Calculate Relative Elevation Grid
The Calculate Relative Elevation Grid (from Terrain and line...) tool creates 'flattened' digital terrain models called Relative Elevation Grids (REM), also called River Elevation Models, based on the cross sectional elevations along a linear feature (such as a river) in order to highlight the changes within the feature and/or the feature and the surrounding terrain.
Create Relative Elevation Grids... by choosing this option from the Analysis Menu.
Compare a standard Digital Elevation Model (Left) to a River Elevation Model (Right) of the same area. We know that rivers flow down hill, so an REM removes that trend from the data. They are useful for measuring the change in elevation as compared to the river, instead of comparing it to sea level.
Select the line feature(s) to be used in processing before opening this tool from the Analysis drop down menu. If multiple line features are selected, Global Mapper will attempt to join the lines into one feature. The Specify Maximum Combination Distance window (pictured below) lets you determine how close together line end points need to be in order to be joined into one line feature. Any number of line features can be used. If the lines are not close enough to be joined, you can manually draw a feature to join them by using the Digitizer Tool.
The Relative Elevations Grid dialog (pictured below) allows the user to specify the output resolution. The generated REM will be displayed with the REM shader by default. Previous settings will be remembered by default when the tool is rerun, if still valid to the new application, otherwise it will repopulate with the default values when the tool is reopened.
REM Grid X/Y-axis spacing - These values determine the resolution or grid cell size of the generated REM. Will be same as input terrain by default.
Reference Line spacing - The center line will be resampled at this spacing. Smaller is usually more accurate, but will result in a longer REM generation time.
Generate Averaged Elevation Layer - Enable this option to display the intermediate average grid that's used in calculations. The final REM grid is the difference between the original input terrain and this grid. Assessing this layer can indicate the source of any problems and needed adjustments in input parameters. Elevations are calculated using the Inverse Distance Weighted (IDW) method, where distances are from grid cell to a set of nearest center line sample points.
Save Detrended Reference Line Elevation Statistics to File - Enable this option to save a CSV file listing the calculated average elevations along the re-sampled center line and residuals. This can indicate problem areas, such as near sharp bends in river, that may require adjustments in input parameters for best results.
Detrended Elev Grid Spacing - This sets the resolution of the intermediate IDW average elevation grid as a multiplier of input terrain resolution. This grid does not need to have the same resolution as the final REM. Generting this step in the process can take more processing time and can have a significantly larger spacing/lower resolution. Default is a factor of 10; for example, if input spacing is 1m, intermediate grid will get a spacing of 10m.
IDW Number of Samples - This is the number of nearest center line sample points used to calculate the detrended elevation at a given cell. More is better, but at the cost of computation time.
IDW Search Precision - When searching for closest reference line samples, this controls how close to last closest point(s) is good enough to stop looking for a better point. The default of 0m will look for points that are exactly the closest and will produce the best results at the expense of longer calculation times. (A K-D Tree is used to find the set of nearest points; generally, the default of 0m is best unless creating a very large REM.)
The Bounds button allows the user to set up the portion of the loaded elevation grid data they wish to consider when generating the contours.