This Tech Tip discusses an error that can occur when a user attempts to run the Extract > River Network routine.
To extract a river network from a DEM, a user must run the
first four routines in the Extract menu, namely:
When new users run the Extract > River Network routine, they sometimes see the following error dialog:
The cause of this error is that new users don't always understand how the Extract > Basin Outlet dialog works. In that dialog, users are supposed to first click on the Start button, and then click in the resulting image (and possibly move the slider) in order to graphically select a basin outlet pixel. Once the outlet has been selected, the user is supposed to click on the Save Outlet button. By using the flow grid, this tool allows users to select pixels in a DEM very precisely by first selecting a streamline and then using a slider (and the arrow keys) to select a pixel that lies on the streamline. This method is much more precise and user-friendly than the "snap to grid" method that some programs use to select pixels (especially when the image on the screen is much smaller than the size of the DEM itself). The outlet coordinates are printed in the Output log, and can be made to coincide with the coordinates of a gauging station or other feature.
If a user does not click on the Start button, but simply clicks on the Save Outlet button, then a default "basin info" file is created (e.g. Small_basin.txt) that has the pixel in the upper left corner of the DEM (column = 0, row = 0) as the outlet. Due to the existence of this file, the Extract > RT Treefile dialog (with the "Basin above saved outlet" option selected) executes without error, by extracting the (usually tiny) network that drains to pixel (0,0). If the basin info file did not exist, then the Extract > RT Treefile dialog would have issued an error message to that effect.
You can use the File > View Basin Info dialog to see the default basin info that was created. Using the Small data set in the RiverTools sample data, for example, the dialog will look like this:
This problem is easily fixed by returning to the Extract > Basin Outlet dialog and following the instructions above.
Note that the Extract > Basin Outlet step can be skipped if you intend to use any other option in the Extract > RT Treefile dialog besides "Basin above saved outlet."
If you have any additional questions or comments, please contact Rivix Technical Support for assistance.
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