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

<h2>DESCRIPTION</h2>

<em>v.sample</em> samples a GRASS raster map at the point
locations in the input file by either cubic convolution
interpolation, bilinear interpolation, or nearest neighbor
sampling (default).

<p>This program may be especially useful when sampling for
cross validation of interpolations whose output is a raster
map.

<h2>NOTES</h2>

The output points will have the easting and northing of the input points.
The input category value is used. The input attribute, raster value 
and difference is written to output.

<p>When NULL values are encountered for a cell, zero value is used
instead.  In these cases, more acurrate results may be obtained
by using the default nearest neighbor comparisons.

<p>This program may not work properly with lat-long data when
the <b>-bc</b> flags are used.

<p>When interpolation is done (i.e., the <b>-bc</b> flags are
used), values are assumed to be located at the centroid of
grid cells.  Therefore, current resolution settings are
important.

<h2>EXAMPLE</h2>

Comparison of "elev_ned_30m" and "elev_srtm_30m" North Carolina
sample dataset elevation models at random positions:

<div class="code"><pre>
# set computational region:
 g.region raster=elev_srtm_30m -p
# generate random points:
 v.random output=random n=100
# add table with one column:
 v.db.addtable random col="elev_srtm30 double precision"
# transfer elevations at random points into table:
 v.what.rast map=random rast=elev_srtm_30m col=elev_srtm30
# verify:
 v.db.select random

# perform sampling on other elevation map:
 v.sample in=random col=elev_srtm30 rast=elev_ned_30m out=elev_samples

#verify:
 v.db.select elev_samples

#univariate statistics of differences between elevation maps:
 v.univar elev_samples column=diff type=point
</pre></div>


<h2>SEE ALSO</h2>

<em>
<a href="g.region.html">g.region</a>,
<a href="v.random.html">v.random</a>,
<a href="v.what.rast.html">v.what.rast</a>
</em>


<em>Image Sampling Methods</em> - GRASS Tutorial on <em>s.sample</em>
(available as 
<a href="http://grass.osgeo.org/gdp/sites/">s.sample-tutorial.ps.gz</a>) 


<h2>AUTHOR</h2>

<a href="http://mccauley-usa.com/">James Darrell McCauley</a>
<br>when he was at: 
<a href="http://ABE.www.ecn.purdue.edu/ABE/">Agricultural Engineering</a>
<a href="http://www.purdue.edu/">Purdue University</a>
<p>Updated for GRASS 5.0 by Eric G. Miller
<br>
Updated for GRASS 5.7 by Radim Blazek

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