Bug report #3162
GRASS raster display inaccurate
|Affected QGIS version:||master||Regression?:||No|
|Operating System:||SuSE||Easy fix?:||No|
|Pull Request or Patch supplied:||No||Resolution:||worksforme|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||13222|
A GRASS raster displayed in QGIS changes it's position relative to other layers when the map is moved with the mouse. I see this while doing the following:
_ in QGIS: add a GRASS polygon vector layer (let's call it poly)
_ in GRASS: @v.to.rast poly use=cat out=poly_rast'
_ in QGIS: add GRASS raster layer poly_rast
_ now move the map around with the mouse and watch the coloured areas of raster cells move around within the polygons they were made of.
I have the same polygon in PostGIS. When I add this too, the two vectors do not move relative to each other. So seems to be the raster which is displayed inaccurately. In my case it shifts several meters within a Gauß-Krüger (Zone 4) projection. The raster cell-size is 2 m.
#4 Updated by Tomas Brunclik over 8 years ago
I can confirm this issue still present in QGIS 1.8.0 RC1 on Linux, and in various 1.7 and 1.8 versions on Windows and Mac OSX. The shift between GRASS raster and any other type of map layer is usually within 30m when working with Landsat data (30m resolution). But it still means, that if you place a point in the map, you could not be sure over which pixel it was placed in reality, which makes many types raster analysis (like sampling raster values at point locations, supervised classification) unusable or less precise. In projects with 1m raster resolution I observed shift of several pixels. Maybe the extent of the shift is independent on the raster/region resolution?
The issue is not present in GRASS, if run standalone, or if you start GRASS interface from within the QGIS using GRASSS shell.