[GRASS-dev] d.m/d.gis mysteries..

Maciej Sieczka tutey at o2.pl
Tue Oct 31 11:07:07 EST 2006

Michael Barton wrote:
> Before we make these changes, I'd like to reiterate again what is going on
> with a bit of visual display to better show the issues.
> 1. There is not a systematic, major inaccuracy from zooming the display in
> the GIS Manager.
> 2. This inaccuracy reported only

But is there and it will bite users occasionaly, making them corrupt
their data only once a month or week.

> occurs with interactive zooming when
> displaying a raster map so that the raster grid squares are much larger than
> the pixel size. That is, when you can see only a few grid squares in the
> display.
> 3. It affects the number of grid cells displayed, not the actual
> computational or display regions.
> 4. It is primarily (or likely entirely) a function of using g.region -a to
> align the display region to the raster grid at the current resolution. I
> originally did NOT use this flag, because I was concerned about potentially
> uncontrolled changes to zoom precision, but added this to the zooming a few
> months back at the strong request of Maciej (possibly echoed by a few
> others). 

Another try to blame it on me. Noticed, skipped.

Please consider that gis.m not preserving the res while zooming is as
bad as not setting the extents correctly. Neither is acceptable,
becasue either leads to wrong region settings.


> *****************
> The reported problem with displaying grid cells at high magnification is
> primarily or entirely a function of how g.region -a aligns the region with
> the raster grid cell boundaries. I don't know how this works internally and
> someone with C programming experience will have to deal with it if we want
> it to work differently
> So, what do we want to do? We can either have
> 1. All zooming is aligned with the cell borders, matching the current
> resolution, with the possible result of a bit of rounding error in the
> display when zoomed in to a point where the cells are very large compared to
> pixel size--due to the way that g.region -a works.
> OR
> 2. Zooming is NOT aligned with the cell borders and the current resolution.
> In this case, what you see is very close to the actual region extents when
> zoomed in to a point where the cells are very large compared to pixel size.
> This is how I originally had it.

Both are as bad.


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