QGIS » QGIS Application

Projecting data from Gauß-Krüger 4, DHDN/Bessel (EPSG:31468), to Gauß-Krüger 4, Pulkova/Krasowski (EPSG 2398) and vice versa gives me data that seemingly are projected correctly, but there is a difference of 3 m mainly in one direction (resulting EPSG 2398 data are too far in the west). In the Halle region (Sachsen-Anhalt) the result should be roughly a simple 2D-Transformation of +22m (east) and +589m (west). The difference can be told e.g. by comparing world files of orthophotos delivered by the Landesvermessungsamt for both projections. However, in the example provided projecting in QGIS yields +19.13 (to the east) 588.10 (to the north) instead of +21.97 and +589.05. I measured by loading into ArcView (3.2), which doesn't provide on-the-fly projections.

Minor differences (order of magnitude 0.5 m) are also detectable comparing projections of QGIS (GK4 -> UTM 32) to those by ArcGIS (which projects reliably, albeit not necessarily reliably on the fly). I was quite shocked. I thought those reprojections have been tested for a long time already in the Open GIS community. I understand that minor differences of less than a meter depend e.g. on the type of transformation used, but 3 meters with not having a choice of transformation parameter sets seem too much to me.

Krassowski.tfw:
0.2
0.0
0.0
-0.2
4494022.074 upper left corner (center of pixel, thus 22.074 - 0.1 and 589.039 +0.1 should be difference of upper left coordinate of projected extent)
5708589.039
Bessel.tfw:
0.2
0.0
0.0
-0.2
4494000.100
5707999.900