Basic setup for collecting orthometric heights in the UK

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1

Hi there,

I am having trouble collecting orthometric heights, as opposed to ellipsoidal, heights in QField (surveying in the United Kingdom).

I am an archaeologist using QField with a Trimble TDC600 (Android) handset and DA2 antenna. I am connecting with bluetooth and have Trimble Mobile Manager set as the Mock Location App.

I am successfully storing latitude, longitude and ellipsoidal altitude using

x(@position_coordinate), y(@position_coordinate) and z(@position_coordinate)

I have my QGIS\QField Project set to EPSG 9424 ( I am confused whether this should be set to EPSG 27700 to get the grid shift to work)

I do not know if I have the correct file for the vertical grid shift. I have tried OSTN15_NTv2_ETRStoOSBG.gsb and OSGM15_GTX_ASCII.gtx both of which I downloaded from the Ordnance Survey Website.

Selecting these as the vertical grid shift file in QField doesn’t seem to make any difference to the height I see displayed in QField.

I created a separate field in my digitizing logs layer to record Orthometric Height.

alt_ortho Decimal (Double) Length 10 Precision 3.

I have tried using ‘@elevation’, ‘Z($geometry)’ and ‘$Z’, but I cannot seem to get any values to store in this field.

Any help towards a solution would be greatly appreciated. Many thanks in advance.

2

Please try using `z(geometry)` without the `$` or `@`. That’s how it works for us.

3

Thanks Thomas,

I tried changing the code for my ‘alt_ortho’ field to ‘Z(geometry)’, but I still get the same results:

  1. The ellipsoidal height stores successfully in my altitude field, which uses ‘Z(@position_coordinate)’
  2. No value stored in the ‘alt_ortho’ field that I set up to colelct Orthometric height above Mean Sea Level values. That is, when I log a point or a vertex.
  3. The ellipsoidal height remains displayed in the bottom left of the QField Canvas screen. It does acquire the suffix ‘(grid)’, but the value does not change when I select the OSTN15_NTv2_ETRStoOSGB.gsb as my grid shift file in QField.

Do I perhaps have an incorrect .gsb file? Or my CRS EPSG: 9424 project setup may be incorrect?

Do others users have success using grid shift files in combination with using Trimble Mobile Manager as the Android Mock Location App?

I really love QField, but this problem is proving really tricky to solve. If I can solve it, I would love to make a tutorial for other UK users to follow.

4

It does indeed look like there’s a problem with the *.gsb file. Unfortunately, I can’t help you further with this, as I have no experience with this in your country.

Using the Android Mock Location function is the worst way to use GNSS data in QField. Important information is lost, and we’ve found that, especially with Samsung and Lenovo devices, incorrect values ​​are sometimes inserted.

If Trimble Mobile Manager offers the option to stream the NMEA data to a TCP or UDP port, this would be a significantly better option.

5

I am by no means an expert so please bear that in mind

All my projects in both QGIS and QField are in EPSG:27700 - so OSGB36

Under Vertical Grid Shift in use I have OSGM15.gtx and z( $geometry ) as default value in my altitude field, it all seems to work OK

6

Yes, unfortunately with the DA2 antenna and Catalyst there is no option in the UK to share the NMEA stream from Trimble Mobile Manager. I am not sure, therefore, if it is a Mock Location Issue.

7

Hi fruit-github,

Thanks for your reply. Two questions:

  1. Are you also using Mock Location, or connecting to the antenna directly?
  2. Is that the OSGM15_GTX_ASCII.gtx that one can download from the OS website, here, that you are using?:
8

1

No, I don’t use Mock Location. it always seemd like a long way round and for that reason I don’t use GNSSMaster https://www.gnssmaster.com/ though I tried it in the early days. I don’t know if Mock Location is still a requirement. Too many dropouts in the early days for reliable operation but that may have improved. It may be that I am cofusing some of these points with Lefebure, another I tried for a while.

I use SWMaps app and it has proved very reliable, connects to ArduSimple simpleRTK2Blite by USB and to NTRIP service (rtk2go) via SIM data card. QField connects to the simpleRTK2Blite by Bluetooth.

I imagine that Bluetooth might be an issue for other devices, the UBlox F9P does corrections on the fly from the NTRIP data and passes corrected output through the F9P’s 2nd serial port hence Bluetooth.

There is a lot of useful info on the ArduSimple site but sadly they discontiued their Q&A/forum like pages, they were messy but a wealth of useful stuff

2

I am not sure where I found that correction file but most likely OS but possibly the proj site. I have searched but I think it’s gone for good, see PROJ 6 request - OSGM15_GB.gtx · Issue #92 · OSGeo/PROJ-data · GitHub

Within QField’s proj directory I have OSGM15.gtx, OSTN15_GRB.gsb, OSTN15_NTv2_ETRStoOSGB.gsb, OSTN15_NTv2_OSGBtoETRS.gsb uk_os_OSGM15_GB.tif and uk_os_OSTN15_NTv2_OSGBtoETRS.tif but only the first is selected

Not sure that will have helped a lot but good luck