[R-sig-Geo] sp, area and SpatialPolygons

[Patrick Giraudoux]

I did not find a function in sp to extract area from SpatialPolygons 
objects. Thus I was thinking about writing some function to do the job 
similar to the functions written in order to extract lengths from 
SpatialLines objects (see ?SpatialLines). I have incidentally discovered 
a strange thing. Let us define a list of 3 polygons:

   polylist <- list(structure(c(180016, 180034, 180452, 180588, 180615, 
180533,
     180225, 180016, 332182, 331756, 331774, 332074, 332418, 332518,
     332319, 332182), .Dim = c(8L, 2L)), structure(c(179907, 180325,
     180397, 180152, 179781, 179672, 179735, 179907, 331611, 331611,
     331266, 330931, 330967, 331266, 331466, 331611), .Dim = c(8L,
     2L)), structure(c(179499, 179971, 180343, 180161, 179753, 179418,
     179499, 330577, 330768, 330468, 330096, 330078, 330369, 330577
     ), .Dim = c(7L, 2L)))

And make a simple SpatialPolygons object with them:

 SP<-SpatialPolygons(list(Polygons(list(Polygon(polylist[[1]])),ID="P1"),Polygons(list(Polygon(polylist[[2]])),ID="P2"),Polygons(list(Polygon(polylist[[3]])),ID="P3")),pO=as.integer(c(1,2,3)))
  
It looks like if the area of Polygons #1 was not computed:

SP at polygons[[1]]@area
[1] 0

Whilst the area of the Polygons are:

 SP at polygons[[1]]@Polygons[[1]]@area
[1] 322111

However, no trouble with the next Polygons:
 > SP at polygons[[2]]@area
[1] 384740
 > SP at polygons[[2]]@Polygons[[1]]@area
[1] 384740
 > SP at polygons[[3]]@area
[1] 423937
 > SP at polygons[[3]]@Polygons[[1]]@area
[1] 423937

Did I do something wrong, miss something or is there a bug in the 
SpatialPolygons function ?

Patrick