v.hullgrass - Online in the Cloud

PROGRAM:

NAME

v.hull - Produces a 2D/3D convex hull for a given vector map.

KEYWORDS

vector, geometry, 3D

SYNOPSIS

v.hull
v.hull --help
v.hull [-rf] input=name [layer=string] output=name [cats=range] [where=sql_query]
[--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:
-r
Limit to current region

-f
Create a ’flat’ 2D hull even if the input is 3D points

--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:
input=name [required]
Name of input vector map
Or data source for direct OGR access

layer=string
Layer number or name (’-1’ for all layers)
A single vector map can be connected to multiple database tables. This number
determines which table to use. When used with direct OGR access this is the layer
name.
Default: -1

output=name [required]
Name for output vector map

cats=range
Category values
Example: 1,3,7-9,13

where=sql_query
WHERE conditions of SQL statement without ’where’ keyword
Example: income < 1000 and inhab >= 10000

DESCRIPTION

v.hull computes the convex hull of a vector map and outputs the convex hull polygon as a
vector area map. The convex hull, or convex envelope, for an object or a set of objects is
the minimal convex set containing the given objects. This module creates a vector polygon
containing all vector points or lines of the input map.

In the case of 3D input points, the hull will be a 3D hull as well, unless the user
specifies the -f flag. The 3D hull will be composed of triangular faces.
Fig: Convex hull polygon created with v.hull

EXAMPLE

Example of v.hull 3D output (using two random 3D point clouds, North Carolina sample data
set):
g.region rural_1m -p
r.mapcalc "zero = 0"
v.random -z output=random3d_a n=10 zmin=0 zmax=200
v.random -z output=random3d_b n=15 zmin=400 zmax=600
v.hull input=random3d_a output=random3d_a_hull
v.hull input=random3d_b output=random3d_b_hull
d.mon wx0
d.vect random3d_a_hull
d.vect random3d_a color=red
d.vect random3d_b_hull
d.vect random3d_b color=red
# 3D view in wxGUI (g.gui)
Fig: Convex hull in 3D from 3D points created with v.hull

REFERENCES

· M. de Berg, M. van Kreveld, M. Overmars, O. Schwarzkopf, (2000). Computational
geometry, chapter 1.1, 2-8.

· J. O’Rourke, (1998). Computational Geometry in C (Second Edition), chapter 4.

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