Geografic-Information-System/vector/v.buffer/v.buffer.html

<h2>DESCRIPTION</h2>

<em>v.buffer</em> creates a buffer around features of
given <b>type</b>, which have a category in the
given <b>layer</b>. The <b>tolerance</b> controls the number of vector
segments being generated (the smaller the value, the more vector
segments are generated).

<h2>NOTES</h2>

Internal buffers for areas can be generated with negative distance 
values ("inward buffer" or "negative buffer" or "shrinking").
<p>
<em>v.buffer</em> fusions the geometries of buffers by default.
Categories and attribute table will not be transferred (this would
not make sense as one buffer geometry can be the result of many
different input geometries). To transfer the categories and
attributes the user can set the <b>t</b> flag. This will result in
buffers being cut up where buffers of individual input geometries
overlap.  Each part that is the result of overlapping buffers of
multiple geometries will have multiple categories corresponding to
those geometries, and multiple buffer areas can have the same
category. The buffer for the input feature with category X can thus
be retrieved by selecting all buffer areas with category X (see
example below).

<p>
Buffers for lines and areas are generated using the algorithms from
the GEOS library.

<p>
<i>For advanced users:</i> built-in buffer algorithm no longer
desired, we use GEOS: If GRASS is not compiled with GEOS support
or <a href="variables.html">environmental
variable</a> <tt>GRASS_VECTOR_BUFFER</tt> is defined, then GRASS
generates buffers using built-in buffering algorithm (which is still
buggy for some input data).

<p>
The options <b>minordistance</b>, <b>angle</b>, <b>tolerance</b> are 
kept for backward compatibility and have no effect with GEOS buffering.

<h3>Corner settings</h3>

The following vector line related corners (also called "cap") exist:
<ul>
<li>no cap: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <img src="v_buffer_no_cap.png" valign="middle" alt="line buffer: no cap"></li>
<li>rounded cap: <img src="v_buffer_rounded_cap.png" valign="middle" alt="line buffer: rounded cap"></li>
<li>square cap: <img src="v_buffer_square_cap.png" valign="middle" alt="line buffer: square cap"></li>
</ul>

By default <em>v.buffer</em> creates rounded buffers (blue color on
figure below): 

<center>
  <img src="v_buffer_line.png">
</center>

Straight corners with caps are created by <b>-s</b> flag (red color on
the figure below), while <b>-c</b> flag doesn't make caps at the ends of
polylines (green color on the figure below):

<center>
  <img src="v_buffer_line_s.png">
  <img src="v_buffer_line_c.png">
</center>

Using <b>-s</b> with a point vector map as input data, square buffers are 
created instead of round buffers.

<center>
  <img src="v_buffer_point_s.png">
</center>

<!-- Only support by GRASS buffer 
Flag <b>-s</b> also influences corners around polygons (see red color
on the figure below):

<center>
  <img src="v_buffer_area.png">
  <img src="v_buffer_area_s.png">
</center>
-->

<h2>EXAMPLES</h2>

All examples are based on the North Carolina sample dataset.

<h3>Buffer around input lines</h3>

<div class="code"><pre>
v.buffer input=roadsmajor output=roadsmajor_buffer type=line distance=100
</pre></div>

<center>
<img src="v_buffer_lines.png" border="1"><br>
Buffer of 100m along the "roadsmajor" lines (map subset, original center line
shown in black)
</center>

<h3>Circles around input points</h3>

<div class="code"><pre>
v.buffer input=hospitals output=hospitals_circled type=point distance=2000 
</pre></div>

<center>
<img src="v_buffer_points.png" border="1"><br>
Buffer of 2000m around the "hospitals" points (map subset, original points
shown in black, new area centroids in red)
</center>

<h3>Circles around input points with attribute transfer</h3>

<div class="code"><pre>
v.buffer input=hospitals output=hospitals_circled type=point distance=1000 -t

# display buffer around hospital with category 36,
# this buffer is composed of several areas:
d.vect map=hospitals_circled type=area layer=1 cats=36
# extract this buffer, dissolving boundaries
v.extract in=hospitals_circled output=hospital_36_circled layer=1 cats=36 -d
</pre></div>

<h3>Buffer around input areas</h3>

<div class="code"><pre>
v.buffer input=lakes output=lakes_buffer type=area distance=100
</pre></div>

<center>
<img src="v_buffer_areas.png" border="1"><br>
Buffer of 100m around the "lakes" polygons (map subset, original areas
shown in black)
</center>

<h3>Buffer inside input areas</h3>

In this example, an internal buffer ("inward buffer" or "negative buffer")
is generated using a negative <b>distance</b> value:

<div class="code"><pre>
v.buffer input=lakes output=lakes_buffer type=area distance=-50
</pre></div>

<center>
<img src="v_buffer_areas_int.png" border="1"><br>
Internal buffer of 50m inside the "lakes" polygons (map subset, original areas
shown in black) <br>
Not all features are buffered, only the polygons that allow creation inside
a topological cleaned polygon.
</center>


<h2>REFERENCES</h2>

<ul>
<li><a href="http://trac.osgeo.org/geos">GEOS Library</a></li>
</ul>

<h2>SEE ALSO</h2>

<em>
<a href="r.buffer.html">r.buffer</a>,
<a href="v.parallel.html">v.parallel</a>,
<a href="v.extract.html">v.extract</a>,
<a href="v.type.html">v.type</a>,
<a href="v.patch.html">v.patch</a>,
<a href="v.db.connect.html">v.db.connect</a>
</em>

<h2>AUTHORS</h2>

Radim Blazek<br>
Rewritten by Rosen Matev (with support through the
 Google Summer of Code program 2008)<br>
Rewritten by Markus Metz (2011, 2012)

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