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) $

       Full index

       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                     v.sample(1grass)