map2SpatialPolygons' 'proj4string' in spmaps
On Wed, 15 Oct 2008, Sebastian P. Luque wrote:
Hi, I don't understand what the 'proj4string' argument in map2SpatialPolygons() does. map() in the maps package has a 'projection' argument: ,-----[ *help[R](map)* ] | projection: character string that names a map projection to use. See | 'mapproject' (in the 'mapproj' library). The default is to | use a rectangular projection with the aspect ratio chosen so | that longitude and latitude scales are equivalent at the | center of the picture. `----- so I don't know what projection was actually used in a given map() call with that default 'projection' argument. I suppose 'proj4string' in map2SpatialPolygons() should match the actual projection used in the map() call, shouldn't it? Thanks.
No, not at all. The coordinates returned by map() if projection= is used are only for plotting, are in arbitrary units, and are only documented in code. map2SpatialPolygons() assumes that the data are in geographical coordinates, and ought perhaps to guess an ellipse and datum based on the age of the dataset (world possibly WGS84, US counties probably NAD27), but allows the user to set the CRS object directly. The projection= argument of map() makes the function call mapproject() with the given projection on the data flowing through, I believe. If you do str() of map(projection=) output, you will see the projection name used, but note that the coordinates are arbitrary. Roger
Cheers,
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