[Proj] Transformations with height data to bessel bring unrealistic results

stevejking stevejking at talktalk.net
Fri Sep 22 07:44:40 EST 2017

If you are working with non-ellipsoidal heights you can use grid files. One
for the horizontal (lat/long shifts. Note proj4 internally converts your
units if not in lat long already) and one for the vertical (eg. metres)
datum shift to WGS84 ellipsoid (or a close approximation).

eg. for Great Britain Ordnance Datum heights (above the geoid):
from +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000
+y_0=-100000 +ellps=airy +nadgrids=C:\temp\OSTN15_NTv2_OSGBtoETRS.gsb
+geoidgrids=c:\temp\OSGM15_GTX_OSGBtoGRS80.gtx +units=m +no_defs
to +proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs

Most national mapping agencies publish these grid files, or you can find
them searching online.

Sent from: http://osgeo-org.1560.x6.nabble.com/PROJ-4-f3840930.html

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