011] How to Calculate Elevation and Slope Statistics for Polygons in Global Mapper

Calculate precise numerical terrain statistics for specific boundary areas using the Elevation/Slope Stats tool.

This tutorial explains how to extract quantitative data from a Digital Elevation Model (DEM) based on a selected boundary polygon. You will learn to generate reports including average slope, standard deviation of elevation, and 3D surface area, which are critical for site analysis and development planning.

While we often visualize contour or slope analysis through maps, I will demonstrate a method to obtain terrain characteristics as numerical statistics data.

In this tutorial, you'll learn how to extract terrain statistics from a selected area using Global Mapper.

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

[File Download]

Terrain_Info.zip (Verified and safe file for Global Mapper.)

How to Calculate Terrain Statistics for a Boundary Polygon

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

Control Center showing Terrain, Boundary, and Road layers
img 1 : Control Center showing Terrain, Boundary, and Road layers

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

To extract terrain data, the terrain layer must be active. This function only works when the terrain layer is enabled and the selected feature falls within the DEM coverage area.

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).

Right-click context menu navigating to Analysis/Measurement options
img 2 : Right-click context menu navigating to Analysis/Measurement options

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

Global Mapper Pro dialog box asking to create min/max elevation points
img 3 : Global Mapper Pro dialog box asking to create min/max elevation points

  • 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.

Statistics window showing calculated elevation and slope attributes
img 4 : Statistics window showing calculated elevation and slope attributes

Understanding Elevation and Slope 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 elevation varies from the average, helping you evaluate terrain variability.

  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 first quartile slope value (25th percentile), representing relatively gentle terrain.

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

  14. SLOPE_3QUAR_DEG / PCT: The third quartile slope value (75th percentile), representing steeper terrain conditions.

  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.

Map view showing the points created for the highest and lowest elevations
img 5 : Map view showing the points created for the highest and lowest elevations

How to Review and Export Terrain Statistics

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.

    Modify Feature Info window showing saved terrain attributes
    img 6 : Modify Feature Tool
    Modify Feature Information window
    img 7 : Modify Feature Information window

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

Feature Info tool
img 8 : Feature Info tool
Feature Information Window
img 9 : Feature Information Window

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.

Feature Information window highlighting the Copy to Clipboard button
img 10 : Feature Information window - Copy to Clipboard button

When pasted into Notepad, it will look like this:

Terrain statistics pasted into Windows Notepad
img 11 : Terrain statistics pasted into Windows Notepad

Why Terrain Statistics Matter in Site Analysis

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.

As you can see, Global Mapper makes it easy to extract detailed terrain statistics from DEM data.

In the next tutorial, we’ll explore how to calculate terrain statistics along line features such as roads.

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