Transformation from UTM30 to OSGB36/British National Grid + ODN height: still ellipsoidic heights

[Robert Jurisch]

Hello,

I tried to perform a transformation from UTM30 with ellips. heights to OSGB1936 with ODN heights (https://epsg.io/7405) using the NTv2-transformation grid provided by Ordnance Survey (https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/for-developers).
I also have reference data in both Systems.

I used the following command:
lasdatum.exe -i c:\input\*.laz -epsg 25830 -grid c:\...\OSTN15_NTv2_ETRStoOSGB.gsb -target_epsg 7405 -odir c:\output

and got: ERROR: unknown EPSG code in '-target_epsg 7405'.

Also tried to transform in the more common (?) OSGB36 / British National Grid (Code:27700)

lasdatum.exe -i c:\input\*.laz -epsg 25830 -grid c:\...\OSTN15_NTv2_ETRStoOSGB.gsb -target_epsg 27700 -odir c:\output

While the horizontal transformation seemed to work quite good, the heights stayed ellipsoidic and show a difference of ~47 meter to the ODN-heights.

Did I maybe miss an option to also apply height correction or do I have to perform a second transformation to get the correct heights?

Thank you in advance!

[Eoin Killeen]

Hi Robert,

We do this transformation in 4 steps:

1. las2las: UTM to -target_longlat

2. lasdatum: apply the OSTN15 datum transformation

3. las2las:  -longlat to -target_epsg 27700

4. lasheight: Apply geoid model OSGM15 (-ground_points OSGM15.laz -replace_z)

It's been a while since we set it up and there may be other ways of doing it. I think the OSTN15_NTv2_ETRStoOSGB.gsb file is in geographic coordinates so steps 1 & 4 converting to/from lat lon are required. However we were using a beta version of lasdatum originally, maybe Lastools is handling this now if your data looks good in XY.

The height error you are seeing is the geoid, you need to handle that separately. I've attached a LAZ file which we use.

Regards,

Eoin

[Robert Jurisch]

Hey thanks for your adcive,

we implented same workflow as you described in Python with PROJ.4 (Simply because the transformation works on the EPSG codes there). The resulst were good.

May I ask where the geoid information in the laz is actually from? I really tried to find this information and did only find the NTv2 transformation from Ordnance Survey

Regards
Robert

[Eoin Killeen]

It was quite a while ago when we set this up so I'm not sure exactly which file we used to generate this unfortunately.

Regards,

Eoin

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Eoin Killeen

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