[R-sig-Geo] sample.Spatial question

[Chen, Shaofei]

Dear list members,

Thanks to Dr. Bivand's quick reply on my last question about creating a 
specific polygon. My next question follows that one.
After creating a polygon, I want to sample inside the polygon, I used 
sample.Spatial in sp library. I want to sample 23 points in a hexagonal 
structure.

library(sp)
crds <- cbind(x=c(0, 0, 400, 400, 0), y=c(0, 400, 400, 0, 0))
Pl <- Polygon(crds)
ID <- "400x400"
Pls <- Polygons(list(Pl), ID=ID)
SPls <- SpatialPolygons(list(Pls))
df <- data.frame(value=1, row.names=ID)
SPDF <- SpatialPolygonsDataFrame(SPls, df)
plot(SPDF)
rand.points <- sample.Spatial(SPDF,n=23,type="hexagonal")
plot(rand.points, add=T)


According to my imagination, the point pattern should look like this:

+   +  +  +  +
   +  +  +  +
+   +  +  +  +
   +  +  +  +
+   +  +  +  +

However, everytime, the results are not 23 points, they usually come up with 
16 points.
I changed to regular sampling structure with a specified offset c(0.5,0.5). 
It can give consistent results for 4x4=16 or 5x5=25... points. But for other 
number of points, for example, 20 points, it cannot give a 4x5 point 
pattern. Also offset option is not for other sampling type.

I just wonder why this happens. I look at the source code,  I think it may 
because that the it defines the first point randomly (of course, it uses 
"area = prod(apply(bb, 1, diff))/n" to define the stratum), and then 
construct the following samples.  In this way, some points may be out of the 
boundary.

Is it possible that I can get exactly number of points that I want for 
sampling?  And also I want the offset for hexagon is available, so that I 
can make sure that the sampling points locating in the centroid of the 
hexagon. I checked the archives of the list, Dr. Bivand has provided codes 
that not utilize sample.Spatial  for creating hexagons and centroids (I have 
done it with 23 points in 400 by 400 polygon), but it may not be easy in 
unregular polygon with defined number of points.

Thank you in advance!

Best,
Shaofei