[Qgis-user] transformation

[Andre Joost]

Am 23.02.2015 um 13:57 schrieb Goran Pavic:
> Recently, in Croatia we must use new official reference coordinate system for map projection HTRS96/TM. All data I used before are in
> HR_GK_6 projection.
> I'm interested how to make transformation of shapefiles which I have in HR_GK_6 projection into new one HTRS96/Croatia TM projection?
>
> HR_GK_6 - +proj=tmerc +lat_0=0 +lon_0=18 +k=0.9999 +x_0=6500000 +y_0=0 +ellps=bessel +units=m +no_defs
> HTRS96/Croatia TM - +proj=tmerc +lat_0=0 +lon_0=16.5 +k=0.9999 +x_0=500000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
>
> I tried with on-the-fly CRS transformation but my data didn't "landed" on the right place. All data get a shift Δx=397,85 m and Δy=22,25 m

Your reprojection does not work correctly because the source CRS has no 
dataum shift, hence there will be made no datum shift at all.

Official values for the datum shift can be found here:

<http://www.spatial-analyst.net/wiki/index.php?title=MGI_/_Balkans_coordinate_systems>

The old Gauß-Krueger system is usually named MGI 1901/Balkans zone 6 or 
MGI Balkans zone 6.

QGIS offers these EPSG codes:

EPSG:31276
+proj=tmerc +lat_0=0 +lon_0=18 +k=0.9999 +x_0=6500000 +y_0=0 
+ellps=bessel +towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232 
+units=m +no_defs

EPSG:3908
+proj=tmerc +lat_0=0 +lon_0=18 +k=0.9999 +x_0=6500000 +y_0=0 
+ellps=bessel +towgs84=682,-203,480,0,0,0,0 +units=m +no_defs

which differ in the datum shift (and therefore accuracy). The values 
from the linked article should give best accuracy. Or use those from

<http://spatial-analyst.net/PDF/HR_geoid_transformation_parameters_per_county.pdf>

HTH,
André Joost