Geografic-Information-System/raster/r.to.vect/r.to.vect.html

<h2>DESCRIPTION</h2>

<em>r.to.vect</em> scans the named <b>input</b> raster map
layer, extracts points, lines or area edge features from it, converts data
to GRASS vector format.

<h3>Point conversion</h3>

The <em>r.to.vect</em> program extracts data from a GRASS raster map 
layer and stores output in a new GRASS <em>vector</em> file.  

<h3>Line conversion</h3>
<em>r.to.vect</em> assumes that the <em>input</em> map has been thinned
using <em><a href="r.thin.html">r.thin</a></em>.

<p>
<em>r.to.vect</em> extracts vectors (aka, "arcs") from a
raster map.  These arcs may represent linear features
(like roads or streams), or may represent area edge
features (like political boundaries, or soil mapping
units).  

<p>
<em><a href="r.thin.html">r.thin</a></em> and <em>r.to.vect</em>
may create excessive nodes at every junction, and may create small spurs
or "dangling lines" during the thinning and vectorization process.
These excessive nodes and spurs may be removed using
<em><a href="v.clean.html">v.clean</a></em>.


<h3>Area conversion</h3>

<em>r.to.vect</em> first traces the perimeter of each unique
area in the raster map layer and creates vector data to
represent it.  The cell category values for the raster map
layer will be used to create attribute information for the
resultant vector area edge data.

<p>
A true vector tracing of the area edges might appear
blocky, since the vectors outline the edges of raster data
that are stored in rectangular cells. To produce a
better-looking vector map, <em>r.to.vect</em> smoothes the
corners of the vector data as they are being extracted. At
each change in direction (i.e., each corner), the two
midpoints of the corner cell (half the cell's height and
width) are taken, and the line segment connecting them is
used to outline this corner in the resultant vector map.
(The cell's cornermost node is ignored.) Because vectors
are smoothed by this program, the resulting vector map will
not be "true" to the raster map from which it was created.
The user should check the resolution of the geographic
region (and the original data) to estimate the possible
error introduced by smoothing.

<p>
<em>r.to.vect</em> extracts only area edges from the named raster 
input file. If the raster map contains other data (i.e., line edges, 
or point data) the output may be wrong.

<h2>EXAMPLES</h2>

The examples are based on the North Carolina sample dataset:
<p>
<b>Conversion of raster points to vector points:</b>
<p>
Random sampling of points:
<div class="code"><pre>
g.region raster=elevation -p
# random sampling of points (note that r.random also writes vector points)
r.random elevation raster_output=elevrand1000 n=1000
r.to.vect input=elevrand1000 output=elevrand1000 type=point
# univariate statistics of sample points
v.univar elevrand1000 column=value type=point
# compare to univariate statistics on original full raster map
r.univar elevation
</pre></div>
<p>

<b>Conversion of raster lines to vector lines:</b>
<p>
Vectorization of streams in watershed basins map:
<div class="code"><pre>
g.region raster=elevation -p
r.watershed elev=elevation stream=elev.streams thresh=50000
r.to.vect -s input=elev.streams output=elev_streams type=line
# drop "label" column which is superfluous in this example
v.db.dropcolumn map=elev_streams column=label
v.db.renamecolumn map=elev_streams column=value,basin_id
# report length per basin ID
v.report map=elev_streams option=length units=meters sort=asc
</pre></div>
<p>

<b>Conversion of raster polygons to vector polygons:</b>
<p>
Vectorization of simplified landuse class map:
<div class="code"><pre>
g.region raster=landclass96 -p
# we smooth corners of area features
r.to.vect -s input=landclass96 output=my_landclass96 type=area
v.colors my_landclass96 color=random
</pre></div>

<h2>KNOWN ISSUES</h2>

For type=line the input raster map MUST be thinned by
<em><a href="r.thin.html">r.thin</a></em>;
if not, <em>r.to.vect</em> may crash.

<h2>SEE ALSO</h2>

<em>
<a href="g.region.html">g.region</a>,
<a href="r.thin.html">r.thin</a>,
<a href="v.clean.html">v.clean</a>
</em>

<h2>AUTHORS</h2>
<b>Point support</b><br>
Bill Brown<br>
<br>

<b>Line support</b><br>
Mike Baba<br>
DBA Systems, Inc.<br>
10560 Arrowhead Drive<br>
Fairfax, Virginia 22030<br>
<br>

<b>Area support</b><br>
<em>Original</em> version of <em>r.poly</em>: 
<br>
Jean Ezell and Andrew Heekin, 
<br>
U.S. Army Construction Engineering 
Research Laboratory

<p><em>Modified</em> program for smoothed lines: 
<br>
David Satnik, 
Central Washington University
<br>
Updated 2001 by Andrea Aime, Modena, Italy<br>
<br>

<b>Update</b><br>
Original r.to.sites, r.line and r.poly merged and updated to 5.7 by Radim Blazek

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