
NAME
v.sample - Samples a raster map at vector point locations.
KEYWORDS
vector, raster, resample
SYNOPSIS
v.sample
v.sample help
v.sample input=name layer=string column=name output=name raster=name [method=string] [z=float] [--overwrite] [--verbose] [--quiet]
Flags:
- --overwrite
- Allow output files to overwrite existing files
- --verbose
- Verbose module output
- --quiet
- Quiet module output
Parameters:
- input=name
- Name of input vector point map
- Data source for direct OGR access
- layer=string
- Layer number or name
- A single vector map can be connected to multiple database tables. This number determines which table to use. Layer name for direct OGR access.
- Default: 1
- column=name
- Name of attribute column to use for comparison
- output=name
- Name for output vector map to store differences
- raster=name
- Name of raster map to be sampled
- method=string
- Interpolation method
- Options: nearest,bilinear,bicubic
- Default: nearest
- z=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 "elevation.dem" and "elevation.10m" Spearfish maps
at random places:
#generate random points:
v.random output=random n=100
#add table with one column:
v.db.addtable random col="el10 double"
#transfer elevations at random points into table:
v.what.rast rast=elevation.10m vect=random col=el10
#verify:
v.db.select random
#generate sampling from other elevation map:
v.sample in=random col=el10 rast=elevation.dem out=elev_sample
#verify:
v.db.select elev_sample
#univariate statistics of differences between elevation maps:
v.univar elev_sample col=diff type=point
SEE ALSO
v.random,
g.region
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: 2008-08-14 23:16:42 -0700 (Thu, 14 Aug 2008) $
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