[R-sig-Geo] SpatialLines2PolySet with Albers equal area projection?

[Roger Bivand]

On Sun, 13 Mar 2011, Sharon Baruch-Mordo wrote:

> Hello list,
> ??
> I???m trying to create a SpatialPolygonsDataFrame from an object of class list
> (x), which has coordinates for contour lines in Albers equal area (full
> parameters below). I???m executing the following code:
> ??
> CRSdata <- "+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=37.5 +lon_0=-96 +x_0=0
> +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs +towgs84=0,0,0"
> sldf <- ContourLines2SLDF(x)
> proj4string(sldf) <- CRS(CRSdata)
> ps <- SpatialLines2PolySet(sldf)
> ??
> #so far so good; continue:
> ??
> sp <- PolySet2SpatialPolygons(ps)
> ??
> # I get the following error:
> ??
> Error in PolySet2SpatialPolygons(ps) : unknown coordinate reference system

As was explained in thread starting at:

https://stat.ethz.ch/pipermail/r-sig-geo/2011-March/011152.html

"PBSmapping "knows" unprojected (geographical coordinates) and UTM 
(projected coordinates) with a zone number. It does not provide facilities 
to store any other values."

So unless you project to utm or longlat first and back again afterwards, 
you are stuck. ?PolySet does state the limitation.

Roger

> ??
> #what I was hoping to continue executing (which works great with UTM data sets
> where I defined attr(ps,???projection???) <- ???UTM??? and attr(ps,???zone???) <- ???17N???)
> ??is:
> ??
> dataframe <- as.data.frame(matrix(as.character(1,nrow=1,ncol=1)))
> spdf <- SpatialPolygonsDataFrame(sp,dataframe,match.ID=TRUE)
> ??
> Because Albers isn???t ???UTM??? nor ???LL??? I???m not sure what projection to specify for
> the PolySet object (i.e., attr(ps,???projection???)) in order to properly convert
> PolySet2SpatialPolygons. Any thoughts or suggestions??? I would also welcome any
> comments if I???m using an unnecessary long code to convert the list of contour
> line x/y points into a spatial polygon with the goal of calculating its area in
> R and exporting it into an ESRI shapefile.
> ??
> Thanks!
> Sharon
> ??
> Sharon Baruch-Mordo
> Ph.D. Candidate in Ecology
> Dept. of Fish, Wildlife, and Conservation??Biology
> Colorado State University
> Fort Collins, CO 80523-1474
> Phone: 970-491-7020??
> Fax: 970-491-5091
> E-mail: sharonbm at warnercnr.colostate.edu
> http://www.warnercnr.colostate.edu/research/bear
>
>
>
>
> 	[[alternative HTML version deleted]]
>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no