010] Global Mapper - DEM Arithmetic: Combine/Compare Terrain Layers

Today, we will learn how to perform calculations on DEM (Digital Elevation Model) terrain data in Global Mapper. This feature is particularly useful when you have multiple overlapping terrain datasets and need to analyze the differences or relationships between them.

"Before we begin, if you missed the previous session, check it out here:"

[004] Creating Terrain Path Profiles (Cross-Sections)

009] Global Mapper - Visibility Frequency Analysis

Introduction to DEM Calculation in Global Mapper

15 Available Terrain Operations

Global Mapper provides a total of 15 different operations for combining or comparing terrain layers. Here is a breakdown of what they do:

A. Arithmetic Operations for Elevation Values

• Addition: Adds the values of two input terrain datasets to create a new elevation.
• Subtraction (Difference) - Signed: Calculates the difference between two datasets while maintaining the sign (+/-). This is useful for analyzing the direction of elevation change (increase/decrease). If Layer A is larger than B, the result is positive (+); otherwise, it's negative (-).
• Subtraction (Difference) - Unsigned: Calculates the absolute difference between two datasets. It always returns a positive value, which is useful for measuring the total magnitude of change.
• Average Elevation: Calculates the mean value of the two input datasets to find the average elevation distribution.
• Multiply Values: Multiplies the two inputs. Often used to emphasize specific patterns by multiplying elevation with weighting data.
• Divide 1st Value by 2nd: Divides the first dataset by the second. Used for ratio analysis or calculating relative elevation differences.

B. Comparison Operations

• Minimum Elevation: Selects the lower value between the two input datasets.
• Maximum Elevation: Selects the higher value between the two input datasets.
• Filter (Keep First if Greater than Second): Retains the value from the first dataset only if it is greater than the second.
• Filter (Keep First if Less Than Second): Retains the value from the first dataset only if it is less than the second.

C. Validation & Conditional Filters

• Filter (Keep First if Second Valid): Retains the first dataset's value only if the second dataset has a valid value at that location.
• Cut Layer Out (Keep First if Second Invalid): Retains the first dataset's value only if the second dataset is invalid at that location.
• Filter (Keep First if Second Greater Than Compare): Retains the first dataset's value if the second dataset is greater than a specific comparison value.
• Filter (Keep First if Second Less Than Compare): Retains the first dataset's value if the second dataset is less than a specific comparison value.
• Count Valid Layers at Each Location: Analyzes and counts the number of valid overlapping layers at each coordinate.

While these features might not be used frequently in general urban planning or landscape design, they are vital for specific technical analyses. Let's look at a practical example.

Hands-on Exercise: Combining DEMs

You can practice with the sample DEM_Arithmetic.zip file. Once you open the file in Global Mapper, you will see two layers: the original terrain and the planned elevation (Design Level).

Global Mapper Control Center with two DEM layers loaded

To perform the calculation, you must have at least two DEM layers registered. In this example, the 'Planned Elevation' layer is overlaid on the 'Original Terrain' layer.

Step 1: Addition Operation

Navigate to Terrain Analysis -> Combine/Compare Terrain Layers...

Menu path for Combine/Compare Terrain Layers

The "Combine Terrain Options" window will appear. Enter a name for the resulting layer (e.g., "DEM Addition") and select Addition as the Operation. Select the layers you want to add while holding the Ctrl key.

Combine Terrain Options dialog for Addition

Click OK, and a new layer representing the sum of the two elevations will be added to the Control Center.

Resulting layer after Addition operation

Step 2: Subtraction (Signed vs. Unsigned)

For subtraction, the order of layers is crucial. Let's subtract the 'Planned Elevation' from the 'Original Terrain'.

Settings for Signed Subtraction

In the "Subtraction (Difference) - Signed" result, you will see values ranging from negative to positive. This represents areas that were cut or filled.

Result of Signed Subtraction

Now, let's compare this with Unsigned Subtraction. This operation converts all differences into absolute values.

Settings for Unsigned Subtraction

Visual comparison of Signed vs Unsigned result

To verify the difference, right-click on the 'Subtraction' layer and select Options. Under the Alter Elevation Values tab, you can see the elevation range.

signed :

Elevation range for Signed subtraction showing -10 to 29.57m

Unsigned :

Elevation range for Unsigned subtraction showing 0.0 to 29.57m

As you can see, the 'Unsigned' version turns all negative changes into positive values, focusing only on the amount of change rather than the direction.

Step 3: Range Selection Filter

Finally, let's look at Filter (Keep First if Second Greater than Compare). This extracts areas from the first layer where the second layer's elevation exceeds a certain value.

For example, if we enter 85 as the 'Compare Value', Global Mapper identifies areas in the 'Planned Elevation' layer higher than 85m and extracts the corresponding terrain data from the 'Original Terrain' layer.

Settings for Range Selection Filter with Compare Value 85

Final extracted terrain result

The remaining operations are intuitive once you understand these basics. I encourage you to experiment with each one to see how they can serve your GIS projects.

"In the next session, we'll dive deeper into more advanced features. Stay tuned!"

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