[Proj] Re: Newbie Polar stereographic question

Clifford J Mugnier cjmce at lsu.edu
Fri Jul 14 14:10:32 EDT 2006

Regarding the general philosopy of transformation accuracy alluded to
below, Mr. Evenden is correct in his statement.  The old classical
formulae, oftentimes being a "double" projection to the Equivalent Sphere
first, had an expected computational accuracy of 0.1 meter.

The new inertial datums referenced with GPS/GLONASS tools have projections
typically defined with an accuracy criterion of 0.1mm.

Expecting 0.1mm accuracy to "match" a 19th century projection math model is
as Mr. Evenden states, pointless.  It may be satisfying, but really has not
any realistic need.

Furthermore, most transforms and parameters offered by the EPSG have the
standard "oil patch" level of necessary computational accuracy, which is
0.1 meters.  Doing better than that does not do any better in finding where
to drill for oil and gas.  It's nice to have, but it's not necessary.

Clifford J. Mugnier C.P.,C.M.S.
National Director (2006-2008)
Photogrammetric Applications Division
Chief of Geodesy
Center for GeoInformatics
Dept. of Civil Engineering
CEBA 3223A
Baton Rouge, LA  70810
Voice:              (225) 578-8536
Facsimile:          (225) 578-8652
Corps of Engineers: (504) 862-1094 [Summertime]

> From: "Gerald I. Evenden"
>>>> I wonder, can all 3 variants of the EPSG (OGP) stereographic
>>>> be calculated with (lib)proj's stere?
>>> +proj=stere is probably OK for most polar aspects.  However, most
>>> oblique stereographic projections probably use +proj=sterea.
>> I am afraid this is no useful answer for
>>>> EPSG coordinate operation method 9830:
>>>> Polar Stereographic (variant C)
> I an going to make a single comment: worrying about differences in the 1
> micron range are pointless.

Probably a point is missed here.
(lib)proj is NOT capable of computing said projection.
I found out that a trick is needed: use an adapted false northing.
I asked for an equation, I got a micron :-)

Don't bother, in the mean time I've derived such an equation myself.
Anyone interested can mail me or post a reply here.
Variant B poses no difficulty if you know how to do it.

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