## 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