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
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