Geografic-Information-System/scripts/v.dissolve/v.dissolve.html

<H2>DESCRIPTION</H2>

The <EM>v.dissolve</EM> module is used to merge adjacent areas in a vector
map which share the same category value. The resulting merged area(s) retains
this category value. Alternatively an integer or string column can be defined
which is used to find adjacent polygons with identical attribute for common
boundary dissolving. In this case the categories are not retained, only the
values of the new key column. See the <em>v.reclass</em> help page for details.


<h2>NOTES</h2>

GRASS defines a vector area as composite entity consisting of a set of
closed boundaries and a centroid. The centroids must contain a
category number
(see <em><a href="v.centroids.html">v.centroids</a></em>), this number
is linked to area attributes and database links.
<P>
Multiple attributes may be linked to a single vector entity through
numbered fields referred to as layers. Refer
to <em><a href="v.category.html">v.category</a></em> for more details.

<P>
Merging of areas can also be accomplished using
"<em>v.extract <b>-d</b></em>" which provides some additional options.
In fact, <a href="v.dissolve.html">v.dissolve</a> is simply a
front-end to that module. The use of the <EM>column</EM> parameter
adds a call to <em><a href="v.reclass.html">v.reclass</a></em> before.


<h2>EXAMPLES</h2>

<h3>Basic use</h3>
<div class="code"><pre>
v.dissolve input=undissolved output=dissolved
</pre></div>

<h3>Dissolving based on column attributes</h3>
<div class="code"><pre>
# check column names:
v.info -c polbnda_italy

# dissolve based on column attributes:
v.dissolve input=polbnda_italy output=pol_italy_regions col=vmap_nam
</pre></div>

<h3>Dissolving adjacent SHAPE files to remove tile boundaries</h3>

If tile boundaries of adjacent maps (e.g. CORINE Landcover SHAPE files)
have to be removed, an extra step is required to remove duplicated
boundaries:

<div class="code"><pre>
# patch tiles after import:
v.patch -e `g.mlist type=vect pat="clc2000_*" sep=,` out=clc2000_patched

# remove duplicated tile boundaries:
v.clean clc2000_patched out=clc2000_clean tool=snap,break,rmdupl thresh=.01

# dissolve based on column attributes:
v.dissolve input=clc2000_clean output=clc2000_final col=CODE_00
</pre></div>


<H2>SEE ALSO</H2>

<EM>
<A HREF="v.category.html">v.category</A>,
<A HREF="v.centroids.html">v.centroids</A>,
<A HREF="v.extract.html">v.extract</A>,
<A HREF="v.reclass.html">v.reclass</A>
</EM>


<H2>AUTHORS</H2>

module: M. Hamish Bowman, Dept. Marine Science, Otago University, New Zealand<br>
Markus Neteler for column support<BR>
help page: Trevor Wiens

<P>
<i>Last changed: $Date$</i>