[R] intersecting polygons and conversion from decimal degree to km

[White.Denis@epamail.epa.gov]

Dr. M.Sawada wrote:
> Here is a simple Albers-Equal Area Conic program in R that you can
> use (actually I wrote it for S+ but should be the same - you may
> have to modify the dimnames() function for R).  This is preferred
> over an equidistant projection.  Long/Lat, as you are probably aware,
> are a spherical system and so distances are not isotropic (1 degree
> of long at the equator is 116 miles vs 0 miles at the pole) ...

At the equator, 1 degree of longitude is 111+ km on the Clarke 1866
ellipsoid.  Choosing a map projection depends on the problem.
Equal-area may not be preferable to conformal or equidistant.  And
simplicity may be fine depending on precision requirements.  For
background on map projections try a web search, e.g.
http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html.

There have been a number of discussions on r-help on map projections.
See threads about 20 April 2001, and 22 June 2001.

Denis White
   US EPA, 200 SW 35th St, Corvallis, Oregon, 97333 USA
   voice: 541.754.4476, email: white.denis at epa.gov
   web: www.epa.gov/wed/pages/staff/white/

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