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, 3 insertions, 18 deletions

diff --git a/test/unit/test_c_api.cpp b/test/unit/test_c_api.cpp
index 8ef426ac..d535e412 100644
--- a/test/unit/test_c_api.cpp
+++ b/test/unit/test_c_api.cpp
@@ -507,36 +507,21 @@ TEST_F(CApi, proj_as_proj_string_incompatible_WKT1) {
 
 // ---------------------------------------------------------------------------
 
-TEST_F(CApi, proj_as_proj_string_etmerc_option_yes) {
+TEST_F(CApi, proj_as_proj_string_approx_tmerc_option_yes) {
     auto obj = proj_create(m_ctxt, "+proj=tmerc +type=crs");
     ObjectKeeper keeper(obj);
     ASSERT_NE(obj, nullptr);
 
-    const char *options[] = {"USE_ETMERC=YES", nullptr};
+    const char *options[] = {"USE_APPROX_TMERC=YES", nullptr};
     auto str = proj_as_proj_string(m_ctxt, obj, PJ_PROJ_4, options);
     ASSERT_NE(str, nullptr);
     EXPECT_EQ(str,
-              std::string("+proj=etmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 "
+              std::string("+proj=tmerc +approx +lat_0=0 +lon_0=0 +k=1 +x_0=0 "
                           "+y_0=0 +datum=WGS84 +units=m +no_defs +type=crs"));
 }
 
 // ---------------------------------------------------------------------------
 
-TEST_F(CApi, proj_as_proj_string_etmerc_option_no) {
-    auto obj = proj_create(m_ctxt, "+proj=utm +zone=31 +type=crs");
-    ObjectKeeper keeper(obj);
-    ASSERT_NE(obj, nullptr);
-
-    const char *options[] = {"USE_ETMERC=NO", nullptr};
-    auto str = proj_as_proj_string(m_ctxt, obj, PJ_PROJ_4, options);
-    ASSERT_NE(str, nullptr);
-    EXPECT_EQ(str, std::string("+proj=tmerc +lat_0=0 +lon_0=3 +k=0.9996 "
-                               "+x_0=500000 +y_0=0 +datum=WGS84 +units=m "
-                               "+no_defs +type=crs"));
-}
-
-// ---------------------------------------------------------------------------
-
 TEST_F(CApi, proj_crs_create_bound_crs_to_WGS84) {
     auto crs = proj_create_from_database(m_ctxt, "EPSG", "3844",
                                          PJ_CATEGORY_CRS, false, nullptr);