[GRASS-dev] Zooming issue in gis manager

Glynn Clements glynn at gclements.plus.com
Thu Nov 2 19:52:23 EST 2006

Hamish wrote:

> > Given that the origin for most coordinate systems is a completely
> > arbitrary location arising from various completely arbitrary constants
> > (e.g. whichever round numbers where chosen as the false easting and
> > northing), I have no idea what the intended purpose of the -a flag is,
> > but I'm fairly sure that it shouldn't be used by default.
> it keeps you from needlessly resampling (degrading) your raster data as
> you zoom+pan around & create new maps using the new map bounds?

Oh, I get the point about preserving the resolution, but I would have
thought that there would have been a better way to choose the

If the resolution doesn't change, you probably want to retain the
alignment. Similarly, if the resolution increases or decreases by an
integer multiple (the former has an unambiguously correct answer; the
latter involves an arbitrary choice, but not as arbitrary as the
coordinate system's origin).

In the case where the resolution changes arbitrarily, there isn't any
"correct" alignment, so I think I see where the behaviour of -a comes
from (i.e. a cop-out on the part of the programmer).

But in the case where the resolution doesn't change, I'm even more
confident that "g.region -a" is the wrong approach; the original grid
(both resolution and alignment) should be preserved, IMHO.

Glynn Clements <glynn at gclements.plus.com>

More information about the grass-dev mailing list