[GRASS-dev] suggestion for g.region -a

[Glynn Clements]

Michael Barton wrote:

> Currently, if you use the -a flag with g.region it will align the  
> region exactly to match the resolution of a map (e.g. g.region -a  
> rast=myrast will set the region to exactly the resolution of myrast  
> regardless of the extents) or the resolution specified in the res or  
> ewres/nwres fields (e.g. g.region -a n=523 s=15 res=10 will adjust  
> the extents a little to make sure the resolution is exactly 10).
> 
> However, -a is ignored if the map argument (rast=) or resolution  
> arguments are blank.

Actually, you need either vect= or res= (or nsres= or ewres=). rast=
doesn't have any effect. Also, the interpretation is different for
vect= compared to the resolution options.

> This means that if you want to change the  
> extents and keep your current resolution, you need to specify the -a  
> flag AND explicitly specify the resolution. Otherwise, the resolution  
> will be adjusted slightly.
> 
> Could this be modified so that when the -a flag is used and  
> resolution is NOT specified and a map is NOT specified (i.e., only  
> changing extents) it aligns to the current resolution? That is,  
> respect the -a flag when res/ewres/nsres= is blank and rast= is blank.

It would be possible to implement the -a flag unconditionally,
although you would need to exclude the cases which already have a
defined meaning (e.g. "g.region -a nsres=..." only aligns the top and
bottom edges, leaving the left and right edges unchanged).

-- 
Glynn Clements <glynn at gclements.plus.com>