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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
       # calculcate statistics. Use v.univar.sh for extended 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

       Last changed: $Date: 2017-02-19 22:47:27 +0100 (Sun, 19 Feb 2017) $

SOURCE CODE

       Available at: v.distance source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual