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>

3 files changed, 40 insertions, 28 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);
diff --git a/test/unit/test_io.cpp b/test/unit/test_io.cpp
index 69ef9073..3cb50352 100644
--- a/test/unit/test_io.cpp
+++ b/test/unit/test_io.cpp
@@ -7654,12 +7654,39 @@ TEST(io, projparse_etmerc) {
         crs->exportToPROJString(
             PROJStringFormatter::create(PROJStringFormatter::Convention::PROJ_4)
                 .get()),
-        "+proj=etmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0 "
+        "+proj=tmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0 "
         "+datum=WGS84 +units=m +no_defs +type=crs");
 
     auto wkt1 = crs->exportToWKT(
         WKTFormatter::create(WKTFormatter::Convention::WKT1_GDAL).get());
-    EXPECT_TRUE(wkt1.find("EXTENSION[\"PROJ4\",\"+proj=etmerc +lat_0=0 "
+    EXPECT_TRUE(wkt1.find("EXTENSION[\"PROJ4\"") == std::string::npos)
+        << wkt1;
+}
+
+// ---------------------------------------------------------------------------
+
+TEST(io, projparse_tmerc_approx) {
+    auto obj =
+        PROJStringParser().createFromPROJString("+proj=tmerc +approx +type=crs");
+    auto crs = nn_dynamic_pointer_cast<ProjectedCRS>(obj);
+    ASSERT_TRUE(crs != nullptr);
+    auto wkt2 = crs->exportToWKT(
+        &WKTFormatter::create()->simulCurNodeHasId().setMultiLine(false));
+    EXPECT_TRUE(
+        wkt2.find("METHOD[\"Transverse Mercator\",ID[\"EPSG\",9807]]") !=
+        std::string::npos)
+        << wkt2;
+
+    EXPECT_EQ(
+        crs->exportToPROJString(
+            PROJStringFormatter::create(PROJStringFormatter::Convention::PROJ_4)
+                .get()),
+        "+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");
+
+    auto wkt1 = crs->exportToWKT(
+        WKTFormatter::create(WKTFormatter::Convention::WKT1_GDAL).get());
+    EXPECT_TRUE(wkt1.find("EXTENSION[\"PROJ4\",\"+proj=tmerc +approx +lat_0=0 "
                           "+lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m "
                           "+no_defs\"]") != std::string::npos)
         << wkt1;
diff --git a/test/unit/test_operation.cpp b/test/unit/test_operation.cpp
index 6c1ecf1c..586226e2 100644
--- a/test/unit/test_operation.cpp
+++ b/test/unit/test_operation.cpp
@@ -1237,9 +1237,9 @@ TEST(operation, tmerc_export) {
 
     {
         auto formatter = PROJStringFormatter::create();
-        formatter->setUseETMercForTMerc(true);
+        formatter->setUseApproxTMerc(true);
         EXPECT_EQ(conv->exportToPROJString(formatter.get()),
-                  "+proj=etmerc +lat_0=1 +lon_0=2 +k=3 +x_0=4 +y_0=5");
+                  "+proj=tmerc +approx +lat_0=1 +lon_0=2 +k=3 +x_0=4 +y_0=5");
     }
 
     EXPECT_EQ(conv->exportToWKT(WKTFormatter::create().get()),
@@ -6061,21 +6061,21 @@ TEST(operation, compoundCRS_to_compoundCRS_with_vertical_transform) {
               "+ellps=WGS84");
     {
         auto formatter = PROJStringFormatter::create();
-        formatter->setUseETMercForTMerc(true);
+        formatter->setUseApproxTMerc(true);
         EXPECT_EQ(op->exportToPROJString(formatter.get()),
-                  "+proj=pipeline +step +inv +proj=etmerc +lat_0=1 +lon_0=2 "
+                  "+proj=pipeline +step +inv +proj=tmerc +approx +lat_0=1 +lon_0=2 "
                   "+k=3 +x_0=4 +y_0=5 +ellps=WGS84 +step "
                   "+proj=vgridshift +grids=bla.gtx +multiplier=0.001 +step "
-                  "+proj=utm +zone=32 "
+                  "+proj=utm +approx +zone=32 "
                   "+ellps=WGS84");
     }
     {
         auto formatter = PROJStringFormatter::create();
-        formatter->setUseETMercForTMerc(true);
+        formatter->setUseApproxTMerc(true);
         EXPECT_EQ(op->inverse()->exportToPROJString(formatter.get()),
-                  "+proj=pipeline +step +inv +proj=utm +zone=32 +ellps=WGS84 "
+                  "+proj=pipeline +step +inv +proj=utm +approx +zone=32 +ellps=WGS84 "
                   "+step +inv +proj=vgridshift +grids=bla.gtx "
-                  "+multiplier=0.001 +step +proj=etmerc +lat_0=1 +lon_0=2 "
+                  "+multiplier=0.001 +step +proj=tmerc +approx +lat_0=1 +lon_0=2 "
                   "+k=3 +x_0=4 +y_0=5 +ellps=WGS84");
     }