[GRASS-user] Converting raster to polygon creates too many polygons

Hanlie Pretorius hanlie.pretorius at gmail.com
Wed Sep 29 09:41:26 EDT 2010

2010/9/29, Markus Metz <markus.metz.giswork at googlemail.com>:
> In GRASS, you have full control of the computational region,
> consequently you must be able to explain (to the client/supervisor)
> every aspect of the computational region. Why is the horizontal
> resolution not only slightly off from exactly one meter, but also
> different for north-south and east-west? If these are the settings of
> the input DEM, why was the DEM created with these settings? If these
> resolution settings are different from the DEM's settings, there is
> again lots of explaining and justification to do. Life is much easier
> with, in this case, exactly 1m resolution which might need to be set
> when preprocessing and creating the high-res DEM. The resolution
> settings are just slightly different from 1m which can cause rounding
> errors later on (export, import, file formats storing resolution in
> single and not double precision, other GIS software using single and
> not double precision for resolution, etc.)

Thanks for the extensive comments. I don't really know why the
resolution in the x and y directions are not the same. Even when I set
them explicitly, I get a slight difference:
g.region -p vect=c83a_border at PERMANENT nsres=25 ewres=25
projection: 99 (Transverse Mercator)
zone:       0
datum:      ** unknown (default: WGS84) **
ellipsoid:  wgs84
north:      -3122229.69982072
south:      -3155672.30728643
west:       -66483.27873573
east:       -33474.90929218
nsres:      24.99447494
ewres:      25.00634049
rows:       1338
cols:       1320
cells:      1766160

The original resolution of the DEM was at 25m resolution. I just
assumend that the differences in x and y resolutions are a result of
the projected surface being skew with respect to the cartesian axes. I
say this because vectors that looked square in a latlong GCS display
slightly skewed when projected to the Transverse Mercator PCS that I
use in this location.

My reason for running r.watershed at a much finer resolution was that
I would get a smoother boundary when I then convert to vector.
Obviously not a good idea.

> Markus M

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