HEC-RAS Generate Terrain Command

Digital terrain surfaces can be used for the following tasks:

Extracting cross section geometry
Computing 2D flow areas
Determining bank locations
Assigning levees and lateral structures
Constructing roadway crossings
Defining dam structures
Extracting storage area volumes
Generating flood maps
Watershed delineation
Detention and retention pond design
Stream realignments and restoration
Flow control structures

While the software can read in a variety of terrain surface types, it is sometimes necessary to construct a terrain surface from newly acquired elevation data. This section describes how to do that.

From the Terrain ribbon menu, select the Generate Terrain command.
The Generate Terrain dialog box will be displayed.
The following sections describe the Generate Terrain command and how to interact with the above dialog box.

Terrain Elevation Source

This section controls the selection of the elevation data source to be used for generating the terrain surface. If a map data layer was selected prior to selecting this command, the map data layer will be already selected within this section.

From the Terrain source entry, select the terrain source type to be used for generating the terrain surface. The following terrain source types are supported:

CAD Drawing
GIS Data
LandXML Data
LIDAR Elevation Data
Point Elevation Data
TIN Surface

Based upon the terrain source type selected, additional fields are provided for selecting the map data layer and attribute information.

Terrain Grid Limits

The terrain surface that is constructed using this command is a rectangular grid. The user can define the rectangular extents of the constructed terrain surface to an area of interest. The following options are available for defining the rectangular extents of the generated terrain surface:

Elevation source extents: The generated terrain surface grid will have the same extents as the original source data.
User-defined limits: The user can draw the rectangular extents of the generated terrain surface grid to limit to an area of interest. This has the effect of making the generated terrain surface file 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 generated terrain surface. 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 generated terrain. Click the [Pick] button. The dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on 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 within the selected clipping polygons will be considered the terrain to be processed by the software. In addition, 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 the extents of the defined model, plus an additional buffer boundary.

By default, the Crop terrain surface to TIN boundary option is checked if the user selects TIN surface or LandXML Data from the Terrain source entry. The generated terrain surface grid will not extend outside of the TIN boundary.

Terrain Grid Specifications

This section is used to define the specifications for the generated terrain surface.

Click the […] button for the Terrain file entry to specify the file name and directory location to save the generated terrain grid file.

By default, the Load terrain as map layer checkbox option is checked to load the terrain file 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 generated terrain grid. If there is only one CRS being used in the project, the software automatically selects it.

By default, the Overwrite existing terrain layer checkbox option is checked. This option controls overwriting an existing terrain layer with the same layer name.

Terrain Grid Resolution

This section is used to define the terrain quality and the cell size for the terrain grid being generated.

From the Predefined terrain quality entry, choose the resolution option to be used. Elevation grid resolutions range from low to ultra-high quality for the terrain surface grid being generated.

From the User-defined 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 generated terrain surface. However, a smaller cell size results in more cells generated for the grid, creating a larger grid file and taking longer to generate.

After the options have been defined, click the [Generate] button and the software will create the elevation grid file and load the grid as a new layer in the Map Data Layers panel.