Jeffrey Evans
Quantitative Methods in Spatial Ecology
Flexible alpha convex hull
An implementation of the alpha hull model. This methods provides more flexibility over traditional convex functions. You can adjust the alpha parameter to relax or increase the tension between boundary points. This provides the ability to create and "exact" or "generalized" envelope.
PROGRAM
ConvexHull
USE
CREATE CONVEX HULL USING THE Pateiro-Lopez ALPHAHULL MODEL
REQUIRES
sp, alphahull, maptools
ARGUMENTS
x sp Points CLASS OBJECT
alpha CONVEX HULL ALPHA VALUE (SEE ?alphahull)
increment DEFAULT 360
rnd DEFAULT 2
VALUE
A sp SpatialPolygonsDataFrame OBJECT
NOTES
THIS METHOD PROVIDES FLEXIBILITY OVER TRADITIONAL CONVEX FUNCTIONS. YOU CAN
ADJUST THE ALPHA PARAMETER TO RELAX OR INCREASE TENSION BETWEEN EDGE-BOUNDARY
POINTS TO CREATE MORE OF AN EXACT FIT OF THE ENVELOPE (SEE EXAMPLE).
REFERENCES
Pateiro-López & Rodríguez-Casal (2009) Generalizing the Convex Hull of a Sample: The R Package alphahull. Journal of Statistical
Software 34(5):1-28 http://www.jstatsoft.org/v34/i05/paper
EXAMPLES
require(sp)
data(meuse)
coordinates(meuse) = ~x+y
a <- ConvexHull(meuse, alpha=100000)
plot(a)
points(meuse, pch=19)
TEST MULTIPLE alpha VALUES
a=c(1000, 100000)
par(mfcol=c(1,2))
for (alpha in a) {
ch <- ConvexHull(meuse, alpha=alpha)
plot(ch)
points(meuse, pch=19)
}