v.distancegrass - Online in the Cloud

PROGRAM:

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 is written to stdout but may be uploaded to a new table created by this module;
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
Default: -1

dmin=float
Minimum distance or -1 for no limit
Default: -1

upload=string[,string,...] [required]
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 of table created when the ’distance to all’ flag is used

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.

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 -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.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 is in degree units
v.distance from=pnt1 to=pnt2 upload=dist col=distance -p --q
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 col=dist

North Carolina sample data location:
v.distance -pa from=hospitals to=hospitals upload=dist \
col=dist separator=comma
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 ...
...

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