[Proj] Libproj4 manual opinion poll

Clifford J Mugnier cjmce at lsu.edu
Mon Sep 29 13:34:56 EDT 2008

Normal Mercator can indeed have "2 standard parallels," but they are always symmetric about the Equator.  I have done one that is legally recognized in Ecuador for the city of Guayaquil to cover a larger area of interest outside of the old city limits (for the greater Metro area), with minimal scale distortion.  It is simply done by having a scale factor at origin less than 1.0 at the Equator.
Watch your blood pressure.
C. Mugnier
Louisiana State University


From: proj-bounces at lists.maptools.org on behalf of Gerald I. Evenden
Sent: Mon 29-Sep-08 10:35
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Libproj4 manual opinion poll

One thing quickly becomes apparent: the confusion as to what is the difference
between proj.4 and libproj4.

In terms of cartographic projections, nearly nothing.  In terms of datum
shifts and other miscellany: everything.  libproj4 as with the original
proj.4 software had nothing to do with datum shifts other than the original
distribution had a separate section that dealt only with the NAD datums.  The
original proj program also had a mechanism for generating Tchebyshev
polynomials that were designed for use with the mapgen map graphic system.

For the end user, the difference between running proj and lproj is trivial and
the user can use the original report for proj when using lproj without
getting into trouble.  There are a few more projections in libproj4 library
and lproj than the old system.

There have been a few peculiarities introduced into the proj.4 material, some
I can understand and some that I cannot (to put it very mildly).

1. Alternate longitude origins.  Are not supported in libproj4 (nor original
proj).  Some usefulness and a valid suggestion for libproj4.

2. Alternate origins for some projections.  I shudder when I see "two standard
parallels" listed for plain ol' Mercator.  There is not even one standard
parallel, let alone two.  There is a latitude of true scale and that only
adjusts the k_0 factor so that the scale factor is 1. at the selected
latitude.  If one wants to shift the origin then use x_0, y_0.  Stuff like
that really raises blood pressure.

I am sure there are others but that is a starter list.
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
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