Using QGIS V 2.2.0 (OS-Windows 8), I need to prepare raster files (ESRI ArcInfo Grids) to be later used in a species distribucion model using MaxEnt. Each of the original raster files (environmental variables for Guadalquivir, Spain) has a different spatial extension (but all share the same pixel size of 200, -200). However, to be able to use these layers in MaxEnt, all of them need to have the exact same spatial extent. Therefore, I clipped every raster using a polygon of Guadalquivir. After clipping, the new rasters have their original pixel size (200,-200) slightly altered (200.001,-199.942), which means trouble when running the maximum entropy algorithm.
The Layer Spatial Reference System for Guadalquivir is:
+proj=utm +zone=30 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
The Layer Spatial Reference System for the raster layers is:
+proj=utm +zone=30 +ellps=GRS80 +units=m +no_defs
I'm using the function
raster >> Extraction >> Clipper followed by the options...
- No data value = 0
- Mask layer: Guadalquivir
Following, I save the new raster as GeoTIFF.
Should the clipping be done using a polygon with the exact same projection? I am not sure to what extent projections differ in the layers above.
In case response to question 1 was "Yes" (same projections required), I did the clipping using both options: first by saving Guadalquivir as .shp with the raster's projection, and later the other way around, that is saving raster files with Guadalquivir's projection. Regardless of the option described in point 2), pixel size remains altered.
How can I keep the original pixel size of 200,-200?
I wouldn't bother too much about this seemingly slight difference if it weren't for the warning message that I get (see below) when trying to convert the GeoTIFF rasters to ASCII (a requirement for MaxEnt to work):
Warning 1: Producing a Golden Surfer style file with DX and DY instead of CELLSIZE since the input pixels are non-square. Use the FORCE_CELLSIZE=TRUE creation option to force use of DX for even though this will be distorted. Most ASCII Grid readers (ArcGIS included) do not support the DX and DY parameters.