[R] intersecting polygons and conversion from decimal degree to km
White.Denis at epamail.epa.gov
Sat May 25 01:51:16 CEST 2002
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.
There have been a number of discussions on r-help on map projections.
See threads about 20 April 2001, and 22 June 2001.
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