Aldo,
Please see below
On Feb 3, 2009, at 9:19 AM, <grass-user-request@lists.osgeo.org> wrote:
Date: Tue, 3 Feb 2009 16:24:17 +0100
From: "Aldo Clerici" <aldo.clerici@unipr.it>
Subject: [GRASS-user] Map Display problems
To: <grass-user@lists.osgeo.org>
Message-ID: <F31B8E2D65FB4E0B86BC2E32F5A69C6E@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