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.

NOTES

If a nearest feature does not have a category, the attribute column is updated to null. This is true also for areas, which means for example, that if a point is in an island (area WITHOUT category), v.distance does not search for the nearest area WITH category; the island is identified as the nearest and category updated to null.

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

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

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 a polygon 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 -p from=pnt to=ln out=connections upload=dist column=dist

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 from=pnt to=map_to_query upload=cat col=somecol -p

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.addcol 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 v=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 matrix

v.distance -pa from=archsites to=archsites upload=dist col=dist

Note: Matrix-like output is enabled only for flag -a and one given upload option.

AUTHOR

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
Martix-like output by Martin Landa, FBK-irst, Trento, Italy

Last changed: $Date$