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

1 files changed, 9 insertions, 0 deletions

diff --git a/src/proj_internal.h b/src/proj_internal.h
index c102d910..253c647b 100644
--- a/src/proj_internal.h
+++ b/src/proj_internal.h
@@ -339,6 +339,13 @@ struct CoordOperation
     }
 };
 
+enum class TMercAlgo
+{
+    AUTO, // Poder/Engsager if far from central meridian, otherwise Evenden/Snyder
+    EVENDEN_SNYDER,
+    PODER_ENGSAGER,
+};
+
 /* base projection data structure */
 struct PJconsts {
 
@@ -741,6 +748,8 @@ struct projCtx_t {
 
     int projStringParserCreateFromPROJStringRecursionCounter = 0; // to avoid potential infinite recursion in PROJStringParser::createFromPROJString()
 
+    TMercAlgo defaultTmercAlgo = TMercAlgo::PODER_ENGSAGER; // can be overriden by content of proj.ini
+
     projCtx_t() = default;
     projCtx_t(const projCtx_t&);
     ~projCtx_t();