.TH v.sample 1grass "" "GRASS 7.8.5" "GRASS GIS User's Manual"
.SH NAME
\fI\fBv.sample\fR\fR  \- Samples a raster map at vector point locations.
.SH KEYWORDS
vector, sampling, raster
.SH SYNOPSIS
\fBv.sample\fR
.br
\fBv.sample \-\-help\fR
.br
\fBv.sample\fR \fBinput\fR=\fIname\fR  [\fBlayer\fR=\fIstring\fR]  \fBcolumn\fR=\fIname\fR \fBoutput\fR=\fIname\fR \fBraster\fR=\fIname\fR  [\fBmethod\fR=\fIstring\fR]   [\fBzscale\fR=\fIfloat\fR]   [\-\-\fBoverwrite\fR]  [\-\-\fBhelp\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR]  [\-\-\fBui\fR]
.SS Flags:
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-help\fR" 4m
.br
Print usage summary
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.IP "\fB\-\-ui\fR" 4m
.br
Force launching GUI dialog
.SS Parameters:
.IP "\fBinput\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of input vector point map
.br
Or data source for direct OGR access
.IP "\fBlayer\fR=\fIstring\fR" 4m
.br
Layer number or name
.br
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.
.br
Default: \fI1\fR
.IP "\fBcolumn\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of attribute column to use for comparison
.IP "\fBoutput\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name for output vector map to store differences
.IP "\fBraster\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of raster map to be sampled
.IP "\fBmethod\fR=\fIstring\fR" 4m
.br
Sampling interpolation method
.br
Options: \fInearest, bilinear, bicubic\fR
.br
Default: \fInearest\fR
.br
\fBnearest\fR: Nearest\-neighbor interpolation
.br
\fBbilinear\fR: Bilinear interpolation
.br
\fBbicubic\fR: Bicubic interpolation
.IP "\fBzscale\fR=\fIfloat\fR" 4m
.br
Scaling factor for values read from raster map
.br
Sampled values will be multiplied by this factor
.br
Default: \fI1.0\fR
.SH DESCRIPTION
\fIv.sample\fR 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).
.PP
This program may be especially useful when sampling for
cross validation of interpolations whose output is a raster
map.
.SH 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.
.PP
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.
.PP
This program may not work properly with lat\-long data when
the \fB\-bc\fR flags are used.
.PP
When interpolation is done (i.e., the \fB\-bc\fR flags are
used), values are assumed to be located at the centroid of
grid cells.  Therefore, current resolution settings are
important.
.SH EXAMPLE
Comparison of \(dqelev_ned_30m\(dq and \(dqelev_srtm_30m\(dq North Carolina
sample dataset elevation models at random positions:
.br
.nf
\fC
# 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=\(dqelev_srtm30 double precision\(dq
# 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
\fR
.fi
.SH SEE ALSO
\fI
g.region,
v.random,
v.what.rast
\fR
\fIImage Sampling Methods\fR \- GRASS Tutorial on \fIs.sample\fR
(available as
s.sample\-tutorial.ps.gz)
.SH AUTHOR
James Darrell McCauley
.br
when he was at:
Agricultural Engineering
Purdue University
.PP
Updated for GRASS 5.0 by Eric G. Miller
.br
Updated for GRASS 5.7 by Radim Blazek
.SH SOURCE CODE
.PP
Available at: v.sample source code (history)
.PP
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.PP
© 2003\-2020
GRASS Development Team,
GRASS GIS 7.8.5 Reference Manual