Introduction
Engineers and scientists are using GIS tools to develop HEC-RAS models more and more. As a result, imported cross-sections may have more than 500 points (the limit per cross-section in HEC-RAS). In addition, interpolated cross-sections can easily contain more than 500 points if the bounding cross-sections have more than 255 points. Luckily, the HEC-RAS program contains a tool called the Cross-Section Points Filter. This tool allows users to quickly and easily remove points from cross-sections containing more than 500 points.
How do I know if a cross-section has more than 500 points?
Many models contain many cross-sections. Fortunately, HEC-RAS lists the River, Reach, and River Station for each “offending” cross-section.
Using the Cross-Section Points Filter
If you receive an error message like the one above, navigate to the Geometric Data Editor. Then click Tools → Cross Sections Points Filter.
The Cross Sections Points Filter tool will appear
There are a few options when using this tool. Firstly, the user can choose whether to apply changes to a single location or multiple locations. Do this by using the tabs at the top of the dialog box.
Secondly, the user can remove points with the Near and Colinear Filter tab or the Minimize Area Change tab. The Near and Colinear Filter allows users to specify tolerances for points that are very close together. When using this option, I recommend checking the Only filter cross-sections with more than 500 points box.
In contrast, the Minimize Change Area option allows the user to specify the number of points in the filtered cross-section. HEC-RAS will remove points sequentially to minimize the change in the cross-section area. This option is easier to use and is more convenient. If there are several cross-sections with more than 500 points, I recommend using the Multiple Locations option as shown below. The user can select all cross-sections even the ones with more than 500 points, and HEC-RAS will only filter the ones with more than 500 points. Finally, click Filter Points on Selected XS.
If cross-sections started with significantly more than 500 points, it is a good idea to do a “before and after” comparison. This will ensure that the “filtered” cross-section still represents the appropriate conditions.