[Qgis-user] controlling cell size during raster warp

[Janneke Qgis]

Dear list,

I've downloaded aster dem data (25 tiles) and I want to use the tiles 
for watershed analysis (using grass r.watershed). I would like to get 
some general advise about how best to approach this (and haven't found 
something yet on the internet).

1. Would it be better to work with the original or with projected data 
(wgs84/utm36s)?
2. Is it necessary to work with the merged data, or is it 
possible/adviseable to use a virtual raster?
3. if it is necessary to reproject and merge the data, should the data 
be warped first before merging? In that case, what are the consequences 
of the different cell sizes resulting from the warp process for the 
merge process?

I'm thinking of doing all these preparational steps using the GDAL 
plugin because I'm more familiar with it, but if the grass plugin gives 
better results I'll give that a try.

Any hints/links/advice welcome, thanks,

Janneke