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

SOURCE CODE

       Available at: v.sample source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual