[R] map issues

[Paul Hiemstra]

Dear Alexander,

You can use the function readShapePoly function from maptools. This 
directly reads the shapefile into a SpatialPolygons object. You also 
need to install the sp package (don't know if you did that already).

You could consider joining the r-sig-geo mailing list. These kinds of 
questions are better suited for that mailing list.

hope this helps,

Paul

Alexander Nervedi schreef:
> 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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-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone: 	+31302535773
Fax:	+31302531145
http://intamap.geo.uu.nl/~paul