Tuesday, July 13, 2010

Cross Section Points Filter

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 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.

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12 comments:

  1. quite useful, Chris!, thank you very much!

    Mijail

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  2. Thanks Chris, that was helpful.

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  3. Is there a maximum number of cross section points that may read into HEC-RAS from GEO-RAS? I am wondering whether to change the cell size of the LiDAR or filter in HEC-RAS. LCR 02/15/2014

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    1. Not that I'm aware of. I generally keep my LiDAR resolution as fine as possible/practical and filter in HEC-RAS after importing. Filtering in HEC-RAS is very easy to do.

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  4. When using the "Minimize Area Change" type of filter, does RAS still preserve the bank station and n-value break point locations?

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  5. Has anyone ever recieved an error message saying GUI sent too many points to river station XXXX ? I think it has something to do with this subject, but I could be wrong.

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  6. Hi Chris,
    I am currently working on a 1D/2D project and I'm facing an issue when it comes to perform the simulation. I get a message saying that the maximum of station-elevation points is 500 [...]
    So i did everything as you explained on this post, with the cross section filter on the "minimize area change" mode.
    All my XS are now with a fixed number of 500 station-elevation points each but i still get the same error message...
    Do you know what can i do ?
    Thanks a lot in advance
    Best regards
    Victor

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    1. The most common mistake is that when you have a project with multiple reaches, you forget to look at one of the reaches. This is the only thing that I can think of that you might check.

      Make sure that in the Cross Section Points Filter window, you select "All Reaches" in the Reach dropdown box. Then scan through all of the RSs listed and make sure that they are all 500 points or less. There has to be one you are missing somewhere.

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    2. Thanks for the reply !
      I only have one reach on my project and unfortunately it still doesn't work...
      All my XSs are have 5OO points but i still get the error message...
      Is there a way I can send you my project so you can take a look at it ?
      Thanks in advance
      Victor

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    3. Sure, I'll take a look at it. cgoodell@westconsultants.com

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