[R-sig-Geo] kernels, contours, points?

[Jeebz]

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			ashton at msu.edu
> 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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>