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Merge DEMs Command

The Merge DEMs command is used to merge multiple elevation grids (sometimes called rasters, DEMs or Digital Elevation Models) into a single elevation grid. The user can prioritize and define preference for use of different elevation grids so that higher resolution (smaller cell size) elevation grids, for example, are used where possible and lower resolution (larger cell size) elevation grids are used to fill-in areas where high resolution elevation data is lacking. In addition, this command allows the user to specify the coordinate reference system (CRS) and grid resolution of the final merged elevation grid.

This command should not be used to merge elevation grids that have different elevation units (i.e., intermixing feet and meter elevations). The Convert Elevations command should be used to create compatible elevation raster grids and to ensure consistency in elevation units before use of the Merge DEMs command.

Follow these steps to display the Merge DEMs command:

  1. From the Terrain ribbon menu, select the Merge DEMs command.
    Merge DEMs Command
  2. The Merge DEMs dialog box will be displayed.
    Merge DEMs Dialog box

The following sections describe the Merge DEMs command and how to interact with the above dialog box.

Select DEMs to Merge

This section controls the selection of elevation grids to be used for merging into a single elevation grid.

From the Existing DEM layer dropdown combo box, select the elevation grids, one at a time, that need to be merged and click the [Add] button. The selected elevation grid will be added to the table listing the elevation grids to be merged.

To change the merge order of the listed elevation grids, select the appropriate row and right-click to display a context menu. Then, select the Move Layer Up or Move Layer Down context menu command to change the merge order of the highlighted elevation grid. Layers that are higher in the listing have precedence over layers that appear lower in the list.
DEM Layer panel

Merge DEM Limits

The elevation grid that is constructed using this command is a rectangular grid. The user can define the rectangular extents of the merged elevation grid using the criteria described below.
Merged DEM Limits panel

The following options are available to define the rectangular extents of the merged elevation grid:

  • DEM source extents: The merged elevation grid will have the same extents as the original source data.
  • User-defined limits: The user can draw the rectangular extents of the merged elevation grid to correspond to a specific area defined by the user. This can make the merged elevation grid smaller and easier to work with. Click the [Pick] button to define the limits of the terrain surface. The dialog box will temporarily disappear, and an information message will be displayed on the status line. Click and drag a rectangular region to define the limits of the merged elevation grid. After releasing the mouse, the user will be returned to the dialog box. A layer will be created with a rectangular box to represent the user-defined selected region.
  • Clipping polygons: The user can select polygon shape regions to define the processing area for the merged DEM. Click the [Pick] button. The dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next. Select the clipping polygons on the Map View and press the [Enter] key or right-click and select Done from the displayed context menu. The user will be returned to the dialog box. The area underneath selected clipping polygons will be considered by the software in terms of processing the terrain. Additionally, the user can check the Crop terrain to polygon boundary checkbox to crop the merged terrain to the selected polygon boundary.
  • Model extents: If a project model has been defined, this option will create a bounding rectangular region to correspond to the extents of the defined model, plus an additional buffer boundary.

Fill Small Gaps Between Grids

This section allows the user to fill-in the small gaps between adjacent elevation grids by interpolating the elevations from the edge cells of each grid. By default, the Fill Small Gaps Between Grids checkbox is checked.
Fill Small Gaps Between Grids

From the Maximum search distance entry field, the user can define the maximum search radius that will be used for gap filling. The spin control button can be used to define a threshold value between 1 to 50 cells. A gap that is farther away from any cell than this threshold value will remain unfilled.

The Smoothing iterations entry field allows the user to define the number of iterations that the software will perform for smoothing the filled gaps in the grid.

Overlapping Cells Mosaic Merging

This section allows the user to mosaic merge multiple terrain surface DEMs together. Select the Overlapping Cells Mosaic Merging checkbox to enable this section.
Overlapping Cells Mosaic Merging panel

The user can select from two available options to define how the two surfaces should be merged when there is an overlapping space between terrain surfaces that are to be merged.

The following options are provided for the mosaic merge operation:

  • Use minimum elevation – On selecting this option, the merged cell elevation for the overlapping areas will be the minimum elevation contained in the original terrain surfaces.
  • Use maximum elevation – On selecting this option, the merged cell elevation for the overlapping areas will be the maximum elevation contained in the original terrain surfaces.

DEM Processing Specifications

This section is used to define the specifications for the merged elevation grid.

Click the […] button for the Merged DEM file entry to specify the file name and directory location for purposes of saving the merged elevation grid file.

By default, the Load DEM as map layer checkbox option is checked to load the merged elevation grid as a layer in the Map Data Layers panel. Click the pencil icon to rename the layer.

The user can select the CRS to be used for the merged elevation grid. If there is only one CRS in the project, the software automatically selects it.

From the DEM cell size entry, the user can manually define the elevation grid cell size. The finer the grid resolution (or smaller the cell size) defined, the greater the detail that can be represented in the merged elevation grid. However, there is no benefit to selecting a cell size smaller than the smallest cell size contained within the elevation grids to merge. The software will automatically select the smallest cell size.

Different Coordinate Reference Systems

The Merge DEMs command supports merging DEMs that have different coordinate reference systems (CRS). For example, the project might be in a State Plane CRS in US Survey Foot and the DEMs being merged might be in a geographic (lat-long) or metric CRS. The software will perform the necessary reprojections to align the selected DEMs so that they can be merged together.

Merging the Elevation Grids

After the options have been defined, click the [Merge] button and the software will merge the elevation grids into a new elevation grid file and, if desired by the user, load the grid layer in the Map Data Layers panel.

About the Author Chris Maeder

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