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NAME

       v.distance  - Finds the nearest element in vector map ’to’ for elements in vector map ’from’.

KEYWORDS

       vector, distance, database, attribute table

SYNOPSIS

       v.distance
       v.distance --help
       v.distance    [-pa]    from=name     [from_layer=string]      [from_type=string[,string,...]]     to=name
       [to_layer=string]     [to_type=string[,string,...]]     [output=name]      [dmax=float]      [dmin=float]
       [upload=string[,string,...]]         [column=name[,name,...]]        [to_column=name]        [table=name]
       [separator=character]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -p
           Print output to stdout, don’t update attribute table
           First column is always category of ’from’ feature called from_cat

       -a
           Calculate distances to all features within the threshold
           Output may be written to stdout using the ’-p’ flag or uploaded to a new table created by the ’table’
           option; multiple ’upload’ options may be used.

       --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:
       from=name [required]
           Name of existing vector map (from)
           Or data source for direct OGR access

       from_layer=string
           Layer number or name (from)
           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

       from_type=string[,string,...]
           Feature type (from)
           Input feature type
           Options: point, line, boundary, centroid, area
           Default: point,line,area

       to=name [required]
           Name of existing vector map (to)
           Or data source for direct OGR access

       to_layer=string
           Layer number or name (to)
           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

       to_type=string[,string,...]
           Feature type (to)
           Input feature type
           Options: point, line, boundary, centroid, area
           Default: point,line,area

       output=name
           Name for output vector map containing lines connecting nearest elements

       dmax=float
           Maximum distance or -1 for no limit
           Map units, meters for ll
           Default: -1

       dmin=float
           Minimum distance or -1 for no limit
           Map units, meters for ll
           Default: -1

       upload=string[,string,...]
           Values describing the relation between two nearest features
           Options: cat, dist, to_x, to_y, to_along, to_angle, to_attr
           cat: category of the nearest feature
           dist: minimum distance to nearest feature
           to_x: x coordinate of the nearest point on the ’to’ feature
           to_y: y coordinate of the nearest point on the ’to’ feature
           to_along: distance to the nearest point on the ’to’ feature along that linear feature
           to_angle:  angle  along the nearest linear feature in the ’to’ map, measured CCW from the +x axis, in
           radians, between -Pi and Pi inclusive
           to_attr: attribute of nearest feature given by to_column option

       column=name[,name,...]
           Column name(s) where values specified by ’upload’ option will be uploaded

       to_column=name
           Column name of nearest feature (used with upload=to_attr)

       table=name
           Name for new attribute table

       separator=character
           Field separator for printing output to stdout
           Special characters: pipe, comma, space, tab, newline
           Default: pipe

DESCRIPTION

       v.distance finds the nearest element in vector map (to)  for  elements  in  vector  map  (from).  Various
       information  about the vectors’ relationships (distance, category, etc.) may be uploaded to the attribute
       table attached to the first vector map, or printed to ’stdout’. A new vector map  may  be  created  where
       lines connecting nearest points on features are written. dmin and/or dmax can be used to limit the search
       radius (in lat-long locations to be given in meters since they are calculated as geodesic distances on  a
       sphere).

       For lines to lines, say line A to line B, v.distance calculates the shortest distance of each vertex in A
       with each segment (not vertex) in B. The module then calculates the shortest distance of each vertex in B
       to  each segment in A. The overall shortest distance of A points to B segments and B points to A segments
       is used. Additionally,  v.distance  checks  for  intersections.  In  case  of  intersections,  the  first
       intersection found is used and the distance set to zero.

       For  lines to areas, the distance is set to zero if a line is (partially) inside an area. The first point
       of the line that is inside the area is used as common point. The distance is also set to zero if the line
       intersects  with the outer ring or any of the inner rings (isles), in which case the fist intersection is
       used as common point.

       For areas to areas, the module checks first for overlap or if one area is (partially)  inside  the  other
       area.  This  is  computationally quite intensive. If the outer rings of the two areas do not overlap, the
       distance is calculated as above for lines to lines, treating the outer rings as  two  lines.  Again,  the
       first point encountered falling into an area is used as common point, or the first intersection point.

       For anything else than points to lines, there can be several common locations with zero distance, and the
       common location would then be the result of an overlay consisting of several  points,  lines,  or  areas.
       v.distance  selects in these cases a single point, and does not create an overlay like v.overlay. In this
       implementation, any shared point is as good as any other. Calculating an intersection is costlier than to
       check  if  a  vertex  is  inside  a polygon. For example, if a vertex of the boundary of the ’to’ area is
       inside the ’from’ area, it is a common location. For speed reasons, the distance is then set to zero  and
       no further tests are done.

NOTES

       If a nearest feature does not have a category, the attribute column is updated to NULL.

       The upload column(s) must already exist. Create one with v.db.addcolumn.

       In  lat-long  locations v.distance gives distances (dist, from_along, and to_along) not in degrees but in
       meters calculated as geodesic distances on a sphere.

