Make tmerc an alias for etmerc. (#1234)

* Make tmerc an alias for etmerc

This switches the algorithm used in tmerc to the Poder/Engsager
tmerc algorithm. The original tmerc algorithm of Evenden/Snyder
origin can still be accessed by adding the +approx flag when
initializing a tmerc projection. The +approx flag can also
be used when initializing UTM projections, in which case the
Evenden/Snyder algorithm is used as well.

If a tmerc projection is instantiated on a spherical earth
the Evenden/Snyder algorithm is used as well since the
Poder/Engsager algorithm is only defined on the ellipsoid.

+proj=etmerc can still be instantiated for backwards compatibility
reasons.

Co-authored-by: Kristian Evers <kristianevers@gmail.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>

1 files changed, 4 insertions, 7 deletions

diff --git a/src/iso19111/c_api.cpp b/src/iso19111/c_api.cpp
index 6fafa2c8..240d11b3 100644
--- a/src/iso19111/c_api.cpp
+++ b/src/iso19111/c_api.cpp
@@ -1207,9 +1207,8 @@ const char *proj_as_wkt(PJ_CONTEXT *ctx, const PJ *obj, PJ_WKT_TYPE type,
  * @param type PROJ String version.
  * @param options NULL-terminated list of strings with "KEY=VALUE" format. or
  * NULL.
- * The currently recognized option is USE_ETMERC=YES to use
- * +proj=etmerc instead of +proj=tmerc (or USE_ETMERC=NO to disable implicit
- * use of etmerc by utm conversions)
+ * The currently recognized option is USE_APPROX_TMERC=YES to add the +approx
+ * flag to +proj=tmerc or +proj=utm
  * @return a string, or NULL in case of error.
  */
 const char *proj_as_proj_string(PJ_CONTEXT *ctx, const PJ *obj,
@@ -1243,10 +1242,8 @@ const char *proj_as_proj_string(PJ_CONTEXT *ctx, const PJ *obj,
     try {
         auto formatter = PROJStringFormatter::create(convention, dbContext);
         if (options != nullptr && options[0] != nullptr) {
-            if (ci_equal(options[0], "USE_ETMERC=YES")) {
-                formatter->setUseETMercForTMerc(true);
-            } else if (ci_equal(options[0], "USE_ETMERC=NO")) {
-                formatter->setUseETMercForTMerc(false);
+            if (ci_equal(options[0], "USE_APPROX_TMERC=YES")) {
+                formatter->setUseApproxTMerc(true);
             }
         }
         obj->lastPROJString = exportable->exportToPROJString(formatter.get());