On Thu, 8 Jan 2009, Corey Sparks wrote:
Dear list,
I have a point pattern that consists of wells on an island. I am interested
doing some exploratory point pattern analysis on this data (such as Gest in
spatstat). The island as you can imagine is an irregular polygon defined by
1815 points. When I try to create the observation window for the point
pattern in spatstat using
owin(x=range(westcoords$X), y=range(westcoords$Y), poly=list(x=westcoords$X,
y=westcoords$Y))
where westcoords are the coordinated of the polygon
I get:
Error in owin(x = range(westcoords$X), y = range(westcoords$Y), poly = list(x
= westcoords$X, :
Area of polygon is negative - maybe traversed in wrong direction?
Since the origin of the coordinates is a shapefile I created myself, how can
I get around this "traversed in the wrong direction" problem? I can make the
data this is based on available if anyone would need it.
Could you try using readShapeSpatial() in maptools to read in the
shapefile, then coerce the polygon object to an owin object, something
like:
library(maptools)
pl_obj <- readShapeSpatial("my_file.shp")
class(pl_obj)
plot(pl_obj, axes=TRUE)
owin_obj <- as(as(pl_obj, "SpatialPolygons"), "owin")
plot(owin_obj)
The reason for doing this is that if a ring in a SpatialPolygons...
object, created by nested calls to Polygon, does not seem to be a hole -
that is that it is a singleton ring in this case, its ring direction will
be reversed automatically. The heuristics are only approximate, but may
get you where you want to go. If this doesn't help, see if you can make
the offending shapefile available for helpers to hack on.
Roger
I'm using R 2.8.1 on a Mac with Leopard OS
Thanks,
Corey
Corey Sparks
Assistant Professor
Department of Demography and Organization Studies
University of Texas at San Antonio
One UTSA Circle
San Antonio, TX 78249
210 458 6858
[email protected]
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--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: [email protected]
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