Merge pull request #1105 from kbevers/inv-mercator-precision

Improve numerical precision of inverse spherical mercator
OSGeo · Aug 24, 2018 · ee57ac5 · ee57ac5
2 parents 502bc55 + 62b8164
commit ee57ac5
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diff --git a/src/PJ_merc.c b/src/PJ_merc.c
@@ -57,7 +57,7 @@ static LP e_inverse (XY xy, PJ *P) {          /* Ellipsoidal, inverse */
 
 static LP s_inverse (XY xy, PJ *P) {           /* Spheroidal, inverse */
     LP lp = {0.0,0.0};
-    lp.phi = M_HALFPI - 2. * atan(exp(-xy.y / P->k0));
+    lp.phi = atan(sinh(xy.y / P->k0));
     lp.lam = xy.x / P->k0;
     return lp;
 }

diff --git a/test/gie/builtins.gie b/test/gie/builtins.gie
@@ -2928,6 +2928,16 @@ accept  -200 -100
 expect  -0.001790493 -0.000895247
 
 
+-------------------------------------------------------------------------------
+operation +proj=merc +R=1
+-------------------------------------------------------------------------------
+Test the numerical stability of the inverse spherical Mercator
+-------------------------------------------------------------------------------
+tolerance   1e-15 m
+accept  0   1e-15
+expect  0   1e-15
+
+
 ===============================================================================
 Miller Oblated Stereographic
 	Azi(mod)