Projecting shp from GK3 to UTM32 with BeTA2007

Question

I've experimented with projections of shapes, used the official provided grid beta2007 for germany and noticed differences between transforming by double exporting over custom CRS in QGIS and ogr2ogr of around 0.8-1.0 metres...
Perhaps it's happening because of false strings... I don't see it :)

1. Exporting I've got the following custom CRS DHDN GK3 with the grid (EPSG:31467):

   +proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +units=m +no_defs +nadgrids=X:\gridshift\BETA2007.gsb +wktext

   Exported the shape with this CRS, loaded it into QGIS and exported it again to an custom CRS ETRS89 / UTM zone 32N with leading 32 (EPSG:4647), this is also the project CRS, on-the-fly projection is enabled:

   +proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=32500000 +y_0=0 +ellps=GRS80 +units=m +no_defs +towgs84=0,0,0,0,0,0,0

2. Doing the same thing shorter with ogr2ogr

   ogr2ogr -f "ERSI Shapefile" utm32.shp gk3.shp -s_srs "+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +units=m +no_defs +nadgrids=X:\gridshift\BETA2007.gsb +wktext" -t_srs "+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=32500000 +y_0=0 +ellps=GRS80 +units=m +no_defs +towgs84=0,0,0,0,0,0,0"

Why does this happen?

Answer 1

There is/was a bug in gdal regarding nadgrids. A supposed workaround should be to add +nadgrids=@null to the part of the transformation without gsb file.

See

http://trac.osgeo.org/proj/ticket/22

http://osgeo-org.1560.n6.nabble.com/Difference-between-nadgrids-null-and-towgs84-0-0-0-td3841972.html

Another point of distortion is the missing height in "normal" reprojections. A point 10 metres above the ellipsoid A has different coordinates on ellisoid B than a point 200 metres above. It's just because the ellipsoids are not in parallel.That distortion gets greater the more reprojections you do.