[PROJ] Assistance with GDALWarp - ESPG:27700 to ESPG:4326 - Elipsod level not sea level

[Nicolas Cadieux]

Hi,
You may be able to remove the geiod height with Gdal but I would use vdatum https://vdatum.noaa.gov/

Nicolas Cadieux
https://gitlab.com/njacadieux

> Le 18 avr. 2021 à 11:17, Raife Crookes <raife at tp-regen.co.uk> a écrit :
> 
> 
> Good evening,
>  
> I am hoping that someone could assist me with the completion of a reprojection of a geotiff lidar data set to WGS84.
>  
> The transformation should me making an elevation change of roughly 50m for known points however after running GDAL warp the elevations are staying the same when viewed through QGIS.
>  
> For some reason it seems to be defaulting to the WGS84 geoid height not the ellipsoid height I require for import onto our drone. (My knowledge is that EPSG:4326 should equate to the wgs lat/long and ellipsoid height.)
>  
> GDALWARP instructions:
> gdalwarp  -s_srs EPSG:27700 -t_srs EPSG:4326
> gdalwarp  -s_srs -t_srs EPSG:4326
>  
>  
>  
> Proj info return:
> C:\>projinfo -s EPSG:27700 -t EPSG:4326 -o PROJ
> Candidate operations found: 1
> Note: using '--spatial-test intersects' would bring more results (7)
> -------------------------------------
> Operation No. 1:
>  
> unknown id, Inverse of British National Grid + OSGB 1936 to WGS 84 (9), 1 m, UK - Britain and UKCS 49┬░46'N to 61┬░01'N, 7┬░33'W to 3┬░33'E
>  
> PROJ string:
> +proj=pipeline +step +inv +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +step +proj=hgridshift +grids=OSTN15_NTv2_OSGBtoETRS.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
>  
>  
>  
> I can send copies of the original file if requested.
>  
> Hoping that its something stupid like im using the wrong EPSG code but have a terrible feeling that a catch all to prevent users accidentally using the ellipsoid height is catching me out.
>  
> (Ground height OSGB – 30m  in WGS84 = 82m)
>  
> Really look forward to your help/
>  
> Kind regards
>  
> Raife
>  
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20210418/4b0fd74d/attachment.html>