That's exactly the issue I think: anyone know how to turn a contour
into a polygon??
On Jul 17, 2008, at 6:48 AM, Ashton Shortridge wrote:
I think creative use of the overlay() function would accomplish what
you want.
Some example code that is a bit like what you propose (uses the sp
library):
# I have a polydataframe object:
my.poly.data <- SpatialPolygonsDataFrame(my.poly, area.lu.table)
# I have a set of point objects (derived, in this case, from
spsample):
samgrid <- spsample(us.spPolys, 500, 'regular')
# I'd like to know which poly in my.poly.data each point in samgrid
falls in:
sample.area <- overlay(my.poly.data, samgrid)$name
For this to work, you would need to figure out how to turn contours
into
polys, which might be tricky, especially if they aren't closed and/
or have
holes (pits) within them.
I admit that I am not very comfortable working with the sp objects,
and I
would generally advise someone doing GIS-type work to use a GIS.
Hawth's
(not "Hawk's") Analysis Tools are very useful in an ESRI
environment, as are
built in commands and extensions in many GIS.
yours,
Ashton
--
Ashton Shortridge
Associate Professor [EMAIL PROTECTED]
Dept of Geography http://www.msu.edu/~ashton
235 Geography Building ph (517) 432-3561
Michigan State University fx (517) 432-1671
On Wednesday 16 July 2008, Jeebz wrote:
Hi
I'm new to the R spatial packages, but I have a spatial-oriented task
I would like to accomplish.
I have a 2d grid of points and each point has a class (13 classes
total in the dataset). The grid is small-ish, only 256x256. I have
run a 2d kernel density estimation (using kde2d) and can create a
contour plot easily (per class).
What I would like to do is use a contour line to extract all the
points from the original grid that are bounded by that contour.
Ultimately, I wish to calculate a sort of "purity" measure, so
given a
particular contour (e.g. a contour level that encloses 95% of the
classes points), how many points from other classes fall in with the
classes that were used to define the contour.
I know there has to be a way to do this with R... right? A colleague
of mine showed my how it could be done using ArcGIS and "Hawkes
tools", by calculating percent volume contours, but I don't know
where
to begin in R.
Anyone have any thoughts or examples on this?
Much thanks in advance,
bct
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