[GRASS5] Outstanding issues for the 5.0 stable release

Glynn Clements glynn.clements at virgin.net
Fri Jun 28 06:25:09 EDT 2002

Eric G. Miller wrote:

> > 1. This seems self-contradictory. For most GRASS raster commands, the
> > size of the output map is determined from the current region settings,
> > regardless of the portion which will be non-null.
> Errm.  I was mixed up... I never use r.proj due to its memory
> requirements so was remembering it as being run from the input mapset.
> Therefore, it's seems like it should be treated like an import/export
> routine since it's run in the target location.  
> The problem I think most folks run into is setting the current region
> for the projection but not having a good idea what it should be set at.
> So, as a user, do I have to project some points or a vector box first
> just to guess what the current region extents should be?  Maybe the
> behavior is what some users want, but I'd bank that in general users
> would want to project the entire input at its data resolution into the
> current mapset (much like any raster import).  If I want/need to cut it
> down or reduce the resolution, I'd make those modifications before or
> after the projection (depending on what I'm using to make those
> determinations).  

Two problems with that:

1 (Boundaries). Projecting an entire map may result in the output
being infinitely large (e.g. projecting a "global" lat/lon map to e.g. 
Mercator), or at least infeasibly large (try applying one of the "Mod. 
Stererographics" projections to a map which extends significantly
beyond the intended region).

2 (Resolution). Unless the transformation consists solely of scaling
and translation (and, if you're using r.proj, it probably doesn't),
there isn't a "magic" output resolution which results in no loss of

> > 2. Bear in mind that r.proj "works backwards". For each output cell,
> > it projects that cell's coordinates via the inverse projection
> > function to obtain the corresponding coordinates in the input map, and
> > copies the corresponding cell value from the input map to the output
> > map. Forward projection is only used to compute an optimal bounding
> > box.
> As a user, I don't care how it works internally ;-)

This isn't really "internal"; it's fundamental to what it means to
"project" a raster.

Glynn Clements <glynn.clements at virgin.net>

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