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 "elevation.dem" and "elevation.10m" Spearfish maps
at random places:

<div class="code"><pre>
#generate random points:
 v.random output=random n=100
#add table with one column:
 v.db.addtable random col="el10 double"
#transfer elevations at random points into table:
 v.what.rast rast=elevation.10m vect=random col=el10
#verify:
 v.db.select random

#generate sampling from other elevation map:
 v.sample in=random col=el10 rast=elevation.dem out=elev_sample

#verify:
 v.db.select elev_sample

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


<H2>SEE ALSO</H2>

<EM>
<A HREF="v.random.html">v.random</A>,
<A HREF="g.region.html">g.region</A>
</EM>


<EM>Image Sampling Methods</EM> - GRASS Tutorial on <EM>s.sample</EM>
(available as 
<A HREF="http://grass.itc.it/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>