Spilhaus: better use lon_0 and lat_0

OSGeo · Feb 18, 2025 · 85e7d8e · 85e7d8e
1 parent 05f6183
commit 85e7d8e
Show file tree

Hide file tree

Showing 3 changed files with 41 additions and 12 deletions.
diff --git a/docs/source/operations/projections/spilhaus.rst b/docs/source/operations/projections/spilhaus.rst
@@ -35,7 +35,7 @@ Parameters
 
 .. note:: All parameters are optional.
 
-.. option:: +lon_c=<value>
+.. option:: +lon_0=<value>
 
     Longitude of projection centre.
 
@@ -49,7 +49,7 @@ Parameters
 
         See :ref:`Projection Units <projection_units>` for more information.
 
-.. option:: +lat_c=<value>
+.. option:: +lat_0=<value>
 
     Latitude of projection centre.
 

diff --git a/src/projections/spilhaus.cpp b/src/projections/spilhaus.cpp
@@ -135,25 +135,30 @@ PJ *PJ_PROJECTION(spilhaus) {
                    ? pj_param(P->ctx, P->params, ("r" + name).c_str()).f
                    : def * DEG_TO_RAD;
     };
+
     // Values from https://github.com/OSGeo/PROJ/issues/1851
-    const double lat_center = param_rad("lat_c", -49.56371678);
-    const double lon_center = param_rad("lon_c", 66.94970198);
+    if (!pj_param(P->ctx, P->params, "tlon_0").i) {
+        P->lam0 = 66.94970198 * DEG_TO_RAD;
+    }
+    if (!pj_param(P->ctx, P->params, "tlat_0").i) {
+        P->phi0 = -49.56371678 * DEG_TO_RAD;
+    }
     const double azimuth = param_rad("azi", 40.17823482);
 
     const double rotation = param_rad("rot", 45);
     Q->cosrot = cos(rotation);
     Q->sinrot = sin(rotation);
 
-    const double conformal_lat_center = pj_conformal_lat(lat_center, P->e);
+    const double conformal_lat_center = pj_conformal_lat(P->phi0, P->e);
 
     Q->sinalpha = -cos(conformal_lat_center) * cos(azimuth);
     Q->cosalpha = sqrt(1 - Q->sinalpha * Q->sinalpha);
-    Q->lambda_0 = lon_center + atan2(tan(azimuth), -sin(conformal_lat_center));
+    Q->lambda_0 = atan2(tan(azimuth), -sin(conformal_lat_center));
     Q->beta = M_PI + atan2(-sin(azimuth), -tan(conformal_lat_center));
 
-    Q->conformal_distortion =
-        cos(lat_center) / sqrt(1 - P->es * sin(lat_center) * sin(lat_center)) /
-        cos(conformal_lat_center);
+    Q->conformal_distortion = cos(P->phi0) /
+                              sqrt(1 - P->es * sin(P->phi0) * sin(P->phi0)) /
+                              cos(conformal_lat_center);
 
     P->fwd = spilhaus_forward;
     P->inv = spilhaus_inverse;

diff --git a/test/gie/spilhaus.gie b/test/gie/spilhaus.gie
@@ -367,11 +367,12 @@ expect   -15390625   -15546875
 ------------------------------------------------------------
 # Stable for default parameters
 ------------------------------------------------------------
-operation  +proj=spilhaus +rot=45 +k_0=1 +lat_c=-49.56371678 +lon_c=66.94970198 +azi=40.17823482
+operation  +proj=spilhaus +rot=45 +k_0=1 +lat_0=-49.56371678 +lon_0=66.94970198 +azi=40.17823482
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3733410.0118   -9320.8573
+roundtrip 1
 
 ------------------------------------------------------------
 # Sentitive to input parameters
@@ -381,36 +382,42 @@ tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3733410.0118   -9320.8573
+roundtrip 1
 ------------------------------------------------------------
-operation  +proj=spilhaus +lon_c=10.1
+operation  +proj=spilhaus +lon_0=10.1
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   4343770.7991   -3701935.6242
+roundtrip 1
 ------------------------------------------------------------
-operation  +proj=spilhaus +lat_c=30.1
+operation  +proj=spilhaus +lat_0=30.1
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3637341.2895   -2571368.8666
+roundtrip 1
 ------------------------------------------------------------
 operation  +proj=spilhaus +azi=9.1
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3061806.4542   -1678791.7428
+roundtrip 1
 ------------------------------------------------------------
 operation  +proj=spilhaus +rot=40.1
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3720561.6630   309609.603620
+roundtrip 1
 ------------------------------------------------------------
 operation  +proj=spilhaus +k_0=0.9
 tolerance  1 mm
 ------------------------------------------------------------
 accept   130.4   -16.2
 expect   3360069.0106   -8388.7716
+roundtrip 1
 
 ------------------------------------------------------------
 # Sphere
@@ -422,4 +429,21 @@ accept   130.4   -16.2
 expect   3737644.5177   -7049.7883
 roundtrip 1
 
+------------------------------------------------------------
+# vs Adams WS2
+------------------------------------------------------------
+operation  +proj=adams_ws2 +R=6378137
+tolerance  1 mm
+------------------------------------------------------------
+accept   130.4   -16.2
+expect   8199312.0391   -1392652.9172
+roundtrip 1
+------------------------------------------------------------
+operation  +proj=spilhaus +R=6378137 +lon_0=0 +lat_0=0 +azi=0 +rot=0
+tolerance  1 mm
+------------------------------------------------------------
+accept   130.4   -16.2
+expect   8199312.0391   -1392652.9172
+roundtrip 1
+
 </gie-strict>