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Geoid for TMM using SBAS

  • 1.  Geoid for TMM using SBAS

    Posted 06-19-2024 07:47

    When using SBAS with the ESRI default coordinate system of WGS 1984 Web Mercator Auxiliary Sphere in Field Maps, which geoid should be used in TMM?

    Will that have any affect on the horizontal positions? 

    Thanks for any help.

    Paul Lohr

  • 2.  RE: Geoid for TMM using SBAS

    Posted 06-20-2024 11:31

    No, it does not have any effect on the horizontal positions. The Geoid only determines the orthometric height sent to Field Maps (which is recorded as the z-value of the point, not in the Elevation field). If you're in the US, the most recent Geoid is GEOID18 (CONUS). The default EGM96 Geoid used in TMM is a worldwide Geoid model. It is better than nothing but not very granular.

    Depending on your location in the world, different Geoids may not matter much if you're only using SBAS corrections for elevation. You're going to be more limited by the overall accuracy than the differences between regionalized Geoids.

    Jamie Maxwell

  • 3.  RE: Geoid for TMM using SBAS

    Posted 06-20-2024 15:13

    Hi Paul. Technically, GEOID18 (CONUS) as an implementation of the NAVD88 vertical datum is only supposed to be used with NAD 1983 (2011) as the horizontal datum. If you use it with WGS84 as the horizontal datum, your Z values will be incorrect by the difference in height between the two reference ellipsoids (GRS80 vs WGS84) at that location...which in some cases could be a bigger source of error than the inheritent limitations of SBAS as a correction source. Probably what I'd do in this case is configure TMM to output "Auto" which will be NAD 1983 (2011) and then pick the GEOID18 (CONUS) option for orthometric heights. Then in Field Maps, I'd setup a location profile and set the incoming GNSS as GCS NAD 1983 2011 (EPSG:6318) and the map coordinate system as WGS84 Web Merc Aux Sphere (EPSG:3857)...with the first transformation selected from the list (using the ITRF08 parameters). That would at least give you correct Z values within the limitations of SBAS accuracy.

    Matthew Morris

  • 4.  RE: Geoid for TMM using SBAS

    Posted 06-21-2024 06:06

    @Jamie Maxwell - thank you.

    @Matthew Morris:

    Thank you.

    I stepped through the settings in the TMM Configuration page but there must be some differences. Is that what you mean:

    GNSS correction source = Auto (SBAS in our case)

    Output = Select from list

    GNSS output = NAD83(2011) (EPOCH:2010)

    Geoid = GEOID18(Conus)

    Field Maps Location Profile:

    I am curious - is there not a setting for the geoid of the input source (SBAS in my case)? Would that be something like EGM96 or ITRF2014? Especially when the source could be RTX or SBAS?

    Paul Lohr

  • 5.  RE: Geoid for TMM using SBAS

    Posted 06-21-2024 06:58

    Paul, your images look good to me. Those are the settings we use with Catalyst receivers and Field Maps.

    There is no setting for the input Geoid source in Field Maps because, unlike the horizontal coordinates, Field Maps does not have the ability to do any transformations or conversions to the orthometric height/elevation values. It just assigns the value sent by TMM as the z value (in meters). That's my understanding, at least. Technically the z values stored in the points do not match the vertical datum of the layer. That only really matters if you're doing 3D analysis on the points. We only care about having orthometric height values available.

    Jamie Maxwell