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Why is the Geoid menu visible if collecting raw GNSS (WGS84)?

  • 1.  Why is the Geoid menu visible if collecting raw GNSS (WGS84)?

    Posted 05-16-2021 02:31
    Edited by Higher Logic System 09-20-2021 09:00
    I am trying to troubleshoot a discrepancy between Drone RTK coordinates, and the coordinates of my GCPs that are collected with Trimble Catalyst and the Trimble VRS subsctiption.

    My drone RTK is using corrections from Leica SmartNet and outputs true ellipsoidal values. However, I noticed that the profile of my Timble Mobile Manager was setup to output to ETRF2000 and the Geoid setting was set to OSGM15
    So I figure that I should probably resurvey with the GNSS output set to "Same as source (transformation off)" and try to get matching coordinates between drone images and GCPs.

    But why is it that the menu for setting the Geoid is still visible, if i only want to output ellipsoidal heights? Does this menu become erroneous to the workflow?


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    David Brooks
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  • 2.  RE: Why is the Geoid menu visible if collecting raw GNSS (WGS84)?
    Best Answer

    Posted 05-17-2021 23:01

    Hi David,

    the 'GNSS Output' configures your horizontal datum; the vertical datum is independent from this in TMM.
    So technically you can't deselect the geoid in TMM as most workflows require a geoid.

    If you select 'Same as source' it means TMM does not apply any horizontal datum transformation - but we still apply a geoid afterwards. 

    Never the less TMM/ GCP provide both MSL and ellipsoid height - you can verify this in the export of GCP:


    So there is no need to remeasure points - at least from what I've seen above. If points have been collected with GCP the export does contain both heights. 

    Cheers
    Nico



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    Nico Becke
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    Original Message


  • 3.  RE: Why is the Geoid menu visible if collecting raw GNSS (WGS84)?

    Posted 05-21-2021 01:20
    Okay thanks for that info. Im finding a strange Z offset between control points surveyed by Trimble Catalyst and same poitns sureyed by Leica using Smartnet. I'm just checking that the GNSS Output system and datum are the same across the platforms.

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    David Brooks
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    Original Message


  • 4.  RE: Why is the Geoid menu visible if collecting raw GNSS (WGS84)?

    Posted 05-21-2021 02:36
    Hi,

    can you define 'strange offset' in this context? Like a more or less constant offset or changing? In the ellipsoidal height or MSL height? And in which scale? Like cm, m or where?
    The ellipsoidal height should be more or less identical. MSL can vary based on the used geoid - if you have geoid with higher resolution for your area (so not the global one but local to UK) it will differ from the global.

    Also - what's the caster URL you're using? If this is a none-public URL feel free to shoot it via a private message. The mountpoint may also contain further information on the stream settings - and we can check if there is a config difference compared to Trimble VRS network (which is the foundation of TCH). 


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    Nico Becke
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    Original Message