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

[Raife Crookes]

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

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