Hi Ray,
Thanks for responding! However, it would certainly be very non-intuitive
if you're correct about the units for the projected coordinates. As I'm
sure you know, in a GIS geographic coordinates are usually in
degrees--although it's quite possible for these to be in radians instead, I
guess, since it saves converting degrees to radians when computing sines
and cosines--because these are coordinates that locate points on the
surface of a curved surface like a sphere or ellipsoid. Projected
coordinates, on the other hand, are ordinarily in some sort of physical
distance units like km or feet because they represent a projection of
points from the original curved surface onto a flat surface and include
effects due to the local radius of curvature of the surface. I'd be really
surprised if the projected coordinates were in radians because they are
angular units (independent of the radius of curvature), not distance
units. Still, it's possible.
My problem is I've got vertices of a polygon in lat/lon coordinates
(degrees) and am trying to find the approximate area of the polygon (in
km^2 or some other physical units, hence the question about the units) by
projecting the coordinates of the vertices to planar coordinates using an
Albers projection and the mapproject function in the mapproj package. If
you're correct about the units as radians, I guess I'd have to multiply the
area in radians^2 by R^2, where R is the radius of the earth, to get the
area in physical units.
Best regards,
Buck
On Tue, Feb 7, 2012 at 5:29 PM, Ray Brownrigg
<ray.brownr...@ecs.vuw.ac.nz>wrote:
> On Wed, 08 Feb 2012, William Stockhausen wrote:
> > Does anyone know what the units are for projected coordinates obtained
> > using mapproj's mapproject function with an Albers projection? Thanks
> for
> > any and all help!
> >
> > Buck Stockhausen
>
> I don't know for sure, but it looks like radians to me, with some
> unspecified
> origin(depending on the parameters specified). Certainly the maps package
> data is
> specified in radians internally.
>
> Hope this helps,
> Ray Brownrigg
>
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