## DESCRIPTION

The v.dissolve 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 v.reclass help page for details.

## NOTES

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 v.centroids), this number is linked to area attributes and database links.

Multiple attributes may be linked to a single vector entity through numbered fields referred to as layers. Refer to v.category for more details.

Merging of areas can also be accomplished using `v.extract -d` which provides some additional options. In fact, v.dissolve is simply a front-end to that module. The use of the column parameter adds a call to v.reclass before.

## EXAMPLES

### Basic use

    v.dissolve input=undissolved output=dissolved

### Dissolving based on column attributes

North Carolina data set:

    g.copy vect=soils_general,mysoils_general
    v.dissolve mysoils_general output=mysoils_general_families column=GSL_NAME

### Dissolving adjacent SHAPE files to remove tile boundaries

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:

    # patch tiles after import:
    v.patch -e `g.list type=vector pat="clc2000_*" separator=","` 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

## SEE ALSO

v.category, v.centroids, v.extract, v.reclass

## AUTHORS

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