map2SpatialPolygons' 'proj4string' in spmaps

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.

Cheers,

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.

On Wed, 15 Oct 2008 19:24:18 +0200 (CEST),
Roger Bivand <Roger.Bivand at nhh.no> wrote:

[...]

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.

Thanks a lot Roger, so here:

se.baffin <- map("worldHires", xlim=c(-70, -60), ylim=c(62, 69),
                resolution=0, interior=FALSE)

or in any of the world* databases, can we safely assume "+proj=longlat
+ellps=WGS84" (possibly with "+over" in some cases) in the returned
object?  This would be good info to have in ?map.

One problem I also run into with map2SpatialPolygons() with the above
map object, is that the ring is not closed.

Do you know of any examples showing how one may close such rings, based 
on a polygon?  Say we have a rectangle over the map object and want to 
close the ring so we enclose all the land (or water)?

Cheers,

On Wed, 15 Oct 2008, Sebastian P. Luque wrote:

On Wed, 15 Oct 2008 19:24:18 +0200 (CEST),
Roger Bivand <Roger.Bivand at nhh.no> wrote:

If the fill=TRUE argument to map() is omitted, the rings are not
built:

se.baffin <- map("worldHires", xlim=c(-70, -60), ylim=c(62, 69),
resolution=0, interior=FALSE, fill=TRUE, plot=FALSE) baff <-
map2SpatialPolygons(se.baffin,
IDs=se.baffin$names)[match("Canada:Baffin Island", se.baffin$names)]
plot(baff, axes=TRUE)

looks OK to me. Add the CRS later. I don't think that we know
definitely that WGS84 was used, WDBII seems to have been around since
1985. Try overlaying on GE, for example, to get a feel.

Do you know of any examples showing how one may close such rings,
based on a polygon?  Say we have a rectangle over the map object and
want to close the ring so we enclose all the land (or water)?

By hand, you need to insert coordinates into the coords slot of the
Polygon object of interest and re-generate it. A recent question on
the list about houses east of a road was handled this way (stitch the
bounding box to the road and use overlay()).

Otherwise look at the unionSpatialPolygons() code for ideas on
interfacing gpclib classes and methods (also in PBSmapping).