       If one or both of the input vector maps are 3D, the user is notified accordingly.

EXAMPLES

   Find nearest lines
       Find nearest lines in vector map "ln" for points from vector map "pnt" within  the  given  threshold  and
       write related line categories to column "linecat" in an attribute table attached to vector map "pnt":
       v.distance from=pnt to=ln upload=cat column=linecat

   Find nearest area
       For  each point from vector map "pnt", find the nearest area from map "ar" within the given threshold and
       write the related area categories to column "areacat" in an attribute table attached to vector map  "pnt"
       (in the case that a point falls into an area, the distance is zero):
       v.distance from=pnt to=ar upload=cat column=areacat

   Create a new vector map
       Create  a new vector map which contains lines connecting nearest features of maps "pnt" and map "ln". The
       resulting vector map can be used for example to connect  points  to  a  network  as  needed  for  network
       analysis:
       v.distance from=pnt to=ln out=connections upload=dist column=dist

   Create a new vector map with from and to categories in the attribute table
       Create a new vector map that contains lines connecting nearest features of maps "pnt" and map "ln", and a
       new attribute table that contains distances, from and to categories from the input maps:
       v.distance from=pnt to=ln out=connections upload=cat,dist column=to_cat,dist table=connections

   Query information
       Query information from selected point(s). v.distance takes points from a vector map as input  instead  of
       stdin. A new vector map with query points has to be created before the map can be analysed.

       Create query map (if not present):
       echo "123456|654321|1" | v.in.ascii output=pnt
       Find nearest features:
       v.distance -p from=pnt to=map_to_query upload=cat

   Point-in-polygon
       The  option dmax=0 is here important because otherwise for points not falling into any area, the category
       of the nearest area is recorded.
       For each point from vector map "pnt", find the area from vector map "ar" in which  the  individual  point
       falls,  and  write  the  related area categories to column "areacat" into the attribute table attached to
       vector map "pnt":
       v.distance from=pnt to=ar dmax=0 upload=cat column=areacat

   Univariate statistics on results
       Create a vector map containing connecting lines and investigate mean distance to targets. An  alternative
       solution  is  to use the v.distance upload=dist option to upload distances into the bugs vector directly,
       then run  v.univar  on  that.  Also  note  you  can  upload  two  columns  at  a  time,  e.g.  v.distance
       upload=cat,dist column=nearest_id,dist_to_nr.
       # create working copy
       g.copy vect=bugsites,bugs
       # add new attribute column to hold nearest archsite category number
       v.db.addcolumn map=bugs column="nrst_arch INTEGER"
       v.distance from=bugs to=archsites to_type=point upload=to_attr \
         to_column=cat column=nrst_arch out=vdistance_vectors_raw
       # we need to give the lines category numbers, create a table, and create
       #  a column in that table to hold the distance data.
       v.category vdistance_vectors_raw out=vdistance_vectors type=line op=add
       g.remove -f type=vector name=vdistance_vectors_raw
       v.db.addtable map=vdistance_vectors column="length DOUBLE"
       v.to.db map=vdistance_vectors option=length column=length
       # calculate statistics
       v.univar vdistance_vectors column=length

   Print distance between points
       Example for a Latitude-longitude location (EPSG 4326):
       # points along the equator
       echo "0|-61|1" | v.in.ascii output=pnt1 input=-
       echo "0|-58|1" | v.in.ascii output=pnt2 input=-
       # here, distances are in degree units
       v.distance -p --q from=pnt1 to=pnt2 upload=dist
       from_cat|distance
       1|3

   Print distance matrix
       Note: Matrix-style output is enabled only for flag -a and one given upload option.

       Spearfish sample data location:
       v.distance -pa from=archsites to=archsites upload=dist

       North Carolina sample data location:
       v.distance -pa from=hospitals to=hospitals upload=dist separator=tab
       from_cat to_cat       dist
                     1          2          3          4          5 ...
       1             0    7489.10  339112.17   70900.39   70406.23 ...
       2       7489.10          0  345749.12   76025.46   75538.87 ...
       3     339112.17  345749.12          0  274153.19  274558.98 ...
       4      70900.39   76025.46  274153.19          0     501.11 ...
       5      70406.23   75538.87  274558.98     501.11          0 ...
       ...

SEE ALSO

        r.distance, v.db.addcolumn, v.what.vect

AUTHORS

       Janne Soimasuo 1994, University of Joensuu, Faculty of Forestry, Finland
       Cmd line coordinates support: Markus Neteler, ITC-irst, Trento, Italy
       Updated for 5.1: Radim Blazek, ITC-irst, Trento, Italy
       Matrix-like output by Martin Landa, FBK-irst, Trento, Italy
       Improved processing speed: Markus Metz
       Distance from any feature to any feature: Markus Metz
       New table without the -p flag: Huidae Cho

SOURCE CODE

       Available at: v.distance source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual