GRASS GIS manual: v.dissolve

NAME

v.dissolve - Dissolves boundaries between adjacent areas sharing a common category number or attribute.

KEYWORDS

vector, area, dissolve

SYNOPSIS

v.dissolve
v.dissolve help
v.dissolve input=name output=name [layer=string] [column=string] [--overwrite] [--verbose] [--quiet]

Flags:

--overwrite: Allow output files to overwrite existing files
--verbose: Verbose module output
--quiet: Quiet module output

Parameters:

input=name: Name of input vector map
output=name: Name for output vector map
layer=string: Layer number or name. If -1, all layers are extracted.; A single vector map can be connected to multiple database tables. This number determines which table to use.; Default: 1
column=string: Name of column used to dissolve common boundaries

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

# 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

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.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

AUTHORS

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

Last changed: $Date: 2008-12-10 15:39:46 -0800 (Wed, 10 Dec 2008) $

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