R: [GRASS-user] Map Display problems

[Aldo Clerici]

Michael,
many thanks for your detailed and very clear explanations.
In my case, as I specified in my last message of yesterday, the problem was
simply due to the default setting of the display mode. I compiled GRASS6.4
and I did not realize that the "Map fill display window" was on by default.
As I usually use the "Constrain map to region geometry" option I didn't
remember the other display mode behaviour and I needed some time to realize
what was happening (probably I'm becoming too old).

Sorry again.

Aldo Clerici



-----Messaggio originale-----
Da: Michael Barton [mailto:michael.barton at asu.edu] 
Inviato: martedì 3 febbraio 2009 20.18
A: grass-user at lists.osgeo.org
Cc: Aldo Clerici
Oggetto: Re: [GRASS-user] Map Display problems

Aldo,

Please see below


On Feb 3, 2009, at 9:19 AM, <grass-user-request at lists.osgeo.org> wrote:

> Date: Tue, 3 Feb 2009 16:24:17 +0100
> From: "Aldo Clerici" <aldo.clerici at unipr.it>
> Subject: [GRASS-user] Map Display problems
> To: <grass-user at lists.osgeo.org>
> Message-ID: <F31B8E2D65FB4E0B86BC2E32F5A69C6E at GEO.UNIPR.IT>
> Content-Type: text/plain; charset="us-ascii"
>
> Dear GRASS community,
>
>
>
> I'm having some problem with the Map display (GRASS6.4.0RC3 with Tcl/ 
> Tk
> GUI). It seems that the number of row and columns used to display is  
> fixed
> (482 x 642) and the resolution is changed consequently.

By default, the display will fill the map window and maintain a fixed  
number of pixels. This makes normal display much faster, especially  
with large maps. However, it you want the display to match the region  
geometry, there is a button on the upper right that will change this.  
Note that regardless of how you set the display, it will not change  
the computational region. This is set separately.

>
>
> For example, using Spearfish data:
>
>
>
>> g.region -pd
>
>
>
> projection: 1 (UTM)
>
> zone:       13
>
> datum:      nad27
>
> ellipsoid:  clark66
>
> north:      4928010
>
> south:      4913700
>
> west:       589980
>
> east:       609000
>
> nsres:      30
>
> ewres:      30
>
> rows:       477
>
> cols:       634
>
> cells:      302418
>
>
>
>> r.info -gs geology
>
>
>
> north=4928000
>
> south=4914000
>
> east=609000
>
> west=590000
>
> nsres=100
>
> ewres=100
>

You seem to be confounding the computational region with the inherent  
properties of a raster map. If you set the region to 30x30m (using  
g.region), then all maps will be treated as if they have a 30x30m  
resolution. That is, GRASS will reinterpolate the cells on the fly to  
30x30 regardless of whether the 'original' resolution of the map is  
30x30m or larger. Above, your region is set to 30x30m but the geology  
map's data is stored at 100x100m. You can set the region to match the  
resolution and extents of any map using g.region.

>
>
> To display the map geology on the Map Display, I choose the map with  
> the GIS
> Manager and I select the option "Zoom display to selected map" in  
> the Map
> Display. The map is displayed but with 482 rows and 642 columns and a
> resolution of 29.595 m (instead of 100).
>
>

Again, this is only the display, not the computational geometry. In  
fact, when you select "zoom display to selected map", it only is  
changing the extents viewed in the display. That is, the selected map  
will now fill the display. The display geometry is not connected to  
the geometry of the map unless you specifically set it to be so. This  
is because there are only so many pixels in the display, regardless of  
the number of cells in a map. If there are a lot more cells than  
pixels (often the case), the rendering system simply has to internally  
interpolate those cells down to the correct number of pixels to  
display. The opposite happens when there are more pixels than cells.

>
> I also tried  a region modification as follows:

What is it that you want to see in the geology map that is different  
from what you do see?

If I so g.region rast=geology then pick 'zoom display to computational  
region' the map looks like the geology map and the status bar at the  
bottom reads "Display: rows=140 columns=190 resolution=100 meters"

Michael