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OSTN02 or OSTN15 Grid Shift - UK Ordnance Survey

 
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Luke Benjamin Cummins



Joined: 18 Oct 2016
Posts: 2

London, United Kingdom

PostPosted: Tue 18 Oct, 2016 3:52 am    Post subject: OSTN02 or OSTN15 Grid Shift - UK Ordnance Survey Reply with quote

In order to go between British National Grid and London Survey Grid a OSTN02 or OSTN15 grid shift file is used.

Documentation here:
https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html

Has anyone looked at doing this?

Cheers
Luke
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Matthew Monk



Joined: 21 Oct 2005
Posts: 1851

Victoria, Australia

PostPosted: Tue 18 Oct, 2016 7:49 am    Post subject: Reply with quote

When I was 12d Support for the UK, I had previously used the OSTN02 grid shift file with 12d's general transformation option (Survey->Conversions->General transformation). This panel can perform transformations using 7 parameters or a grid shift file (*.gsb) in the NTv2 format.

For OSTN02, there was a NTv2 GSB made available- https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/ostn02-ntv2-format.html

If an NTv2 GSB file was available, then 12d Model could use that (the NTv2 is more compatible and useful for most software). I can't speak to the ease of integrating the available library into 12d Model.
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Luke Benjamin Cummins



Joined: 18 Oct 2016
Posts: 2

London, United Kingdom

PostPosted: Thu 10 Nov, 2016 3:45 am    Post subject: Reply with quote

Thanks Matthew.

I have managed to get this far with a Grid Shift from British National Grid (BNG) to ETRS89 using the grid shift file (NTv2 available from https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html )as you suggested). It appears that points line up but this has to rely on a conversion from Northings and Eastings to Long/Lat (BNG to ETRS89) and precision appears to be limited.

I then need to undertake a 7 parameter transformation from ETRS89 to get to London Survey Grid (LSG). I have to do this in Lat/Long. It splits the data set in an odd manner (Greenwich 0 Long and sign convention) but it appears inconsequential.

Conversion Name ETRS89 > LSG
dX 19.019m
dY 115.122m
dZ -97.287m
rX (arc seconds) -3.577824
rY (arc seconds) 3.484437
rZ (arc seconds) 2.767646
Scale 18.60847540

Projection name: London Survey Grid
Projection type: Transverse Mercator rigin latitude (dms): N051-10-00 rigin longitude (dms): W000-09-30
False easting: 78,250.000m
False northing: -2,800.000m
Scale factor: 0.9999999
Ellipsoid: WGS84

I then need to convert these again to get to Northings and Eastings. This places the points between 300-40m out.

I have tried every combination I can think of with parameter sign convention, lat/long - easting northing and projection changes earlier or later in the process but to no avail. I am unsure if the switch to Long/Lat is removing the precision required or I am doing something else wrong?
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Roger Walker



Joined: 03 Nov 2016
Posts: 25

New Zealand

PostPosted: Mon 21 Nov, 2016 2:46 pm    Post subject: Reply with quote

So you want to convert from OS grid to London Grid? And you don't want to use the approx (400mm accuracy method) in the linked document? (section 7.3) http://www.engineeringsurveyor.com/software/1-026%20-%20Topographical%20Surveys%20and%20Mapping.pdf

So you are trying to convert OS grid coordinates to lat/long and then back to London Grid?

Don't know enough about 12D to say why your attempt isn;t working but you could use ordnance survey "grid inquest" to check the conversion to lat/long. Then check the projection to LSG using a spreadsheet like this one
https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/coordinate-spreadsheet.html

300m sounds like the shift grid file is not being used. Just a guess.
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