[GRASS-dev] Zooming issue in gis manager

Michael Barton michael.barton at asu.edu
Wed Nov 1 17:30:06 EST 2006

I've looked even further and found that as Glynn notes, d.rast also will
align with cells, even if the -a flag is not set in g.region. It also seems
to do so somewhat more accurately than with the -a flag set. That is, it
produces a much better (exact?) match between the display and computational
region in a more predictable way, as you can see in the graphics I showed
last night.

However, if  you want to *really* see where the grid cells are at high
magnification, try the explore mode. Because it increases the resolution as
you zoom in (to keep a constant number of cells/pixel) you can set the
display to zoom across original cell boundaries. It also shows a point
overlaying cells in a slightly differently place even than "normal" display
mode with -a not set--by about a quarter of a cell on the display I'm
looking at. With -a set, the difference is more than 1/2 cell in the other

I can't tell which is "right" for certain. That is, I'm not sure how to tell
mathematically where the cell boundaries ought to lie with respect to their
actual coordinates and where a point should lie within a cell. I'm betting
that explore mode is the most "accurate" in terms of the relationship
between cells and a point. How can we determine what the cell boundaries
ought to be? Since this is a function of the interaction between g.region
and the display driver, do we need to do anything about it (as Joel asks)?
If so, what? Also, I assume that this is only affecting the graphics created
out of d.rast (and d.vect??), and does not affect any overlay operations. So
how should it be prioritized?


Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics and Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

> From: Joel Pitt <joel.pitt at gmail.com>
> Reply-To: <joel.pitt at gmail.com>
> Date: Thu, 2 Nov 2006 10:08:48 +1300
> To: Glynn Clements <glynn at gclements.plus.com>
> Cc: Maciej Sieczka <tutey at o2.pl>, Michael Barton <michael.barton at asu.edu>,
> grass-dev <grass-dev at grass.itc.it>
> Subject: Re: [GRASS-dev] Zooming issue in gis manager
> On 11/2/06, Glynn Clements <glynn at gclements.plus.com> wrote:
>> I think that the core problem is that d.rast cannot render an
>> arbitrary portion of a map; the rendered image has to be aligned to
>> the cell boundaries.
>> The region always has an integer number of rows and columns. d.rast
>> always ensures that the entire region just fits inside the frame; one
>> pair of edges will always exactly touch the corresponding edges of the
>> region.
>> Thus, the rendered image will only ever show whole cells. You will
>> never get a situation where part of a cell is shown and the other part
>> cropped.
> How much difficulty would it be to alter the behaviour of d.rast to
> allow partial
> cells to be rendered?
> If someone were to make the change, can anyone think of what flow on
> effects this would have for other components of GRASS? (Not too much
> of an issue since a flag could be added to d.rast to have this as
> optional behaviour).
> -- 
> -Joel
> "Wish not to seem, but to be, the best."
>                 -- Aeschylus

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