v.sample(1grass) GRASS GIS User's Manual v.sample(1grass)
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 ac-
cess 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 valida-
tion 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 in-
stead. 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, cur-
rent 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 Tuto-
rial 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)
Accessed: unknown
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GRASS 7.8.7 v.sample(1grass)
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