NAME

       v.sample  - Samples a raster map at vector point locations.

KEYWORDS

       vector, raster, resample

SYNOPSIS

       v.sample
       v.sample help
       v.sample    [-bcq]    input=name    column=string   output=name   raster=name    [z=float]
       [--overwrite]  [--verbose]  [--quiet]

   Flags:
       -b
           Bilinear interpolation (default is nearest neighbor)

       -c
           Cubic convolution interpolation (default is nearest neighbor)

       -q
           Quiet

       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Vector map defining sample points

       column=string
           Vector map attribute column to use for comparison

       output=name
           Vector map to store differences

       raster=name
           Raster map to be sampled

       z=float
           Option scaling factor for  values  read  from  raster  map.  Sampled  values  will  be
           multiplied by this factor
           Default: 1.0

DESCRIPTION

       v.sample  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).

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

NOTES

       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.

       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.

       This program may not work properly with lat-long data when the -bc flags are used.

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

EXAMPLE

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

SEE ALSO

         g.region,  v.random,  v.what.rast  Image  Sampling  Methods - GRASS Tutorial on s.sample
       (available as s.sample-tutorial.ps.gz)

AUTHORS

       James Darrell McCauley
       when he was at: Agricultural Engineering Purdue University

       Updated for GRASS 5.0 by Eric G. Miller
       Updated for GRASS 5.7 by Radim Blazek

       Last changed: $Date: 2012-07-19 03:10:03 -0700 (Thu, 19 Jul 2012) $

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       © 2003-2013 GRASS Development Team