tmerc/utm: add a +algo=auto/evenden_snyder/poder_engsager parameter

The default remains +alg=poder_engsager.
This default value can be changed in proj.ini

+algo=auto will use Evenden Synder implementation if the error in
doing so remains below 0.1 mm on Earth-sized ellipsoid

2 files changed, 30 insertions, 0 deletions

diff --git a/docs/source/operations/projections/tmerc.rst b/docs/source/operations/projections/tmerc.rst
index 2866517b..cf428cff 100644
--- a/docs/source/operations/projections/tmerc.rst
+++ b/docs/source/operations/projections/tmerc.rst
@@ -83,6 +83,21 @@ Parameters
     It is faster than the default algorithm, but also diverges faster
     as the distance from the central meridian increases.
 
+.. option:: +algo=auto/evenden_snyder/poder_engsager
+
+    .. versionadded:: 7.1
+
+    Selects the algorithm to use. The hardcoded value and the one defined in
+    :ref:`proj-ini` default to ``poder_engsager``, that is the most precise
+    one.
+
+    When using auto, a heuristics based on the input coordinate to transform
+    is used to determine if the faster Evenden-Snyder method can be used, for
+    faster computation, without causing an error greater than 0.1 mm (for an
+    ellipsoid of the size of Earth)
+
+    Note that :option:`+approx` and :option:`+algo` are mutually exclusive.
+
 .. include:: ../options/lon_0.rst
 
 .. include:: ../options/lat_0.rst
diff --git a/docs/source/operations/projections/utm.rst b/docs/source/operations/projections/utm.rst
index b45dafc1..82715a02 100644
--- a/docs/source/operations/projections/utm.rst
+++ b/docs/source/operations/projections/utm.rst
@@ -76,6 +76,21 @@ Optional
 
     Use faster, less accurate algorithm for the Transverse Mercator.
 
+.. option:: +algo=auto/evenden_snyder/poder_engsager
+
+    .. versionadded:: 7.1
+
+    Selects the algorithm to use. The hardcoded value and the one defined in
+    :ref:`proj-ini` default to ``poder_engsager``, that is the most precise
+    one.
+
+    When using auto, a heuristics based on the input coordinate to transform
+    is used to determine if the faster Evenden-Snyder method can be used, for
+    faster computation, without causing an error greater than 0.1 mm (for an
+    ellipsoid of the size of Earth)
+
+    Note that :option:`+approx` and :option:`+algo` are mutually exclusive.
+
 .. include:: ../options/ellps.rst
 
 Further reading