Hi !
I have a shapefile that I can easily read into R using library(maptools). My
problem stems from some warning messages that come even though everything seems
to work fine.
library(maptools)
districts <- read.shape(filen = "a_ds", dbf.data = TRUE)
length(districts$Shapes)
so far so good. when I try and plot this I get a nice plot and a warning
message:
> plot(districts)
Warning message:
'plot.Map' is deprecated.
Use 'plot.Spatial' instead.
See help("Deprecated") and help("maptools-deprecated").
so i tried the obvious:
> ?plot.Spatial
No documentation for 'plot.Spatial' in specified packages and libraries:
you could try 'help.search("plot.Spatial")'
I went back and read the help on plot.Map and it says "This function is
deprecated. It is difficult to maintain and there are several
alternatives, either by converting Map objects to sp class objects or polylist
etc. objects" and so i gather we can either try to move things into sp class. A
little search of this archive tells me that there is a function out there
called map2SpatialPolygons that is of some help. So after reading the help on
this i tried to convert the Map object to an sp object. But I keep failing.
> ids <- as.character(districts$att.data$STATE)
> ids[1:2]
"A AND B" "A AND B"
> temp <- map2SpatialPolygons(districts, IDs = ids)
Error in to[nParts] <- nrow(xy) : incompatible types (from NULL to integer) in
subassignment type fix
In addition: Warning message:
is.na() applied to non-(list or vector) in: is.na(object)
I went back to the help on the function map2SpatialPolygons. And the example
code there is:
library(maps)
nor_coast_poly <- map("world", "norway", fill=TRUE, col="transparent",
plot=FALSE)
nor_coast_poly$names
IDs <- sapply(strsplit(nor_coast_poly$names, ":"), function(x) x[1])
nor_coast_poly_sp <- map2SpatialPolygons(nor_coast_poly, IDs=IDs,
proj4string=CRS("+proj=longlat +datum=wgs84"))
This works fine. So I was most puzzled. And then i did
> is(nor_coast_poly)
[1] "map"
> is(districts)
[1] "Map"
So the object i have created is a Map object while the map2SpatialPolygons
seems to be using a map object. Can it also handle Map objects? In which case
can someone tell me whats wrong with my sepcification of the
map2SpatialPolygons call?
your man
Al
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