xenial (1) v.what.rast.1grass.gz

NAME

       v.what.rast  - Uploads raster values at positions of vector points to the table.

KEYWORDS

       vector, sampling, raster, position, querying, attribute table

SYNOPSIS

       v.what.rast
       v.what.rast --help
       v.what.rast  [-ip]  map=name   [layer=string]    [type=string[,string,...]]   raster=name   [column=name]
       [where=sql_query]   [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -i
           Interpolate values from the nearest four cells

       -p
           Print categories and values instead of updating the database

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       map=name [required]
           Name of vector points map for which to edit attributes
           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

       type=string[,string,...]
           Input feature type
           Options: point, centroid
           Default: point

       raster=name [required]
           Name of existing raster map to be queried

       column=name
           Name of attribute column to be updated with the query result

       where=sql_query
           WHERE conditions of SQL statement without ’where’ keyword
           Example: income < 1000 and inhab >= 10000

DESCRIPTION

       v.what.rast  retrieves  raster value from a given raster map for each point or centroid stored in a given
       vector map. It can update a column in the linked vector attribute table with the  retrieved  raster  cell
       value or print it. The column type needs to be numeric (integer, float, double, ...).

       If the -p flag is used, then the attribute table is not updated and the results are printed to stdout.

       If  the  -i  flag  is  used,  then the value to be uploaded to the database is interpolated from the four
       nearest raster cells values using an inverse distance weighting method (IDW). This is  useful  for  cases
       when the vector point density is much higher than the raster cell size.

       Points  and  centroid with shared category number cannot be processed.  To solved this, unique categories
       may be added with v.category in a separate layer.

NOTES

       If multiple points have the same category, the attribute value is set to NULL.  If the  raster  value  is
       NULL, then attribute value is set to NULL.

       v.what.rast operates on the attribute table. To modify the vector geometry instead, use v.drape.

       Categories  and  values  are  output  unsorted  with the print flag. To sort them pipe the output of this
       module into the UNIX sort tool (sort -n). If you need coordinates, after sorting use v.out.ascii and  the
       UNIX paste tool (paste -d’|’). In the case of a NULL result, a "*" will be printed in lieu of the value.

       The  interpolation flag is only useful for continuous value raster maps, if a categorical raster is given
       as input the results will be nonsense.  Since the search window is limited to four raster cells there may
       still  be raster cell-edge artifacts visible in the results, this compromise has been made for processing
       speed. If one or more of the nearest four raster cells is NULL, then only  the  raster  cells  containing
       values will be used in the weighted average.

EXAMPLES

   Transferring raster values into existing attribute table of vector points map
       Reading  values  from  raster map at position of vector points, writing these values into a column of the
       attribute table connected to the vector map:

       # work on copy of original geodetic points map
       g.copy vector=geodetic_pts,mygeodetic_pts
       # set computational region to raster map to be queried
       g.region raster=elev_state_500m -p
       # add new column to existing table
       v.db.addcolumn map=mygeodetic_pts column="height double precision"
       v.what.rast map=mygeodetic_pts raster=elev_state_500m column=height
       # compare official geodetic heights to those of elevation model
       v.db.select map=mygeodetic_pts columns=Z_VALUE,height separator=comma

   Transferring raster values into new vector points map
       In case of a vector map without attached attribute table, first add a new attribute table. This table  is
       then populated with values queried from the raster map:

       # create new random vector points map
       v.random pnts n=100
       # add new table, link to map
       v.db.addtable map=pnts column="height double precision"
       # set computational region to raster map to be queried
       g.region raster=elevation -p
       # query raster map and upload values to vector table into specified column
       v.what.rast map=pnts raster=elevation column=height
       # verify new attribute table:
       v.db.select pnts
       # verify statistics of uploaded values:
       v.univar map=pnts column=height type=point

SEE ALSO

        v.category, v.db.addtable, v.db.select, v.drape, v.univar, v.rast.stats, v.what.vect

AUTHORS

       Radim Blazek
       Hamish Bowman (interpolation)

       Last changed: $Date: 2015-09-10 12:51:40 +0200 (Thu, 10 Sep 2015) $

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