sample.Spatial question

This is not a full answer. To start of with, I'd suggest to use spsample
rather than sample.Spatial (maybe we should not make the latter
available to usage).

rand.points <- spsample(SPDF,n=23,type="hexagonal")
plot(rand.points, add=T,col='red')

The pattern you sketch below (nice ascii art!) might not fit in a square
area.

I'm afraid we overlooked this, and need to modify the documentation to
state that only for a limited number of sampling methods for square
regions the sample size is guaranteed the requested number. And exclude
the hexagonal...

--
Edzer


Chen, Shaofei wrote:

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