[MetaCRS] Stereographic Projections: stere and sterea

[Richard Marsden]

Following on from equal-area-maps.com, I am putting together some samples
for polar projections. Eventually I'll publish a couple of articles at
GeoWebGuru.com.  I'm still at the survey stage, but it looks like there are
two stereographic projections: "stere" and "sterea" and it isn't clear what
the difference is.

The Proj.4 documenation doesn't mention the "sterea" projection, but pages
found by Google suggest it is a recent alternative addition?
This would explain its absence from the original Snyder documentation and
addenda.

The best description I have found is on the GeoTIFF site which says that
they are two different but valid approaches, with "stere" being the original
(Snyder) approach. However it notes that the two approaches produce
different results away from the centre! To me this suggests that it is
unlikely that are both are equally valid.

Considering both were ported to Proj4JS, I assume this issue has already
come up?
What does "sterea" offer over "stere", and which is usually recommended?

Best regards,

Richard Marsden
Winwaed Software Technology LLC
http://www.winwaed.com
http://www.mapping-tools.com
http://www.geowebguru.com
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