# [Proj] Rhumb lines and Mercator on a triaxial ellipsoid

Noel Zinn (cc) ndzinn at comcast.net
Sat Dec 20 10:47:37 EST 2014

```Nice.  A meridian is a limiting case of a rhumb line.  It's not at all
surprising that a meridian undergoes a N/S reversal passing through a pole.
E/W reversal in the general case is not so intuitive, but Fermat's spiral is
a template for that.  -Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

-----Original Message-----
From: Charles Karney
Sent: Saturday, December 20, 2014 8:33 AM
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Rhumb lines and Mercator on a triaxial ellipsoid

It is well known that a rhumb line arrives at a pole in a finite
distance after encircling the pole infinitely many times.  Craig Rollins
recently asked me what heading a rhumb line has *after* passing through
the pole.

One way of answering this is to consider a rhumb line on a triaxial
ellipsoid and to take the limit as the two large axes approach one
another.  The (somewhat surprising) result is that the heading of the
rhumb line is reversed.  E.g., if the initial heading is NE, the heading
after passing through the pole is SW.

This got me to thinking about the Mercator projection on a triaxial
ellipsoid.  This was given by Jacobi in 1843, see section 28 of

https://www.worldcat.org/oclc/440645889

The integrals that Jacobi gives can be written in terms of elliptic
integrals, see

https://dx.doi.org/10.1007/978-3-642-32618-9_17
http://geographiclib.sf.net/html/triaxial.html#triaxial-conformal

Finally, Jacobi has an interesting take on Gauss' work on conformal

"Among the different ways of representing a curved surface on a plane,
as is necessary for a map, one prefers, above all, the method of
projection in which infinitely small elements remain similar.  In the
preceding century Lambert had been concerned with various aspects of
this projection, of which one can learn in detail from his
contributions to mathematics.  Because of these, Lambert's colleague
at that time, Lagrange, was induced to undertake an investigation from
the same standpoint and gave the solution completely for all surfaces
of revolution.  The Copenhagen Academy which later announced a prize
for the solution of this problem for all curved surfaces awarded it to
the treatise sent in by Gauss.  In this, Lagrange's work, to which
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