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 * *************************************************** * Resource Ecology and Fisheries Management * * Alaska Fisheries Science Center * * National Marine Fisheries Service * * National Oceanic and Atmospheric Administration * * 7600 Sand Point Way N.E. * * Seattle, Washington 98115-6349 * *************************************************** * email: william.stockhau...@noaa.gov * * voice: 206-526-4241 fax: 206-526-6723 * * web : http://www.afsc.noaa.gov * *************************************************** All models are wrong, some are useful.--G.E.P. Box Beware of geeks bearing equations. --W. Buffett *************************************************** Disclaimer: The opinions expressed above are personal and do not necessarily reflect official NOAA policy. On Wed, Feb 8, 2012 at 12:40 PM, Ray Brownrigg <ray.brownr...@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.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 > > > > [[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. > > [[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 and provide commented, minimal, self-contained, reproducible code.
