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Georeferencing HEC-RAS Cross Sections

When the software imports a model, it automatically places the river cross sections on the Map View. However, if the original model was not spatially georeferenced, the river cross sections will not align with any loaded background base map. While the software can operate without any issues in this situation, it is preferable to have the river cross sections georeferenced to the background base map. Therefore, it might become necessary to georeference the imported cross sections.

The Georeference Cross Sections command can be used to manually georeference each of the cross sections to the background base map displayed in the Map View. The process of georeferencing a cross section to the Map View can be a trial and error process—especially when the exact location of the original cross section is not known. Using the Georeference Cross Sections command, this process is accelerated.

Note that a CRS should be assigned prior to running this command. Otherwise, the software will display the below informational dialog box.
CRS not assigned to HEC-RAS model dialog box

Refer to this article in our knowledge base to learn how to assign a coordinate reference system to a project.

Follow the steps below to georeference an existing cross section:

  1. From the Input ribbon menu, click the Cross Sections dropdown menu and then select the Georeference Cross Sections command.
    Georeference Cross Sections Input ribbon menu command
  2. The Georeference Cross Sections dialog box will be displayed.
    Georeference Cross Sections dialog box

The following sections describe how to georeference an existing cross section and interact with the above dialog box.

Selecting Cross Section to Georeference

The Select Cross Section to Georeference section allows the user to interactively select the cross sections to georeference on the Map View. This section allows the user to select the river, corresponding reach, and the cross section from the River, Reach, and Cross section dropdown combo boxes. Alternatively, the user can click the [Pick] button to select the cross section from the Map View.

Once the cross section has been selected, the user can choose between the following options to georeference the cross section:

  • Snap to Alignment Polyline
  • Slide along Alignment Polyline
  • Draw on Map View

Snap to Alignment Polyline

If an existing alignment polyline for the cross section exists on the Map View, the Snap to Alignment Polyline can be used to snap the cross section to the alignment polyline so that the cross section matches that of the alignment polyline.

Follow the steps below to use the Snap to Alignment Polyline option:

  1. Select the Snap to Alignment Polyline radio button option. Note that this option is selected by default when the dialog box is displayed.
  2. Click the [Pick] button.
    [Pick] button
  3. The Georeference Cross Sections dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the alignment polyline.
  4. Select the alignment polyline on the Map View.
  5. The Georeference Cross Sections dialog box will be redisplayed and the status of the Select alignment polyline read-only field will be changed from Not Selected to Selected.
  6. Click the [Snap] button. Alternatively, turn on the Scale to fit checkbox option to scale the cross section to fit within the alignment polyline.
    [Snap] button
  7. The software will snap the cross section to the selected alignment polyline.

Slide Along Alignment Polyline

This option allows the user to manually slide the cross section along the alignment polyline.

Follow the steps below to use the Slide along Alignment Polyline option.

  1. Select the Slide along Alignment Polyline radio button option and click the [Slide] button.
    [Slide] button
  2. The Georeference Cross Sections dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the cross section and drag it along the underlying alignment polyline.
  3. Click and drag the cross section on the Map View to revise its alignment.
  4. When finished, press the [Enter] key or right-click and select Done from the displayed context menu. The Georeference Cross Sections dialog box will be redisplayed.

Notes:

  • The cross section must overlay a continuous alignment polyline for this option to work.
  • For more precision, the user can use the Snap to Alignment Polyline option first and then use the Slide along Alignment Polyline option.

Draw on Map View

This option allows the user to draw an alignment polyline and automatically snap the selected cross section to the drawn polyline.

Follow the steps below to use the Draw on Map View option:

  1. Select the Draw on Map View radio button option and click the [Draw] button. Alternatively, turn on the Scale to fit checkbox option to scale the cross section to fit within the alignment polyline.
    [Draw] button
  2. The Georeference Cross Sections dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to draw the alignment polyline on Map View.
  3. Draw the alignment polyline on the Map View. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
  4. The Georeference Cross Sections dialog box will be redisplayed, and the cross section will automatically snap to the drawn alignment polyline.

Extract Elevation Data

This optional section is used to define the elevation data source(s) to be used for extracting the cross section geometry. The user can use the Primary and Secondary Elevation Data panels to define the primary and secondary (if available in the project) elevation data sources for extracting the cross section geometry. Depending upon the elevation data source type that is selected, different options are provided to specify additional elevation data information.
Extract Elevation Data section

Refer to this article in our knowledge base for information on the types of terrain elevation data that can be used for constructing cross sections.

When a secondary elevation data source is available, the software will form a concave hull around the primary elevation data source to identify its bounds. For locations where elevation data from the primary data source are unavailable, the software will use the elevation data from the secondary data source.

Note that the user cannot utilize the same data source for defining the primary and secondary elevation data.

The user can click the [Swap Sources] button to swap the selected elevation source from primary elevation data to secondary elevation data and vice versa.

If this section checkbox is unchecked, then the subsequent sections below it (excluding the Update cross section flow lengths checkbox of the Cross Section Extraction Option section) will be unavailable (i.e., grayed out).

Cross Section Geometry Extraction Control

This optional section is used to control the amount of cross section geometry to extract for the drawn cross section polylines. This assures that an adequately deep enough cross section is created on both sides of the river reach. The software will attempt to retrieve the cross section geometry data to the depth specified within the specified maximum cross section width.
Cross Section Geometry Extraction Control section

If the drawn polyline does not extend outward far enough to get the cross section depth specified, the software will automatically extend the constructed cross section further outward. Similarly, if the drawn polyline extends too far outward for the depth specified, the software will automatically trim the constructed cross section.

Cross Section Extraction Options

This section is used to control which portions of the cross section(s) should be extracted.
Cross Section Extraction Options section

For example, the user may have survey data for the cross section channel and a LIDAR digital terrain surface for the overbank areas. In this example, the user would only want to extract the cross section geometry for the overbank areas since the surveyed channel geometry is already accurate.

The Extract cross section geometry for dropdown combo box entry contains the following options.

  • Both Overbanks
  • Channel Only
  • Entire Cross Section (default)
  • Left Overbank Only
  • Right Overbank Only

The Update cross section flow lengths checkbox option allows the user to update the existing cross section flow lengths with the surveyed cross section flow lengths.

About the Author Chris Maeder

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