[GRASSLIST:7710] Raster Resolution (help requested)

GRASS GRASS user
1

Hey yall,

In the past I have worked mainly with RASTER layers in GRASS and they just worked (some how). Now I am working alot with vectors in GRASS. For instance I have a region that was created througha v.in.ogr. Now when I bring in the coresponding raster maps they are verry blocky unless I view them from a far out zoom. My region settings are

projection: 0 (x,y)
zone: 0
north: 44.86371257
south: 44.67956501
west: -76.34802196
east: -76.08834984
nsres: 0.00920738
ewres: 0.01298361
rows: 20
cols: 20

I created a DEM from contour lines (v.surf.rst) and the results are undesireable due to the chopyness of the raster. for instance instead of smooth DEM I have lakes that contain one or two raster cells. The same area would contain MANY contour lines. Cheers

A quick explanantion of what I've done and how to increase the resolution of the rasters in my project would be appreciated.

--
+---------------------------------------------------------------+
|Dave Sampson: Geographic Information System (GIS) Technician |
| |
|Centre for Sustainable Watersheds |
|Cooperative Approaches to Protect Canada's Water Resources |
| |
|14 Water St, P.O Box 280. Portland Ontario, Canada. K0G 1V0 |
|Lat: 44.700 Long: -76.19 |
|www.watersheds.ca gis@watersheds.ca |
+---------------------------------------------------------------+

2

Dave,

Change the resolution using g.region (also in GUI).

Set the resolution to what you want using g.region, THEN run v.surf.rst. It
will interpolate to the resolution set using g.region. In other words, if
you do ...

G.region res=1

...it will set your resolution to 1 degree (for your latlon map). V.surf.rst
would then create 1 degree x 1 degree raster cells.

G.region res=.00001

...would create a much higher resolution map--with many more cells of
course.

It may be that your original DEM already has a sufficiently high resolution
and you just can't see it. To make your region resolution match that of your
DEM, use...

G.region rast=[myDEM]

...(where [myDEM] is the name of your raster DEM map)

Using g.region, GRASS can set not only the zoom window (extents), but also
the visible and analytical resolution on the fly. You must remember that
nearly all raster operations take place ONLY in the window set by g.region
and at the resolution set by g.region. However, this can be VERY useful.

Michael

On 7/25/05 11:55 AM, "Dave Sampson" <gis@watersheds.ca> wrote:

Hey yall,

In the past I have worked mainly with RASTER layers in GRASS and they
just worked (some how). Now I am working alot with vectors in GRASS. For
instance I have a region that was created througha v.in.ogr. Now when I
bring in the coresponding raster maps they are verry blocky unless I
view them from a far out zoom. My region settings are

projection: 0 (x,y)
zone: 0
north: 44.86371257
south: 44.67956501
west: -76.34802196
east: -76.08834984
nsres: 0.00920738
ewres: 0.01298361
rows: 20
cols: 20

I created a DEM from contour lines (v.surf.rst) and the results are
undesireable due to the chopyness of the raster. for instance instead of
smooth DEM I have lakes that contain one or two raster cells. The same
area would contain MANY contour lines. Cheers

A quick explanantion of what I've done and how to increase the
resolution of the rasters in my project would be appreciated.

____________________
C. Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
PO Box 872402
Arizona State University
Tempe, AZ 85287-2402
USA

Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>

3

Dave Sampson wrote:

In the past I have worked mainly with RASTER layers in GRASS and they
just worked (some how). Now I am working alot with vectors in GRASS. For
instance I have a region that was created througha v.in.ogr. Now when I
bring in the coresponding raster maps they are verry blocky unless I
view them from a far out zoom. My region settings are

projection: 0 (x,y)
zone: 0
north: 44.86371257
south: 44.67956501
west: -76.34802196
east: -76.08834984
nsres: 0.00920738
ewres: 0.01298361
rows: 20
cols: 20

I created a DEM from contour lines (v.surf.rst) and the results are
undesireable due to the chopyness of the raster. for instance instead of
smooth DEM I have lakes that contain one or two raster cells. The same
area would contain MANY contour lines. Cheers

A quick explanantion of what I've done and how to increase the
resolution of the rasters in my project would be appreciated.

Decrease the region resolution with "g.region res=..."; e.g. using
"g.region res=0.001" would give you roughly 200x200 rather than 20x20.

Any rasters which are created by GRASS are created at the current
resolution, so you will need to regenerate any rasters which were
created at too low a resolution.

This doesn't apply to rasters which are imported with r.in.*; they are
imported cell-for-cell.

--
Glynn Clements <glynn@gclements.plus.com>