[GRASS-user] getting a polygone outline of raster areas

[Tim Michelsen]

Hello,
I have a raster which was calculated from r.mapcalc and therefore contains areas
with NULL values.

Is there any procedure I can use to get an raster (or better vector) with the
outline of all coherent or connected areas where the raster values are not NULL?

Example:
Imagine a raster with all countries of the world.
Then I use r.mapcalc to get the country with ID of Greece which leads to large
areas with NULL values and some areas with values representing the Greek
mainland its islands.
How do I get a polygon outlining the mainland and the islands?

In this case the operation will be fast. But in the case of a raster with
floating oint values (slope, aspect, elevation) raster to vector conversion
takes time and can easily overpass the capabilities of the computer.

See also:
http://news.gmane.org/gmane.comp.gis.grass.user

Thanks and kind regards,
Timmie