v.mapcalc - GRASS GIS manual

NAME

v.mapcalc - Vector map calculator.

KEYWORDS

vector, algebra

SYNOPSIS

v.mapcalc

v.mapcalc --help

v.mapcalc expression=expression [--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:

--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:

expression=expression [required]: Expression to evaluate

DESCRIPTION
- PROGRAM USE
- OPERATORS AND FUNCTIONS
NOTES
EXAMPLES
REFERENCES
SEE ALSO
AUTHORS

DESCRIPTION

v.mapcalc performs overlay and buffer functions on vector map layers. New vector map layers can be created which are expressions of existing vector map layers, boolean vector operations and buffer functions.

PROGRAM USE

The module expects its input as expression in the following form:

result = expression

This structure is similar to r.mapcalc, see r.mapcalc. Where result is the name of a vector map layer that will contain the result of the calculation and expression is any valid combination of boolean and buffer operations for existing vector map layers.
The input is given by using the first module option expression= . This option passes a quoted expression on the command line, for example:

v.mapcalc expression="A = B"

Where A is the new vector map layer that will be equal to the existing vector map layer B in this case.

v.mapcalc "A = B"

will give the same result.

OPERATORS AND FUNCTIONS

The module supports the following boolean vector operations:

 Boolean Name   Operator Meaning         Precedence   Correspondent function
----------------------------------------------------------------------------------
 AND            &        Intersection          1      (v.overlay operator=and)
 OR             |        Union                 1      (v.overlay operator=or)
 DISJOINT OR    +        Disjoint union        1      (v.patch)
 XOR            ^        Symmetric difference  1      (v.overlay operator=xor)
 NOT            ~        Complement            1      (v.overlay operator=not)

And vector functions:

 buff_p(A, size)    	  Buffer the points of vector map layer A with size
 buff_l(A, size)    	  Buffer the lines of vector map layer A with size
 buff_a(A, size)    	  Buffer the areas of vector map layer A with size

NOTES

As shown in the operator table above, the boolean vector operators do not have different precedence. In default setting the expression will be left associatively evaluated. To define specific precedence use parentheses around these expressions, for example:

 v.mapcalc expression="D = A & B | C"

Here the first intermediate result is the intersection of vector map layers A & B. This intermediate vector map layer is taken to create the union with vector map C to get the final result D. It represents the default behaviour of left associativity.

 v.mapcalc expression="D = A & (B | C)"

Here the first intermediate result is taken from the parenthesized union of vector map layers B | C. Afterwards the intersection of the intermediate vector map layer and A will be evaluated to get the final result vector map layer D.

It should be noticed, that the order in which the operations are performed does matter. Different order of operations can lead to a different result.

EXAMPLES

This example needed specific region setting. It should work in UTM and LL test locations.
First set the regions extent and create two vector maps with one random points, respectively:

g.region s=0 n=30 w=0 e=50 b=0 t=50 res=10 res3=10 -p3

v.random --o -z output=point_1 n=1 seed=1
v.random --o -z output=point_2 n=1 seed=2
v.info point_1
v.info point_2

Then the vector algebra is used to create buffers around those points, cut out a subset and apply different boolean operation on the subsets in one statement:

v.mapcalc --o expr="buff_and = (buff_p(point_1, 30.0) ~ buff_p(point_1, 20.0)) & \
                    (buff_p(point_2, 35) ~ buff_p(point_2, 25))"
v.mapcalc --o expr="buff_or  = (buff_p(point_1, 30.0) ~ buff_p(point_1, 20.0)) | \
                    (buff_p(point_2, 35) ~ buff_p(point_2, 25))"
v.mapcalc --o expr="buff_xor = (buff_p(point_1, 30.0) ~ buff_p(point_1, 20.0)) ^ \
                    (buff_p(point_2, 35) ~ buff_p(point_2, 25))"
v.mapcalc --o expr="buff_not = (buff_p(point_1, 30.0) ~ buff_p(point_1, 20.0)) ~ \
                    (buff_p(point_2, 35) ~ buff_p(point_2, 25))"

REFERENCES

The use of this module requires the following software to be installed: PLY(Python-Lex-Yacc)

# Ubuntu/Debian
sudo apt-get install python-ply

# Fedora
sudo dnf install python-ply

AUTHORS

Thomas Leppelt, Soeren Gebbert, Thuenen Institut, Germany

SOURCE CODE

Available at: v.mapcalc source code (history)