[R-sig-Geo] overlay fx, consistent topology, sample a point on a 'patch edge'

[Edzer Pebesma]

Ilona Naujokaitis-Lewis wrote:
> NO
>
> Dear list-serve,
> Thanks in advance to all those who help out with the inquiries, it is has
> helped me numerous times. Here go my questions...
>
> I am trying to vary existing landscapes which are composed of habitat
> (patches) and non-habitat. My goals are to vary the number of patches in a
> landscape, and the size of each patch. 
>
> My landscape files are originally raster files, which I have converted to
> ascii format. When importing into R, they are of class SpatialGridDataFrame,
> are fully gridded. Each cell is represented by a 0: non-habitat, or a value
> ranging between >0 and <=1, which represents varying degrees of habitat
> quality. Patches are simply identified where adjoining cells are
>   
>> 0.
>>     
>
> The approach I have taken is to create a mask of my ascii landscape file so
> that already existing patches are masked. This allows me to sample a point
> in the region of my landscape that is identified as non-habitat. I then
> apply the dilate function to essentially 'grow' a patch.
>
> I have followed the code from the following link:
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/115868.html
> The only difference is that once I have effected the dilate function, which
> is of class 'owin', I then:
> 1) coerce "owin" to "im" object class
> 2) coerce "im" object class to a SpatialGridDataframe
>
> My problems are:
> 1. How to overlay my final 2 layers:  1) dilated object (object class
> SpatialGridDataFrame) and 2) original landscape patch layer (also object
> class SpatialGridDataFrame). I need the resulting object to be an object of
> class SpatialGridDataFrame with only 1 band consisting of original patches,
> the new patches, and non-habitat so I can perform additional data
> manipulations. The overlay function does not appear to work with 2 objects
> of this class. Any alternative suggestions? 
>   
Method overlay only combines objects of different class. If you have two 
grids and want to create a third, and all three have the same 
topology/number of cells, then simply use vectorized expressions like

grd$out = grd$dilated != grd$original
> 2. My dilated points object (SGDF) does not have topology identical to the
> original layer - I think I need them to be the same for the overlay process,
> and if yes, how do I change this. The cell size remains the same but the
> offset and cell dimension are not identical.
> For example:
> 	> getGridTopology(dilate_SGDF)
>   	                  X1  X2
> 	cellcentre.offset 1050 890
> 	cellsize            20  20
> 	cells.dim           15  11
> 	> getGridTopology(original_layer)
>         	             x   y
> 	cellcentre.offset 1030 870
> 	cellsize            20  20
> 	cells.dim           16  12
>   
You could coerce the "wrong" one into a SpatialPointsDataFrame and the 
use function (method) overlay with the good grid to get the right indexes.
--
Edzer