[GRASS5] Re: GRASS dissolve

GRASS GIS GRASS GIS developer
1

Michael,

I also just tried to do a dissolve yesterday and it didn’t work. It SHOULD work using v.extract –d. I wanted demonstrate dissolve while working with a student yesterday and it didn’t work. I simply wanted to dissolve all the common boundaries in a map, so I didn’t specify a where clause. However, nothing was dissolved. I think that it will only dissolve polygons with common cat value, regardless of whether or not you specify. Try specifying your range of cat numbers v.extract –d cat=1-[n] (where [n] is your max cat number).

Michael Barton

On 4/26/05 3:57 PM, “Michael Zoldak” Michael.Zoldak@asu.edu wrote:

Dr Barton

This is mike zoldak at IIS again with another GRASS question. I’m trying to dissolve polygon boundaries based on a shared attribute value. Is there some trick to doing this very basic GIS operation? I’ve tried about a hundred different permutations (literally) of v.extract and v.reclass and nothing seems to come close.

Mike Zoldak

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>

2

I also just tried to do a dissolve yesterday and it didn¹t work. It
SHOULD work using v.extract ­d. I wanted demonstrate dissolve while
working with a student yesterday and it didn¹t work. I simply wanted
to dissolve all the common boundaries in a map, so I didn¹t specify a
where clause. However, nothing was dissolved. I think that it will
only dissolve polygons with common cat value, regardless of whether or
not you specify.

Yes, only "common" (ie redundant) boundaries separating areas of common
category are disolved.

There are several strategies for removing cats from a vector map. After
that 'v.extract -d' should do what you want it to. Luckily it is easier
to remove cats then to add them, e.g. make a new column with common cat
numbers and run 'v.reclass column=' or try 'v.category option=del' +
'v.category option=add cat=1 step=0'.

Hamish