NAME

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

KEYWORDS

       vector, sampling, raster

SYNOPSIS

       v.sample
       v.sample --help
       v.sample  input=name  [layer=string]  column=name output=name raster=name  [method=string]
       [zscale=float]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name [required]
           Name of input vector point map
           Or data source for direct OGR access

       layer=string
           Layer number or name
           Vector features can have category values in different layers. This  number  determines
           which layer to use. When used with direct OGR access this is the layer name.
           Default: 1

       column=name [required]
           Name of attribute column to use for comparison

       output=name [required]
           Name for output vector map to store differences

       raster=name [required]
           Name of raster map to be sampled

       method=string
           Sampling interpolation method
           Options: nearest, bilinear, bicubic
           Default: nearest
           nearest: Nearest-neighbor interpolation
           bilinear: Bilinear interpolation
           bicubic: Bicubic interpolation

       zscale=float
           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  "elev_ned_30m" and "elev_srtm_30m" North Carolina sample dataset elevation
       models at random positions:
       # 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

SEE ALSO

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

AUTHOR

       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: 2014-12-19 22:55:37 +0100 (Fri, 19 Dec 2014) $

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       © 2003-2016 GRASS Development Team, GRASS GIS 7.0.3 Reference Manual