SpatialPolygons decomposition...

Hi Peter,

I think I am three days ahead of you on the learning curve. So come join me! 
Below is a cumulation of suggestions from Rodger Bivand.

Perhaps the code below may help. Start out with your own SpatialPolygon. 
Change it to a dataframe. Then do something data like to the dataframe. Then 
coerce back to a SpatialPolygon dataframe. Then dump the SpatialPolygon in 
various ways. Or you could simply dump the SpatialPolygon. 
# the following code takes an sp to a df to add the two columns
# then the df is coerced back into an sp. This solves a merge
# issue when merging an sp and df together. R thinks the result
# should be a data.frame so good bye SpatialPolygons
  tx2_df <- as(tx2_sp, "data.frame")   #make a sp into a df

  tx2_df1      <- merge(tx2_df, votes2_df, sort=FALSE, by.x="PCT",
  by.y="PCT", all.x=TRUE, all.y=TRUE)
  remove(tx2_df)
  remove(votes2_df)
  # notice that the data frame row IDs we print are sequential
  # 1,2,3,4... and not proper precinct names like 1234....
  rownames(as(tx2_df1, "data.frame"))  #show us the row IDs

  # key here is match.ID = FALSE so that it does not try to take
  # the 1,2,3 data frame sequence numbers and think they are IDs.    # both 
sp and df must have rows aligned the same.
  tx3_sp <- SpatialPolygonsDataFrame(as(tx2_sp,"SpatialPolygons"),
            data=tx2_df1, match.ID = FALSE)
  remove(tx2_sp)
  remove(tx2_df1)

  # debug tx3_sp a little, lets make sure its a SpatialPologonsDataFrame!
  sapply(slot(tx3_sp, "polygons"), function(x) slot(x, "ID")) #what are row 
"ID"s?
  str(as(tx3_sp, "data.frame"))         #show representation of variables
  (str(tx3_sp))                         #shows representation of geometries 
too.
  names(tx3_sp)                         #nice but lengthy 
Let us know if this helps you any.

Good luck,
Jim Burke



Peter S. Hayes wrote:

Hi All,

I'm trying to teach a spatial analysis class using R as a means of learning 
some details... I'm not an expert in R myself, but am learning it while 
using it as a 'tool' for lessons in the class...

In getting the students familiar with some of the SP classes, we began 
looking at the class components and manipulating some of the components 
(such as translating by modifying coordinates..).

Is there a means of decomposing SpatialPolygons to access the list of 
Polygons and contained classes? The help files (for example, polygons()) 
appear to hint so, but not function so... the SpatialPolygons isn't quite a 
traditional R dataframe and won't flatten with a call to as.data.frame() to 
allow access to the individual slots... but there must be a means of doing 
that...

Also, I have "Applied Spatial Data Analysis with R" as one of the texts... 
any thoughts for more references... especially for non-programmers: I've 
some software experience (C/C++...) but most of the students are 
environmental study, environmental science, or biology students and this is 
one of their first experiences with anything having a command line. We've 
been taking things slow, but R is still a bit cryptic... any thoughts on 
that would be appreciated!

Thank you for all!


Pete