Mapping and GIS Solutions Community

We recently used Trimble Catalyst to collect ground control point data for a site we were mapping with our drone and it returned altitudes of around 83-92m after antenna height correction. However all spatial data for this site from lidar surveys and government mapping (GDA 94 Zone 56) shows the altitude to be around 40m AMSL. How do we account for this huge difference? And how do we correct it back to the standard altitudes?

Hi,

the Catalyst system provides heights on the ellipsoid - so not on any Geoid.

A difference of around 40m smells like a Geoid. 

If I may ask - which software did you use for collecting the data and which config did you use. If I search the EPSG databse for GDA 94 Zone 56 I get essentially this:

EPSG Geodetic Parameter Registry (click on 'retrieve by code', then use code 28356).

So the GDA 94 Zone 56 is a mix of geodetic datum and none default height model (EPSG::6422, Ellipsoidal CS).

My GUESS is that this causes the offset. 

To give a better answer we need details on the settings as applied in both the application and Trimble Mobile Manager (used datum etc). 

Regards

Nico

It is indeed a GeoID issue Nico. I used Mobile Manager with UAV Ground Control to collect the data points. There are no settings in either application, as far as I can see, to change the projection system used for data collection.

The thing that annoys me is no so much that it used what for me is an obscure height datum (ie, is not used in the real world) so much as that nowhere does it tell me that it's doing that, so I naturally assumed the elevation data in the GCP records was AHD/AMSL.

We typically work in WGS84 GDA94 Zone 56S AHD heights.

Hi Tony,

neither TMM nor UAV do support GeoID models at the moment.

Its something on our list since quite a while - but the GLONASS release plus other features have consumed and still consume most of the time. 

Therefore the positions as provided by Trimble Catalyst are currently only corrected by the geodetic datum shift occurring while switching between global correction systems (RTX, SBAS etc) and local ones (RTK). Otherwise its a raw Lat/ Long based on the global ellipsoid. 

Do you have a way to 'easily' add the height offset introduced by the GeoID? 

We'll also check how to make the height report more transparent - so you at least see directly that its an ellipsoidal height. 

Nico

It seems that the offset is dependent on the location. There are online converters but they are a bit of a pain to use as the CSV data needs to be edited and reorganised.

The latest version of UAV Ground Control and Trimble Mobile Manager (released 4th May 2020), now allow you to specify the requested output reference frame and to select a local Geoid from a list of pre-defined Geoids that are supported out of the box. If your required Geoid is not in the list of pre-supported models, you can custom load any GGF file to the Android device. Instructions for how to use this workflow are described here in the Trimble Mobile Manager help portal:

Configure GNSS output 

Once configured, UAV Ground Control will display and output MSL heights in your local vertical reference frame.

Im having this issue as well but to the opposite degree. I should be at 26 - 30 m range and currently I'm getting readings ranging from -1 m to 2 m. Any assistance would be appreciated.

Hi,

this all sounds like geoid offsets - or expected geoid heights where we provide raw ellipsoid heights.

So first of all - what 'output datum' selection did you made in TMM. So the datum you have chosen for position output. 

Next - what geoid are you typically working in? Do you know this?

Lastly - whats your rough position (to get an idea where MSE is).

Based on these information we can most likely identify if its indeed a 'missing geoid' issue - or something else.

Regards

Nico

Hi Nico,

I'll try to answer your questions the best I can.

The output datum was set to "auto selection" but in the process of trying to figure this all out I changed it to WGS84(G1762).

As for the geoid, I have no idea what that is.

My position is roughly Lat28°25'N; Long 81°13'W

Thank you

Howdy Tony and Nico,

Thanks for posting the question Tony. The above info led me to find a calculator on our LINZ (Land information New Zealand) website to convert from WGS84 (the Catalyst) to geoid or normal-orthometric (NZVD2016) and I'm now getting the correct altitude . New Zealand Coordinate Conversions 

As you say Tony, it would've been nice to know all of that in the beginning but, certainly the option to use Geoid on the fly would be great! I see Trimble are still working on an update Nico?

Unfortunately, I've now got to go back to the beginning, DroneDeploy, and enter my GCP info there. All my work in Surfer is wrong  

Hi Baron,

I'm sorry you have to redo it seems quite a bit.

To avoid similar traps for others - what would you suggest for improvement?

So a problem I see is that its not super clear which positions are used in GCP. I think thats something on our side.

But as you are saying 'it would've been nice to know all of that in the beginning' - are there more things that have been unclear? 

Cheers

Nico

The latest version of UAV Ground Control and Trimble Mobile Manager (released 4th May 2020), now allow you to specify the requested output reference frame and to select a local Geoid from a list of pre-defined Geoids that are supported out of the box. If your required Geoid is not in the list of pre-supported models, you can custom load any GGF file to the Android device. Instructions for how to use this workflow are described here in the Trimble Mobile Manager help portal:

Configure GNSS output 

Once configured, UAV Ground Control will display and output MSL heights in your local vertical reference frame.