Working in the UK - Want to get OS AOD levels from elipsoid levels

I have just started using an RX which will be used to survey in GCP for drone surveys. Just checking how accurate it is and how it works in the field. I took a reference point at a nearby trig pillar and the elipsoid elevation level was 113m. What settings do I need to get levels recorded to OS Newlyn datum?

Apologies for newbie question!

Thanks

What are you using for corrections?

Been playing around with them - OSGB 1936 + ODN Height was the last I used…

Hi @timetomaketracks,

To use a CS based on OSGB 1936 with the ODN height in Emlid Flow, the base station or NTRIP coordinates should be in ETRS89 LLH. Have you tried this setup? How does it work for you?

I’m using rtk2go to receive NTRIP corrections. How do I ensure that they are in ETRS89LLH?

Just as a prompt for any potential advice - using the OSGB36 and ODN height corrections I get a height figure of 55m (elipsoid height being 102m). If I use the ETRS89 data and correct using the OS coordinate transformation website to convert to OSGB36 then the height is 58m AOD (which is the correct reduced height for the spot).

Any ideas why there is a difference of 3m between the OS website and the Reach conversion?

Thanks!

Hi @timetomaketracks,

Sorry for the delay!

Let’s look closer into the issue:

  • Do the eastings and northings match? I mean, is the issue only in the heights?

  • Was the point collected with a Fix solution?

  • Have you collected a point on a benchmark? Or did you find the correct reduced height another way?

Also, is the Emlid Flow app updated to the latest version? If not, please update it, create a new project, and check if the issue persists.

I’m using rtk2go to receive NTRIP corrections. How do I ensure that they are in ETRS89LLH?

ETRS89 is a standard for base corrections in the UK. So most likely, the base is in this datum. But you can reach out to the correction provider to double-check it. Also, you can try another mount point to ensure the results are the same.

Hi - taking each point;
yes - eastings and northing match, so the height datum is the only issue.

Yes - Fix solution

Yes - from GB trig pillar with known AOD level

Emlid Flow app is ver 8 - is this the latest?

Thanks!

Hi @timetomaketracks,

Emlid Flow app is ver 8 - is this the latest?

That’s right, version 8.0 is the latest one.

If I use the ETRS89 data and correct using the OS coordinate transformation website to convert to OSGB36 then the height is 58m AOD (which is the correct reduced height for the spot).

Just to ensure I got it right – have you used the geographic coordinates from your Emlid Flow OSGB 1936 project in the official converter? If you used other ETRS89 coordinates, how were they obtained?

To find out the reason, we’ll need to look closer into the data. Please share the projected and geographic coordinates from Emlid Flow, together with the OSGB 1936 + ODN coordinates you got from the official converter. You can share them with me in PM or via support@emlid.com if you’d like.