The Clip Raster command allows the user to clip an elevation DEM or image to any shape. All areas of the raster outside of the clipped region are set to a NODATA transparent value. This functionality helps by being able to trim an elevation DEM to the limits of a LIDAR field survey, allowing the user to merge in additional elevation data for the areas outside of the survey limits.
Follow the steps below to use the Clip Raster command:
- From the Terrain ribbon menu, select the Clip Raster command.
- The Clip Raster dialog box will be displayed.
The following sections describe the Clip Raster command and how to interact with the above dialog box.
Select Raster to Clip
This section controls the selection of a raster layer such as a DEM or image raster for clipping based on either the model extents, rectangular envelope, or the polygons.
From the Select raster layer dropdown combo box, select the elevation DEM, or image raster that needs to be trimmed into a smaller elevation grid.
Clipped Raster Limits
This section allows the user to define the extents of the clipped raster.
The following options are available to define the grid limits on the selected elevation grid:
- User-defined limits: The user can draw the rectangular extent of the elevation grid to correspond to a specifically defined area. This makes the elevation grid smaller and easier to work with.
Click the [Pick] button to define the limits of the terrain surface. The Clip Raster dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to create a raster limits rectangle. Click and drag a rectangular region to define the limits. After releasing the mouse button, 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 one or more polygons on the Map View and the software will clip the selected raster to the boundary of the polygons. The raster will set the limits of the grid to the extents of the selected polygons. For regions that are within these limits but outside the selected polygon boundary the cells will be set to a null value.
Click the [Pick] button to select the polygon shape region(s). The Clip Raster dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select the clipping polygons. 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 dialog box will be redisplayed. The area underneath the selected clipping polygons will be considered by the software as it processes 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.
- Elevation range: If the selected raster layer is an elevation DEM, then this option is enabled, and the software shows the maximum and minimum elevations contained within the selected elevation DEM.
The user can define the maximum and minimum elevation limits of a clipped raster by checking the Maximum elevation and Minimum elevation checkboxes. Alternatively, by using the corresponding [Pick] buttons, the user can select locations on the DEM layer from the Map View to represent the maximum and minimum elevations to use.
Raster Processing Specifications
This section is used to define the specifications for the clipped raster.
Click the […] button beside the Clipped raster file entry to specify the file name and directory location to which to save the clipped raster file.
Different file formats are available to save the clipped raster file based on the type of raster layer selected. For example, if the selected raster layer is an elevation DEM, then only elevation grid file types are provided for saving the clipped raster. Similarly, if the selected raster layer is an image, then only image raster file types are provided for saving the clipped raster.
By default, the Load raster as map layer checkbox option is checked to load the clipped raster 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 clipped raster file. If there is only one CRS in the project, the software automatically selects it.
Note that if the clipped raster is an elevation DEM, then grid resolution details will show columns, rows and total cells. If the clipped raster is an image raster, however, then grid resolution details will display width, height, and color depth as shown below.
Clipping the Raster File
After the options have been defined, click the [Clip] button. The software will clip the raster file and create a new raster layer at the specified location. If the Load raster as Map layer checkbox was checked, the newly created raster layer will be loaded in the Map Data Layers panel.