Tuesday, July 13, 2010

Cross Section Points Filter

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © 2010. All rights reserved.

Now that a lot of us use GIS to generate our cross sections, this is becoming a much more “used” feature in RAS: The Cross Section Points Filter. RAS allows a maximum of 500 station-elevation points in any given cross section. It is very common for one or more cross sections cut in GIS to come in with a LOT of station elevation points. And then, if you interpolate cross sections (interpolated cross sections have more-or-less double the station elevation points as their bounding sections) you’ll have even more points. Exceeding 500 points in a cross section is very easy to do.

RAS offers a few ways to filter out points. There is the “Near and Co-linear” filter. This allows you to specify a tolerance for points that are very close to each other, and points that are in a straight, or nearly straight line. RAS will then remove points that are within the tolerance level (i.e., if you have three points in a perfectly straight line, there is no need to include the middle point in your geometry-unless it happens to be a bank station or n-value break point, in which case RAS will preserve it).

However, my preference normally is to use the “Minimum Area Change” option. RAS will remove points sequentially in an effort to minimize the area change of the cross section. You, as the user, simply specify the number of points you want RAS to filter to, and all the work is done for you. This is a much more convenient way to filter points-and much faster, but be aware that you have a lot less control over how the points are filtered. If you have a lot more than 500 points to begin with, it is a good idea to compare the “before” and “after” cross sections to make sure the new cross section preserves the true geometry. In my experience RAS does a great job at filtering using the Minimize Area Change option.

If RAS tells me there are a lot of cross sections that need to be filtered, I’ll use the “Multiple Locations” tab to get them all done at once. Select all the cross sections in your geometry (even the ones that don’t exceed 500 points) and RAS will only filter the cross sections that need filtering.