breakdown of each item provided in the results:

1. PERIMETER: The total boundary length of the selected area.

2. ENCLOSED_AREA: The total 2D area of the selected polygon.

3. MIN_ELEV_M: The lowest elevation point within the area.

   • MIN_ELEV_X / MIN_ELEV_Y: The X and Y coordinates of the lowest point.

4. MAX_ELEV_M: The highest elevation point within the area.

   • MAX_ELEV_X / MAX_ELEV_Y: The X and Y coordinates of the highest point.

5. AVG_ELEV_M: The average elevation of the area.

6. MODE_ELEV_M: The most frequently occurring elevation value.

7. STD_DEV_ELEV_M: The standard deviation of elevation. This indicates how much the terrain height varies from the average, helping you understand the ruggedness.

8. MAX_SLOPE_DEG / PCT: The steepest slope in the area, shown in degrees and percent.

9. AVG_SLOPE_DEG / PCT: The average slope angle of the entire area.

10. SURFACE_AREA_3D: The 3D surface area, taking terrain undulations into account (usually larger than the 2D enclosed area).

11. STD_DEV_SLOPE_DEG / PCT: The standard deviation of the slope, showing how much the steepness varies.

12. SLOPE_1QUAR_DEG / PCT: The bottom 25% (1st quartile) slope value, representing relatively flat areas.

13. SLOPE_MEDIAN_DEG / PCT: The median slope value, useful for understanding the overall steepness.

14. SLOPE_3QUAR_DEG / PCT: The top 75% (3rd quartile) slope value, representing steeper regions.

15. AVG_ASPECT: The average terrain orientation (azimuth) in degrees. This helps estimate sun exposure and wind direction (calculated clockwise from North).

After clicking "OK," the window closes, and this information is saved as attributes within the boundary polygon.

How to Re-check or Export the Data

If you want to view this information again later, you don't need to re-run the analysis.

1. Select the Digitizer Tool and double-click the boundary polygon.
   

   
   
   

2. Alternatively, select the Feature Info Tool and click on the polygon.

If you wish to save this data separately, select "Copy to Clipboard" in the Feature Info window. You can then paste it into Notepad or Excel.

When pasted into Notepad, it will look like this:

Conclusion

With just the terrain data (DEM) and a boundary polygon, there is so much you can analyze during the review stage:

• The exact area of the project site.

• Average slope, which is critical for determining development feasibility.

• Standard deviation of elevation and slope to estimate the scale of earthworks (cut and fill).

• The aspect, showing which direction the site faces.

It’s quite simple, isn’t it?

In the next session, I will demonstrate how to obtain terrain information by selecting a road (line feature), although the resulting items may differ slightly.

That's all for today!