Finding Local Depressions in Rasters

Sometimes referred to as bluespots or sinks, depression terrain features in raster layers are areas of low elevation relative to the surrounding area. They often don’t appear with standard terrain visualization methods. These depressions must be measured locally, so they aren’t negated by the overall slope of the terrain. Locating sinks is important for areas sensitive to water, such as farms, as they can prevent water from draining, leading to ponding and negatively impacting crop growth. The example in the workflow below utilizes a function of the Watershed Analysis tool in Global Mapper® to identify areas in a field where water may pond, potentially impacting agricultural yield.

Follow along with this workflow:

• Use the Watershed tool to create a grid with filled depressions
• Create a depression raster layer by subtracting the filled watershed layer from a DEM
• Optional: Create custom shaders, contour lines, etc., to visualize the depressions

This workflow requires a bareground DEM layer. When starting from Lidar data, first create an Elevation Grid from the ground points before beginning the following workflow.

Step 1: Use the Watershed Tool to create a Fill Depressions Layer

Global Mapper’s watershed tool is designed to estimate how water moves and accumulates across the terrain layer. Essentially, it compares changes in elevation from cell to cell to map the downhill direction of elevation. The workflow for locating sinks takes advantage of a functionality within the watershed tool: Fill Depressions. Before mapping out streamflow, Global Mapper creates a new DEM based on the fill depth value. The generated DEM represents the amount of “fill” Global Mapper had to add to these areas to prevent them from ponding.

*The watershed tool only maps the potential flow of water over the terrain. It does not consider soil type and permeability, existing water, or precipitation.

Watershed Settings:

In this workflow, only two settings need to be adjusted from the default in the watershed dialog box. For more information on other settings, such as stream cell count, see the watershed generation section of the Knowledge Base.

1A. Set the Depression Fill Depth value:

This tool looks at Fill Depth from the perspective of creating a watershed. The original intention of this setting is to input an elevation change that Global Mapper can ignore/smooth over when mapping water flow from cell to cell. The Fill Depression function (which we will be using to create our depression layer) lets you define how deep a “depression” is within this dataset. The Fill Depth value is less of a representation of the depth, and more of a measurement of the hill/berm that it has to flow over.

Choosing a value:

Use a value that is larger than your deepest depression; if the value is too shallow, the deeper areas won’t be accurately represented. In this example, an egregiously large fill depth of 12 feet was used to create a grid where the deepest depression was just over 3 feet.

1B. Save your DEM to GMG file after Filling Depressions

The .gmg (Global Mapper Grid) file created when the “Save DEM to GMG File After Filling Depressions” box is checked will be used in the next step to create the depression layer. When the tool finishes processing, import this layer back into your workspace.

What is it? This layer is what the watershed uses for mapping streamflow. If this option is not checked, the DEM will be created in a temporary folder and deleted upon completion of the watershed generation. For this workflow, the layer needs to be saved.

1C. Optional Setting: Flow Direction Arrows

Flow Direction arrows show the flow of water across the terrain, and a large red X marks where the flow terminates. After generating Flow Direction Arrows, zoom into the data to render the arrows. Many of the flow lines terminate at the edge of the dataset as the water flows out of the field. Some of them terminate in the middle of the field; these spots are likely water accumulation points.

Here is an example of a termination point at an established berm in the middle of the field, likely built to slow water flow.

2. Create a Depression Depth layer

Remember that original DEM the watershed layer is based on? For this next step, use the Combine/Compare Terrain tool to subtract the new .gmg watershed depression layer from that original DEM. This will create a new raster layer representing the difference between the two layers, i.e., the depth of the sink/depression areas.

The resulting grid cells in the new depression depth layer no longer show elevation, but instead display the measured change between the ground elevations and the filled depression grid. Areas that are the same elevation in both rasters will appear as “0” ft in the new depression depth layer.

3. Optional: Data Visualization

Once the raster has been generated, there is a wide range of tools in Global Mapper that can be used to make the data more easily interpreted. Users can create a shader to highlight the depressed areas using color or use the contour or breakline tool to generate area features around the depressions.

Gridded Data Color Display

Creating a Custom Elevation shader in Global Mapper is an easy way to visually add meaning to your data. The shader in the screenshot below describes elevations below 0, while coloring non-sink areas with a uniform brown.

Find the Custom Shaders settings at the bottom of the Shader Options page of the Configuration menu.

Use the Contour Tool to Vectorize the Depression Terrain Features

Contours delineate elevation changes. In the Depression depth map, the sinks are the only changes. Use the Create Contours tool to create contours at -0.01 ft by checking the box to Only Create Contour Lines at Specified Height. Also, check the box at the bottom of the dialog to Discard Closed Contour Lines Shorter than (100) m. This will outline the sinks without creating extra lines.

Technically, the Contour tool can also be used to locate sinks from the original DEM, especially with the Create Spot Elevation Points… option enabled. However, this method requires more manual discernment in finding closed, low-elevation contours in the landscape. However, it could be a good method for verifying that all of the skins in the data were correctly detected.

Use Contours to Crop out Sink Areas

Instead of creating the area features required for cropping, the contour tool creates line features. To work around this, select the line features, then use the Digitizer Toolbox to Create Areas From Lines. Once areas have been created, select the features, open the Options menu for the difference raster, and from the cropping tab, choose Crop To Currently Selected Polygons. Select Crop to All when prompted. Once cropped, the depressions can be layered on top of aerial imagery. If aerial imagery is needed, the Online Data tool offers access to a variety of free data sources.

Mitigation Planning for Terrain Depressions

Global Mapper’s robust Terrain Analysis toolset provides options for measuring and modeling terrain data:

• Use the Cut and Fill tool in Global Mapper to calculate the volume of soil required to fill the depression.
• To model how altering the terrain would impact the flow of water, use the Terrain Paint tool to “fill” the depressions, create berms, etc., before using the watershed tool on the new values.

Explore Terrain Depressions & More!

Use Global Mapper to explore your terrain and raster data by downloading a 14-day free trial. To learn more about terrain analysis in Global Mapper, check out the resources below:

• Simplify Site Suitability & Lidar-Focused Terrain Analysis
• Knowledge Base