Hi Ray (& all),

Many apologies to Ray--apparently my intuition stinks!!  The projection is
based on the unit sphere (R=1), so the projected coordinates really are
dimensionless--as you said! So to scale up to the earth, just multiply
the projected coordinate values from mapproject by your favorite radius of
the earth.

I did try to track down the source code as listed in the mapproj help but
one link was broken and I wasn't able to find the function listing in
the "Plan 9" c code listings.

Best,

Buck
***************************************************
* Dr. William T. Stockhausen                      *
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Disclaimer: The opinions expressed above are personal
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On Wed, Feb 8, 2012 at 12:40 PM, Ray Brownrigg
<Ray.Brownrigg@ecs.vuw.ac.nz>wrote:

> Buck:
>
> [Note this is not sent to the list]
>
> I guess you have two options:
> 1) contact the author of the mapproj package for further information
> 2) read the source :-)
>
> Cheers,
> Ray
>
> On Thu, 09 Feb 2012, William Stockhausen wrote:
> > 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.Brownrigg@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
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>and provide commented,
> > minimal, self-contained, reproducible code.
>
>

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