The file
core/sis-referencing/src/main/java/org/apache/sis/referencing/datum/DefaultEllipsoid.java
contains a routine orthodromicDistance. A more standard (and more
euphonious) name would be geodesicDistance. In addition the statement
"The orthodromic path is always on a great circle" is wrong, since, in
general, there are no great circles on an ellipsoid.
orthodromicDistance (or geodesicDistance) uses Vincenty's algorithm
which is known to fail for nearly antipodal points. My package
net.sf.geographiclib.GeographicLib-Java includes an algorithm which is
more accurate than Vincenty's and (more importantly) always converges.
In the case where algorithm hasn't converged, the routine attempts a
lame patch-up in the case where the two points lie on the equator. This
computes the distance as
abs(lon1-lon2) * getSemiMajorAxis();
This is doubly wrong:
(a) lon1-lon2 needs to be reduced to the range [-pi,pi]. The tests in
DefaultEllipsoidTest.java don't catch this blunder.
(b) the formula assumes that the geodesic runs along the equator;
however this only holds if the longitude difference is less than or
equal to (1-f) * pi. The tests in DefaultEllipsoidTest.java bake in
this incorrect assumption.
--Charles
