SpatialPolygons to SpatialGrid

Thanks again Roger - I see about the slopes assignment.  I'm just learning 
how to work with these spatial objects.  That should make them very nice to 
work with actually, glad to realize this.

I don't suppose there is any way to go from SpatialPolygons to a SpatialGrid 
is there?  I'm having a problem using the shapes to  grid function in SAGA 
via RSAGA geoprocessor, although most functions are working very well for me 
otherwise.  There is a problem with the SAGA source code it seems. 
Essentially I need a relatively rapid conversion from a shapefile to a 
SpatialGridDataFrame (i.e. something not too computationally intensive) as I 
am having to do some expensive formatting to make things work via RSAGA.

Thanks again,

Murray


Roger Bivand wrote:

 On Fri, 15 Aug 2008, Murray Richardson wrote:

 Oh maybe I've misunderstood the function.

 I am using unionSpatialPolygons to dissolve boundaries between adjacent 
 polygons that have a similar attribute (I use cut to classify the ID 
 into say, 10 different categories), i.e.:

 x<-readShapePoly("up1polys", IDvar="ID")
 slopes<-slot(x[(1:length(slot(x, "polygons"))),3], "data")
 breaks<-c(0,5,10,20,30,40,50,60,70,100)
 ID <- cut(slopes[,1], breaks)
 sptmp <- unionSpatialPolygons(x, ID)
 newID<-data.frame(c(1:length(slot(sptmp, "polygons"))))
 merged<-SpatialPolygonsDataFrame(sptmp, data=newID, match.ID=FALSE)
 writePolyShape(merged, "up1merged", factor2char = TRUE, max_nchar=254)

 So I am just using unionSpatialPolygons as a dissolve tool but I would 
 like non-adjacent polys within the same slope class to be separate 
 polygons when I'm done.

 The ID argument groups all, both contiguous and so merged, and
 non-contiguous, Polygon objects in the same Polygons object. So if you
 don't want them merged, only set the same ID values for the slivers to be
 joined, merge the polygons, then go on and do the other things.

 Your assignment to slopes is very strange. What does names(x) say? If the
 third one is "slopes", why not just say x$slopes? The object does behave
 just like a data.frame, after all. I think that you need to do things
 strictly step by step, do the slivers first, then associate the correct
 attributes with the output polygons.

 Roger

 Thanks again

 Murray


 Roger Bivand wrote:

  On Fri, 15 Aug 2008, Murray Richardson wrote:

  Thanks for this Roger.

  One other thing now...if I use unionSpatialPolygons as a 

 dissolve tool, >  is there a way to then explode distinct polygons 
 back to individual >  polygons? i.e. once all the slivers are gone and 
 I have polygons merged >  based on attributes, the result is a 
 multipart polygon for each ID I >  used for the merge, but I need them 
 back as separate polys.

 Each unique ID value should give a separate Polygons object, so look 
 at
  the ID vector before going into unionSpatialPolygons() to make sure 
  it
  does what you want.

  Roger

  Thanks
  Murray

  Roger Bivand wrote:

   On Wed, 13 Aug 2008, Murray Richardson wrote:

   Hello again r.sig.geo list,

   Thanks Roger, for help on my previous question regarding > 

iterating >    through a shapefile.

   I'm sure once I receive my copy of  "Applied Spatial Data 

  Analysis with >  R" I will find answers to simple questions 

 like this > >  on my own, but in >  the meantime....

   Is it possible to merge sliver polygons that fall below a 

certain >   threshold area with adjacent neighbours (e.g. 

 perhaps using > >  >  unionSpatialPolygons but without aggregating any 
 polygons?).  If a > >  >  sliver shares edges with more than one 
 polygon, it doesn't really > >  matter >  which one it merges with, 
 but if I had to choose a rule I > >  would have it >  merge with the 
 largest one.

  Not such a simple question ...
  Both the Polygon and Polygons objects in the 

 SpatialPolygons object > >  have

   "area" slots, with different roles. The Polygon objects have a 
   correct
   naive area in the geometry of the coordinates taken as planar. 

 The

   Polygons objects use the "gross" area of Polygon objects 

 belonging to

   them, but "only" to provide the plot order (plot from largest 

to > >   smallest

   to avoid over-painting).

   If you "trust" the area slot of the Polygons objects 

 (beware of hole

  Polygon objects), you can first find your candidate slivers by 
  retrieving
   the areas by:

   Polygons_areas <- sapply(slot(SPobj, "polygons"),

     function(x) slot(x, "area"))

  and set a cutoff. Then use poly2nb(SPobj, queen=FALSE) in 

 spdep to > >  find

  the neighbours (rook criterion). Next use the output object to 
  identify
   the largest neighbours of the sliver candidates, and build a 
   "new
   Polygons" ID vector. Finally, use unionSpatialPolygons(). I'm 
  assuming you
   wouldn't have asked if there was useful data in the slivers!

   Hope this helps,
   Roger

   Thanks in advance,

   Murray Richardson