I am working with BIOCLIM temperature/precipitation layers and I have noticed that when I clip the global raster down to a specific region, something happens and the raster value at a specific point no longer matches that on the global raster. Here is my working procedure:
Download bioclim layers from http://www.worldclim.org/current. Load e.g. bio_1 layer into QGIS
clip using Raster>Extraction>Clipper. Clip with "By Extent" mode and set the extent to e.g. -50,-50 : 50,50
Using the identify features tool, click on region of the map and compare values for both layers. For some points, the raster values are the same, others are slightly off.
Why does this happen?
My first thought was that the vector I am clipping with is dissecting raster cells and thereby shifting the new layer slightly. I have tried to change the "Snapping Option..." (Settings>Snapping option...) to all layers, snap to vertex and set the sensitivity to different number of pixels (0.5 to 50) but nothing works.
What am I doing wrong?