How is a lines length calculated in GM?

DC KelleyDC Kelley Global Mapper UserPosts: 76Trusted User
edited July 2011 in Technical Support
How is the "Length" shown in the features information dialog calculated for poly lines? When I vary the elevation widely (adding 1km on a run of ~30 meters) the value does not change. I have imported an elevation layer and added that to some line types, but as far as I can tell your tool is using Vincenty’s formula along the surface of the ellipsoid, is that correct?

Is there an way to add the elevation and its changes to that when needed? I more of less need to use a path along the surface as part of this, if I go to an ECEF model I think I lose this and will then get a simple trig solution for a straight line. If I use the GM value as one leg of a triangle and the delta change as another, perhaps that is sufficient for short baselines but I am sure others have this issue as well. And my knowledge of this area of coordinate math is very weak, so perhaps want I am asking here is nonsensical. Suggestions on how to proceed?

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2011
    That is correct, the distance reported for lines is the 2D length along the surface of the ellipsoid using the Vincenty formula, it does not use the height information to get a 3D length. However if you have terrain loaded and select the line, then right-click and select to show a 3D path profile, the 3D length of the line will be available on the Path Details from the Options menu.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 76Trusted User
    edited July 2011
    Understand. In our case (very tight sub meter road data over short baselines), we only see road grade slopes of perhaps 5~6% but the accuracy impact on such a slope over even 50 meters can be ~20 centimeters or so. I think if we just use the delta elevation change and some trig and ignore the "tilt" from common elevation we can cope. I further think that is much better (even for short distances) then a pair of EFEC points, but can not go to a black board and prove it. But I will have to go ask a real cartographer to bound the errors on that one. Thanks for the quick confirmation.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2011
    I decided to update the 'Add Feature Measure Attributes' option to include a LENGTH_3D attribute for the 3D length of 3D line features (if it is different than the 2D length). I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with the change for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 76Trusted User
    edited July 2011
    I decided to update the 'Add Feature Measure Attributes' option to include a LENGTH_3D attribute for the 3D length of 3D line features (if it is different than the 2D length).

    Got it download and running but (sorry to be so new here) do not have any idea where 'Add Feature Measure Attributes' is found can do not see anything likely in any menus. A board search on "Feature Measure Attributes" yields a buncvh of result but not where newbies should look to find the improvement you have made in this release.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2011
    Select the features with the Digitizer Tool, then right-click and one of the options is to add feature measure attributes.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 76Trusted User
    edited July 2011
    Got it, and it seems to work fine (although I just tweak a few number to be outrageous to see it play and a more realistic ~2cm over a 350 meter that rises ~2 meters test seems about right to me). I also learned an important fact, that I need to manually tell it to recalculate each line one at a time when done.
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