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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