Fix ingestion of +proj=cea with +k_0

Fixes #1881

Digging into the implementation of proj=cea, it appears that
k_0 and lat_ts are intended to be exclusive ways of specifying the
same concept. EPSG only models the variant using lat_s.
So if k_0 is found and lat_ts is absent, compute the equivalent
value of lat_ts from k_0.

Note: k_0 should normally be in the [0,1] range. In case creative users
would use something outside, we raise an exception, even if the cea
implementation could potentially deal with any k_0 value. Hopefully
this is a (reasonable) limitation that will address nominal use cases.

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/source/operations/projections/cea.rst b/docs/source/operations/projections/cea.rst
index 4542fe71..80fe1df3 100644
--- a/docs/source/operations/projections/cea.rst
+++ b/docs/source/operations/projections/cea.rst
@@ -29,3 +29,9 @@ Parameters
 .. include:: ../options/x_0.rst
 
 .. include:: ../options/y_0.rst
+
+.. note::
+
+    ``lat_ts`` and ``k_0`` are mutually exclusive. If ``lat_ts``
+    is specified, it is equivalent to setting ``k_0`` to
+    :math:`\frac{\cos \phi_{ts}}{\sqrt{1 - e^2 \sin^2 \phi_{ts}}}`
\ No newline at end of file