The Merge DEMs command is used to merge together multiple elevation grids (sometimes called rasters, DEMs or Digital Elevation Models) into a single elevation grid. The user can define the precedence to be used for each elevation grid so that higher resolution (smaller cell size) elevation grids are will be used where possible and lower resolution (larger cell size) elevation grids 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 first convert differing elevation raster grids so that they have the same elevation units before using the Merge DEMs command.
Follow these steps to display the Merge DEMs command:
- From the Map Edit ribbon menu, select the Merge DEMs command.
- The Merge DEMs dialog box will be displayed.
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.
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 to an area of interest. The following options are available for defining 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 limit to an area of interest. This will can possibly 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.
- HEC‑RAS model extents: If a HEC‑RAS model has been defined, this option will create a bounding rectangular region the same size as the HEC‑RAS model reaches, cross sections, storage areas and 2D flow areas, plus an additional buffer boundary.
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 to save 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 can be represented in the merged elevation grid. However, there is no benefit to define a cell size smaller than the smallest cell size contained within the elevation grids to merge. Therefore, the software will automatically select the smallest cell size.
By default, the Interpolate to fill small gaps between grids checkbox option is checked. This option fills-in the small gaps between adjacent elevation grids by interpolating the elevations from the edge cells of each grid.
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 optionally load the grid as a layer in the Map Data Layers panel.