From 1e8e527c4a4066443946ebf3edb83c43cd60a6e0 Mon Sep 17 00:00:00 2001
From: Kristian Evers <kristianevers@gmail.com>
Date: Wed, 21 Jun 2017 11:06:51 +0200
Subject: Correct calculation of meridian convergence for non-conformal
 projections. Fixes #526.

---
 src/pj_deriv.c   | 16 ++++++++--------
 src/pj_factors.c |  2 +-
 2 files changed, 9 insertions(+), 9 deletions(-)

(limited to 'src')

diff --git a/src/pj_deriv.c b/src/pj_deriv.c
index 6e9e3b82..1c969a9d 100644
--- a/src/pj_deriv.c
+++ b/src/pj_deriv.c
@@ -17,8 +17,8 @@ int pj_deriv(LP lp, double h, PJ *P, struct DERIVS *der) {
 
     der->x_l = t.x;
     der->y_p = t.y;
-    der->x_p = -t.x;
-    der->y_l = -t.y;
+    der->x_p = t.x;
+    der->y_l = t.y;
     lp.phi -= h;
     if (fabs(lp.phi) > M_HALFPI)
         return 1;
@@ -29,8 +29,8 @@ int pj_deriv(LP lp, double h, PJ *P, struct DERIVS *der) {
 
     der->x_l += t.x;
     der->y_p -= t.y;
-    der->x_p += t.x;
-    der->y_l -= t.y;
+    der->x_p -= t.x;
+    der->y_l += t.y;
     lp.lam -= h;
     t = (*P->fwd)(lp, P);
     if (t.x == HUGE_VAL)
@@ -38,8 +38,8 @@ int pj_deriv(LP lp, double h, PJ *P, struct DERIVS *der) {
 
     der->x_l -= t.x;
     der->y_p -= t.y;
-    der->x_p += t.x;
-    der->y_l += t.y;
+    der->x_p -= t.x;
+    der->y_l -= t.y;
     lp.phi += h;
     t = (*P->fwd)(lp, P);
     if (t.x == HUGE_VAL)
@@ -47,8 +47,8 @@ int pj_deriv(LP lp, double h, PJ *P, struct DERIVS *der) {
 
     der->x_l -= t.x;
     der->y_p += t.y;
-    der->x_p -= t.x;
-    der->y_l += t.y;
+    der->x_p += t.x;
+    der->y_l -= t.y;
     der->x_l /= (h += h);
     der->y_p /= h;
     der->x_p /= h;
diff --git a/src/pj_factors.c b/src/pj_factors.c
index fb4a1dd4..6c1b4978 100644
--- a/src/pj_factors.c
+++ b/src/pj_factors.c
@@ -72,7 +72,7 @@ int pj_factors(LP lp, PJ *P, double h, struct FACTORS *fac) {
 
         /* convergence */
         if (!(fac->code & IS_ANAL_CONV)) {
-            fac->conv = - atan2(fac->der.y_l, fac->der.x_l);
+            fac->conv = - atan2(fac->der.x_p, fac->der.y_p);
             if (fac->code & IS_ANAL_XL_YL)
                 fac->code |= IS_ANAL_CONV;
         }
-- 
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