[Proj] "Double Ellipsoid" error, reproduction

Duncan Agnew dagnew at ucsd.edu
Wed Dec 10 13:00:16 EST 2008

Since proj doesn't know where its coordinates come from, +proj=merc 
will do the spherical Mercator as it
stands (I assume)--no need for a new flag. I suppose this illustrates 
Gerald's concerns about confusion resulting from the mixing of datums 
and projections in one routine; I'll say, as a user, that it is 
convenient to be able to combine datum shifts and grid conversions in 
one routine, though it would be
equally convenient to separate them and use pipes. The usual problem of 
path-dependence: see the QWERTY
keyboard and the 4'8.5" railway gauge.

I'll disagree with Gerald about Google Maps--their Mercator is an 
excellent choice for their purpose. It is a terrible choice for a 
global projection of a static map, but an excellent one at large 
scales, which is where all the activity on Google Maps ends up. And for 
doing preliminary locations, a familiar cylindrical projection has 
merit even at a global scale. I do agree with Gerald that for a paper 
global map, equal-area should be the default choice, absent some other 

Duncan Agnew

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