[Qgis-user] Do I need satellite raster layers relative to an extended UTM zone?

[ardi]

Hi,

I've a satellite photo data set which spans across three UTM zones,
and I need to do some tasks in all three zones at once. Each image
tile from the data set is provided in two versions: one is projected
in the UTM coordinate system for the zone it actually belongs. The
other is projected over the central UTM zone, extending such central
zone.

AFAIK, QGIS can deal with layers belonging to different UTM zones,
because you see a lat/long display and layers are reprojected to
lat/long on the fly.

My question is: can I save disk space by storing the image tiles in
just one version, the version projected in its real UTM zone? Or would
I also need the versions projected into the (extended) central UTM
zone?

Just out of curiosity, what's the use of the version projected into
the extended zone? Is it because some GIS packages don't support
on-the-fly reprojection? Or why?

Thank you in advance