[GRASS-dev] [bug #4469] (grass) g.region: nsres & ewres are rounded differently

[Andrew Danner]

I was going to suggest adding a flag to support keeping the resolution
fixed and adjusting the bounds instead, but it appears it is already
there. The -a flag aligns to bounds, so if you type 

g.region res=1.23456 -a

the n,s,e, and w bounds will adjust slightly (less than one pixel width)
to keep the resolution fixed at the user specified value. 

Though the -a flag works in a strange way in my opinion, it shifts the
boundaries instead of expanding/contracting the boundaries.

Example

g.region rast=test -p
north:      603580
south:      590120
west:       2455900
east:       2465980
nsres:      20
ewres:      20
rows:       673
cols:       504

//nsres gets re-adjusted
g.region rast=test res=40 -p
north:      603580
south:      590120
west:       2455900
east:       2465980
nsres:      39.94065282
ewres:      40
rows:       337
cols:       252

//nsres stays fixed, north and south are shifted
//east and west are also shifted even though they didn't need
//to be adjusted
g.region rast=test res=40 -ap
north:      603600
south:      590120
west:       2455880
east:       2466000
nsres:      40
ewres:      40
rows:       337
cols:       253

I would prefer that either the region boundary be expanded or contracted
by a small amount and not shifted. Let say your original raster is just
the right size and data fills every pixel. If you enlarge the boundary
(rounding up) on the res adjustment, some pixels extend into regions of
possible no data. If you contract the region (round down), you will have
valid data for the entire boundary. If you shift, as is currently the
case, the west/south edges are ok, but east and north may extend into a
no data region. 

I think the -a flag is useful for when you want to preserve a particular
res= setting, but the behavior is a little odd. 

-Andy

On Thu, 2006-05-18 at 17:54 +0100, Glynn Clements wrote:
> Request Tracker wrote:
> 
> > this bug's URL: http://intevation.de/rt/webrt?serial_num=4469
> > -------------------------------------------------------------------------
> > 
> > Subject: g.region: nsres & ewres are rounded differently
> > 
> > Platform: GNU/Linux/x86
> > grass obtained from: CVS
> > grass binary for platform: Compiled from Sources
> > GRASS Version: 2006-05-18
> > 
> > Hi
> > 
> > nsres & ewres are rounded differently when setting res to something
> > near the decimal places limit.
> > 
> > In [1] both nsres & ewres are rounded-down to 1.12345678 when
> > requested res=1.123456784. That's OK.
> > 
> > But in [2], when requested one decimal point higher res=1.123456785,
> > nsres is rounded-up to 1.12345679 while ewres is rounded-down to
> > 1.12345678.
> > 
> > Moreover, next g.region run [3] with settings identical like in [2]
> > results in ewres=nsres=1.12345678, and, naturaly, different region
> > extent.
> > 
> > Is this all OK? I thought each single, identical g.region run should
> > result in the exactly same region settings.
> 
> If you only specify the res= option, the new bounds will depend upon
> the previous bounds.
> 
> Ultimately, a region is defined by its bounds and the number of rows
> and columns. The resolution is derived from those values, and it may
> not be possible to get exactly the value which you requested (as the
> result of dividing a floating-point number by an integer may not be
> exactly representable as a floating-point number).
> 
> The alternative (i.e. to define a region by one corner, the
> resolution, and the number of rows and columns, with the other corner
> being derived), would be signficantly more awkward to write code for.
>