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## NAME

**v.net.distance** - Computes shortest distance via the network between the given sets of features.

Finds the shortest paths from a feature 'to' to every feature 'from' and various information about this realtion are uploaded to the attribute table.
## KEYWORDS

vector, network, shortest path
## SYNOPSIS

**v.net.distance**

**v.net.distance help**

**v.net.distance** [-**g**] **input**=*name* **output**=*name* [**from_layer**=*integer*] [**from_cats**=*range*] [**from_where**=*sql_query*] [**to_layer**=*integer*] [**to_cats**=*range*] [**to_where**=*sql_query*] [**to_type**=*string*[,*string*,...]] **afcolumn**=*name* [**abcolumn**=*name*] [--**overwrite**] [--**verbose**] [--**quiet**]
### Flags:

**-g**
- Use geodesic calculation for longitude-latitude locations
**--overwrite**
- Allow output files to overwrite existing files
**--verbose**
- Verbose module output
**--quiet**
- Quiet module output

### Parameters:

**input**=*name*
- Name of input vector map
**output**=*name*
- Name for output vector map
**from_layer**=*integer*
- From layer number or name
- A single vector map can be connected to multiple database tables. This number determines which table to use.
- Default:
*1*
**from_cats**=*range*
- From category values
- Example: 1,3,7-9,13
**from_where**=*sql_query*
- From WHERE conditions of SQL statement without 'where' keyword
- Example: income < 1000 and inhab >= 10000
**to_layer**=*integer*
- Layer number
- To layer number or name
- Default:
*1*
**to_cats**=*range*
- To category values
- Example: 1,3,7-9,13
**to_where**=*sql_query*
- To WHERE conditions of SQL statement without 'where' keyword
- Example: income < 1000 and inhab >= 10000
**to_type**=*string[,**string*,...]
- To feature type
- Options:
*point,line,boundary*
- Default:
*point*
**afcolumn**=*name*
- Arc forward/both direction(s) cost column
**abcolumn**=*name*
- Arc backward direction cost column

## DESCRIPTION

*v.net.distance* finds the nearest element in set *to*
for every point in set *from*.
## NOTES

These two sets are given by the respective **layer**, **where**
and **cats** parameters. The type of *to* features is
specified by **to_type** parameter. All *from* features
are *points*. A table is linked to **output** map
containing various information about the relation. More
specifically, the table has three columns: *cat*, *tcat*
and *dist* storing category of each *from*
feature, category of the nearest *to* feature and the
distance between them respectively. Furthemore, **output** map
contains the shorest path between each *cat*, *tcat*
pair. Each path consist of several lines. If a line is on the
shorest path from a point then the category of this point is
assigned to the line. Note that every line may contain more than one
category value since a single line may be on the shortest path for
more than one *from* feature. And so the shortest paths can
be easily obtained by querying lines with corresponding category
number.

The costs of arcs in forward and backward direction are specified by
**afcolumn** and **abcolumn** columns respectively. If
**abcolumn** is not given, the same cost is used in both directions.
*v.net.distance* will not work if you are trying to find the
nearest neighbors within a group of nodes, i.e. where *to*
and *from* are the same set of nodes, as the closest node
will be the node itself and the result will be zero-length paths. In
order to find nearest neighbors within a group of nodes, you can
either loop through each node as *to* and all other nodes as
*from* or create a complete distance matrix with
v.net.allpairs and select the
lowest non-zero distance for each node.

## EXAMPLES

Find shortest path and distance from every school to the nearest
hospital and show path from school number one.
v.net.distance input=city output=nearest from_where="type=school" \
to_where="type=hospital" afcolumn=SHAPE_LEN
d.vect nearest cats=1

## SEE ALSO

*
v.net.path,
v.net.allpairs,
v.net.distance,
v.net.alloc
*
## AUTHORS

Daniel Bundala, Google Summer of Code 2009, Student

Wolf Bergenheim, Mentor
*Last changed: $Date: 2013-05-23 13:01:55 -0700 (Thu, 23 May 2013) $*

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