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

[Ben Mazzotta]

Extremly helpful. Thanks again.

On 07/19/2010 01:30 PM, Roger Bivand wrote:
> On Mon, 19 Jul 2010, Ben Mazzotta wrote:
>
>> 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.
>
> If they have the same IDs, no problem, just make a 
> SpatialPolygonsDataFrame with data=as(old, "data.frame").
>
>>
>>
>> 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.
>
> Firstly, the input is a planar grid of values of a variable, so the 
> queries on the two output grids need to accommodate the loss of 
> resolution implied there. Try on something simple first. If you can 
> get a wrapper around cartogram() to take a SpatialPolygonsDataFrame, 
> make a grid from it for the chosen variable (including NA handling), 
> then do cartogram, followed by interpolation for Polygons object by 
> Polygons object (remember objects may be multiple Polygon objects, and 
> may have holes), you may be able to get there. Constructing the output 
> objects from computed coordinates looks like the least of your 
> challenges.
>
> Good luck!
>
> Roger
>
>
>
>>
>>
>>
>>
>>
>>
>> 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