v.sample - GRASS GIS manual

Note: This document is for an older version of GRASS GIS that is outdated. You should upgrade, and read the current manual page.

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
NOTES
EXAMPLE
SEE ALSO
AUTHORS

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