Relative Elevation Models in Global Mapper
The latest Global Mapper release (v24.1) includes a new tool for creating Relative Elevation Models (REM), which are also referred to as river elevation models. These grids are different from the more common digital elevation models and therefore have a separate tool in Global Mapper. REMs are used for looking at local fluctuations in elevation. This blog will explore how they can be used, how to make them in Global Mapper, and a look at the new shader options that accompany the tool.
REMs are detrended elevation grids. Using a line feature and a DEM/terrain layer, the elevation values in a REM are changed to exaggerate the lower elevations. Higher elevations that are away from the center of the line feature are ignored and glossed over. Look at the elevation bar in the center image below to see how the shader takes advantage of the detailed lower elevations in a REM with a bright yellow color.
While this tool is most commonly used for rivers, it can be used with any line feature and elevation raster layer. Highways, powerlines, underground cables, glaciers, volcanic features; REMs are great for anything where seeing the terrain relative to a line is important. As always, more information on specific tool settings can be found in the Knowledge Base.
REMs and DEMs (digital elevation models) look similar, but they aren’t quite the same. In REMs, elevation values are displayed relative to the line feature instead of sea level. For example, in REMs based on a river, we know that rivers flow downhill, so displaying that information is unnecessary. By removing that downhill trend from the data (“detrending” the data), the local fluctuations in the terrain are highlighted. This elevation display structure allows patterns in lower elevations to be more easily discerned.
Rivers tend to meander over time. Floods, bank erosion, and other factors can change where the main channel flows. From the patterns in the terrain, we can discern previous paths and future erosion issues. REMs work well with large, slow-moving rivers that have meandered over time.
Now for the technical tool information: Found in the analysis dropdown menu, the ‘Create a Relative Elevation Model tool’ requires a digital elevation model (DEM) and a line feature. To make the REM, Global Mapper first makes an averaged elevation layer by sampling elevations from the DEM along the line feature and interpolating it into a surface. Interpolation is done with the IDW method, the Inverse Distance Weighting method, that more heavily weighs values that are near the line feature, almost ignoring those that are farther away. You can see this layer in Global Mapper by checking the box to Generate Averaged Elevation Layer. The final step is to subtract the averaged layer from the original DEM.
Are purple and yellow not your jam? A new custom shader option lets you create REM shaders with different color pallets. Bright and multi-colored shaders work best for visual analysis. However, if you plan on using the data for cartographic purposes, a monochromatic gradient produces a more aesthetically pleasing effect.
In the Custom Shaders dialog, add the highest and lowest elevation values in the dataset, then click Create REM Shader. Be sure to change the default shader for the REM layer by looking in the Display tab in the layer’s Raster Options.
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USGS. (2017, July 14). Missouri River Corridor geologic mapping completed. Missouri River Corridor Geologic Mapping | U.S. Geological Survey. Retrieved February 28, 2023, from https://www.usgs.gov/centers/geosciences-and-environmental-change-science-center/science/missouri-river-corridor
Montana State Library. (2022, August 10). Relative elevation models. Retrieved February 28, 2023, from https://storymaps.arcgis.com/stories/19b6bfe0c3aa454c853bd6d9b7228adf