[Qgis-user] Why does clipping a raster alters cell size?

[Luciano La Sala]

Dear QGIS users,

I am using QGIS V 2.2.0 (OS-Windows 8), and 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, I save as GeoTIFF but the new rasters have 
their original pixel size (200,-200) slightly altered 
(200.001,-199.942). I wouldn't bother too much about this menial 
difference in cell size 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. .

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

When clipping, 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.

Questions:

Should the clipping be done using a polygon with the exact same 
projection? I am not sure to what extent the projections differ between 
the layers above.

In case response to question is "Yes" (same projections needed), 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, pixel size remains altered/uneven.

How can I keep the original pixel size of 200,-200?

Thank you in advance.

Luciano