[R-sig-Geo] Converting Spatial Polygons to data frame for krige.conv

Mon May 30 07:52:49 CEST 2011

Have you tried

> coordinates(obj)

?

It retrieves the polygon centroids as a matrix.

On 05/30/2011 07:13 AM, Ben Madin wrote:
> G'day,
> 
> I'm re-running some old models with new data, but in the same location as previously. In the past I had a text file with the border coordinates, but this is no longer anywhere to be found - thus I'm coming unstuck with krige.conv() and the borders option.
> 
> I have otherwise moved very happily into using sp classes for everything, so can load a set of boundaries from PostGIS in using rgdal but although I find discussion about this (see below) for SpatialPoints, I am stumped for a method to convert Spatial Polygons to a list of coordinates - I guess I could manually split the object into it's component polygons and then iterate through the coords ... but I was hoping that there might be a method that I've overlooked.
> 
> I confess I have also forgotten how I originally created the text file I had - there must have been a method, but I'm not sure I can remember where it was. (> 5years ago). I thought it was maptools, but all the maptools functions seem to read into sp classes...
> 
> cheers
> 
> Ben
> 
> 
> 
> 
> 
>> 	Subject: 	Re: [R-sig-Geo] Spatial data tower of babel
>> 	Date: 	25 August 2010 8:53:33 PM AWST
>>
> On 25/08/2010, at 8:53 PM, Edzer Pebesma wrote:
> 
>> Barry, what exactly did you try out before you posted?
>>
>> Your claim is not completely true: geoR has a function
>> as.geodata.SpatialPointsDataFrame, so you can do, for instance:
>>
>> library(geoR)
>> data(meuse) # from sp
>> coordinates(meuse) = ~x+y
>> krige.bayes(as.geodata(meuse, "zinc"))
>>
>> and its locations argument can be a SpatialPoints object.
>>
>> Best regards,
>>
>> On 08/18/2010 09:05 PM, Roger Bivand wrote:
>>> On Wed, 18 Aug 2010, Barry Rowlingson wrote:
>>>
>>>> Hi,
>>>>
>>>> Recently while teaching at SFU I hit the problem that infects R when
>>>> many people work on similar projects - the multitude of data formats
>>>> for similar data. The sp project was partly an attempt to give a
>>>> standard format for spatial data but its widespread non-use in older
>>>> packages causes trouble.
>>>>
>>>> So for example I taught the students all about 'sp' objects, and then
>>>> they had to use spatstat and splancs for some point-process stuff,
>>>> then geoR for some kriging, none of which use sp objects.
>>>>
>>>> So I figured maybe we need a whole load of 'as' functions that can
>>>> convert between the various spatial data formats (there are more in
>>>> CRAN, I am sure) to help us all out on this. Some of these functions
>>>> may already exist, indeed I just found something about converting
>>>> fairly raw x-y coordinate objects to SpatialPolygons hidden away in
>>>> the SpatialEpi package (polygons2spatial.polygons).
>>>
>>> Barry,
>>>
>>> I think that spatstat is well provided for, mostly in maptools, but also
>>> in the spatstat vignette on using shapefiles. Of course, the available
>>> functionalities sp <-> spatstat classes could probably be documented
>>> more fully, and the coercion functions updated, but I think that they do
>>> most of what is needed.
>>>
>>> I agree that there may be others out there, and like you come across
>>> them from time to time. Sometimes the CRAN reverse dependencies show who
>>> they might be. Since splancs is largely pre-S3 (right?) and doesn't use
>>> classes, coercion isn't an option, so documentation and a wrapper
>>> function or two might be sensible. I started on this, but only got as
>>> far as the spkernel2d() that uses a call to GridTopology() to set up the
>>> output grid.
>>>
>>> geoR does use S3 classes, so might be closer, and does depend on sp.
>>> There is a method for coercing a SpatialPointsDataFrame to "geodata".
>>> The borders component of a "geodata" object is harder to introduce.
>>> Taking the coordinates() of a SpatialPixels object to pass to locations=
>>> is OK, as are the subsetting of data frame columns for the trend.d= and
>>> trend.l= arguments. I guess Paulo would need to move to a formula= data=
>>> interface to likfit(), krige.bayes() and krige.conv(), at least, to
>>> permit sp objects to be used "closer" to the actual core.
>>>
>>> Probably a good deal could be done by documentation, and by
>>> communicating better about what already is there.
>>>
>>> Useful topic!
>>>
>>> Best wishes,
>>>
>>> Roger
>>>
>>>>
>>>> Would it be a good idea to stick all the conversions we can think of
>>>> into a single package, "spBabel" say (or spConversion to avoid any
>>>> cultural reference), so people have a one-stop shop? And if we find
>>>> routines stuck in other packages (such as polygons2spatial.polygons)
>>>> we rip them out and bundle them?
>>>>
>>>> Yes, its a matter of time and effort and we're all busy, but I'd like
>>>> to put it out as a proposal. It might make a nice intern or GSOC
>>>> project, but we're a bit late for that, so maybe if anyone has a PhD
>>>> student starting who needs to get up to speed with R packages and
>>>> spatial data it would be a good introduction for them. Once its all
>>>> set up (on R-forge or similar) contributing shouldn't be a problem.
>>>>
>>>> Okay, that's my one crazy idea for the day done.
>>>>
>>>> Barry
>>>>
>>>>
>>>>
>>>
>>
>> -- 
>> Edzer Pebesma
>> Institute for Geoinformatics (ifgi), University of Münster
>> Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
>> 8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
>> http://www.52north.org/geostatistics      e.pebesma at wwu.de
>>
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> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de