[Rspatial-devel] sp change in handling gridded data

[Edzer Pebesma]

[r-spatial-devel included, but please continue any discussion about sp
on the r-forge list]

I'm proposing a change in the way sp handles gridded data. Right now, a
data set with coordinates

x y
1 1
1 2
1 3

cannot be coerced to SpatialPixels (gridded(x) = TRUE) because from the
x coordinate we cannot figure out the grid cell size.

My proposal is to change this, and when another coordinate (y) can give
the grid cell size, to take that one as grid cell size. This would then
generate the following warning:

Warning message:
In points2grid(points, tolerance, round) :
  cell size from constant coordinate 2 possibly taken from other coordinate

library(sp)
data(meuse.grid)
gridded(meuse.grid) = ~x+y
meuse.grid[2:3,]

used to generated an error, but would now pass with this warning.

As the major nuissance with this is when selecting few or single pixels,
I modified the behaviour of [ on SpatialPixels[DataFrame]s. The default
is drop=FALSE, which will keep the original @grid GridTopology, so that

meuse.grid[1,]

will result in a single-pixel grid, with grid topology from meuse.grid,
whereas

meuse.grid[1,drop=TRUE]

will result in an error (cannnot derive grid from single point), and

meuse.grid[2:3, drop=TRUE]

will result in the warning message above. SpatialPixels() and
SpatialPixelsDataFrame() now have a grid=NULL argument, that, when
specified, will take these grid topologies instead of those derived from
the points.

I committed this to r-forge, and hope you like it. Reactions more than
welcome. An appropriate message to r-sig-geo at (or before?) next
release of sp is useful, i suppose.
-- 
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