[R-sig-Geo] How to replace coordinates of Polygons

[Ben Mazzotta]

Thank you very much, Roger.

I was afraid that might be the answer. Once I have the new Polygon and 
Polygons objects, how can I merge all the slots from the old Polygon and 
Polygons to the new Polygon and Polygons objects? Ultimately I'd like to 
bring the other fields from the old object into the new one.


The context is this: I am transforming the coordinates using the 
cartogram() function from {Rcartogram}. The argument to cartogram() is a 
rectangular matrix, which contains sample densities of a statistic at 
regular intervals over the map. Cartogram() returns a lookup table in 
the form of two matrices, which specify x_new as a function of (x_old, 
y_old) and y_new, again as a function of (x_old, y_old). After cleaning 
up missing observations, I can interpolate new coordinates for every 
pair of old coordinates.

Therefore, I'd like to create a new version of the world map by 
transforming every pair of coordinates in the original polygon.






On 07/19/2010 12:09 PM, Roger Bivand wrote:
> On Mon, 19 Jul 2010, Ben Mazzotta wrote:
>
>> Dear R geographers:
>>
>> I'm able to replace the coordinates of any given polygon, but not 
>> construct apply() family commands or for() loops to accomplish the 
>> same task. How can I construct a simple loop or apply() command that 
>> will sequentially replace all of the coordinates in a SpatialPolygons 
>> object?
>
> Just build lists of new Polygon then Polygons objects, and build a new 
> SpatialPolygons at the end. I don't think that you'll see big memory 
> problems, and the approach you propose will probably have poor 
> timings, because the containing objects will probably be rebuilt for 
> each update.
>
> Are you looking for the elide() method for SpatialPolygons in 
> maptools? Do you need an extra operation added to it?
>
> Roger
>
>
>>
>> As far as I understand, the coordinates of a polygon are kept in a 
>> slot referred as follows:
>> map at polygons[[x]]@Polygons[[y]]@coords
>>
>> Using which, I can create mapply(foo, indexp, indexP) commands to 
>> extract the original coordinates and transform them.
>>
>> # Define a pair of vectors indexp, indexP that together index every 
>> Polyon in the SpatialPolygons object map.
>> # Then extract coordinates as follows:
>> coords <- function(x,y) 
>> {map at polygons[[indexp]]@Polygons[[indexP]]@coords}
>> # Or operate a function foo on the old coordinates
>> coords.alt <- mapply(foo, 
>> map at polygons[[indexp]]@Polygons[[indexP]]@coords[,1], 
>> map at polygons[[indexp]]@Polygons[[indexP]]@coords[,2])
>>
>> So I can create lists and matrices of coordinates, and transform the 
>> coordinates.
>>
>> It is possible to write a command that replaces coordinates using 
>> integers; but these commands fail when I attempt to use mapply() to 
>> replace existing coordinates. For example,
>>
>> p at polygons[[1]]@Polygons[[1]]@coords <- foo(1,1)
>>
>> will work fine, but the following will not.
>>
>> newcoords <- function(indp, indP) {
>>    p at polygons[[indp]]@Polygons[[indP]]@coords <- foo(indp, indP)
>>    }
>> mapply(newcoords, indexp, indexP)
>>
>>
>> Have I missed something in the manual on coordinates that would 
>> enable me to transform the coordinates of all the Polygons in a 
>> SpatialPolygonsDataFrame according to some regular function of the 
>> old coordinates (x,y)? Is the only alternative to build a new 
>> SpatialPolygons data frame from scratch, beginning with the new 
>> coordinates?
>>
>> If it would be useful, I can provide sample code that works on 
>> wrld_simpl.
>>
>> Please advise. Thank you!
>>
>>
>>
>

-- 
Ben Mazzotta, PhD Candidate
The Fletcher School, Tufts University
benjamin.mazzotta at tufts.edu