ESRI GNSS Altitude vs Z values

[CarsonG]

Would one interpret the following statment from Prepare for high-accuracy data collection—ArcGIS Field Maps | Documentation that Z values are recorded as ellipsoidal height or orthometric heigh?

• Altitude—Ellipsoidal height received from the GPS receiver (not orthometric height, as used for z-values)

We have data that was GPS collected and when the Z value is calculated, it appears to be calculating the orthometric height. We compared these points to nearby control points, and they are within inches of each other.

[CarsonG]

Anyone have insight into this?

[RhettZufelt]

The documentation you linked to says the Altitude metadata field records Ellipsoidal height, but at the bottom of the same page, it says:

"When recording altitude, Field Maps records orthometric height based on the geoid model of your receiver (often EGM96, but see your device's user manual). If you require a different geoid model, postprocess the values to use the required geoid model."

So, if you have the geoid model in your GPS profile, it seems that the z-value will be adjusted as such?????

Would be interested to know if the GPS metadata Altitude value is the same as the Z-value.

R_

[CarsonG]

Thank you Rhett, that clarifies the statement. It does raise additional questions like you had.

Now I am wondering if the Geoid model being set in Trimble Mobile manager (for this case it was set to GEOID18 (Connus)) is dictating what Field Maps does, which it sounds like it does per the following:

Obtaining Orthometric Heights with Trimble GNSS and Collector for ArcGIS - YouTube

I will be testing the ESRI_GNSS_Altitude vs the z-value to make sure this is correct.

Back when I was using GOE7x's and ArcPad the z value that was calculated from a gps collected point was being recorded as HAE which is what made me question all of this to begin with.

[Anonymous User]

I think the Trimble Mobile Manager app is probably doing the conversion for you. Here's a related Idea which is in the product plan but not yet implemented Measure the orthometric height in Field Maps - Esri Community

[mykola]

Hi CarsonG,

Z is certainly an orthometric height which you can derive using arcade expression in arcgis online or in postprocessing. As it was pointed out in the docs, The field maps/collector uses the geoid model from the receiver. I've done several tests with GNSS receiver and arcgis field maps but unfortunatelly there is no control over the vertical datum in the Field Maps. I have tested the same GPS receiver with the same RTK correction service in Arcgis Field Maps and in another app where I have control over the vertical datum. Both measured the same ellipsoidal height (ALTITUDE in the field maps). The external app gave me the correct values of Z but the Field Maps had an error of roughly 20-22 cm in Z (note that the ellipsoidal height was the same in both). I suspect that the Field Maps uses some sort of global geoid model (most likely EGM96 if we should believe the docs) which results in wrong ortho height. 

I am still haven't found the solution and I guess it won't change until Field Maps will offer a manual selection of the local geoid model. 

[CarsonG]

While there is not control over the vertical datum in the profile settings, would it be controlled by the base map that is being used if both the base map and TMM are using the same vertical datum?

Also, if you look at the link provided above by AnonymousUser, Measure the orthometric height in Field Maps - Esri Community

The last post at that link

"To help prioritize this real-time orthometric height issue, please contact your Esri Support and reference this Esri Enhancement to tell them to include you in their "Subscribed" list for this enhancement:

ENH-000140091

Provide the possibility to measure and save orthometric height values when saving points to a web feature layer using Field maps app and GNSS antenna.

"Under Consideration""

Has me confused. Unless the plan is to add a GNSS Metadata Field that collects orthometric height?

[mykola]

Yes, you set up the webmap accordingly. But because of no control over the vertical datum, I get slightly different results for ortho height because I guess the FM uses global geoid model to calculate the ortho height. I am not sure how it works with TMM because you are not connecting the receiver directly to the FM. Maybe it will give the right ortho height that is calculated before the data goes to the FM. I used the receiver directly with the FM via bluetooth so maybe that is a part of the issue. 

Ideally, I would like to see an option to chose my own geoid model  in the field maps. From the link it seems that the work is in progress (hopefully).

Regarding the last question, you can add the gnss metadata fields with the available geoproccesing tool but there is no ortho. Just the altitude (ellipsoidal height). I used Round(Geometry($feature).Z,2) in arcade to extract Z value in the webmap and in the field maps. You can also extract it by postprocessing your data.