1 files changed, 63 insertions, 0 deletions
diff --git a/src/projections/eck4.cpp b/src/projections/eck4.cpp
new file mode 100644
index 00000000..4fa4c21f
--- /dev/null
+++ b/src/projections/eck4.cpp
@@ -0,0 +1,63 @@
+#define PJ_LIB__
+
+#include <math.h>
+
+#include "projects.h"
+
+PROJ_HEAD(eck4, "Eckert IV") "\n\tPCyl, Sph";
+
+#define C_x .42223820031577120149
+#define C_y 1.32650042817700232218
+#define RC_y    .75386330736002178205
+#define C_p 3.57079632679489661922
+#define RC_p    .28004957675577868795
+#define EPS 1e-7
+#define NITER   6
+
+
+static XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
+    XY xy = {0.0,0.0};
+    double p, V, s, c;
+    int i;
+    (void) P;
+
+    p = C_p * sin(lp.phi);
+    V = lp.phi * lp.phi;
+    lp.phi *= 0.895168 + V * ( 0.0218849 + V * 0.00826809 );
+    for (i = NITER; i ; --i) {
+        c = cos(lp.phi);
+        s = sin(lp.phi);
+        lp.phi -= V = (lp.phi + s * (c + 2.) - p) /
+            (1. + c * (c + 2.) - s * s);
+        if (fabs(V) < EPS)
+            break;
+    }
+    if (!i) {
+        xy.x = C_x * lp.lam;
+        xy.y = lp.phi < 0. ? -C_y : C_y;
+    } else {
+        xy.x = C_x * lp.lam * (1. + cos(lp.phi));
+        xy.y = C_y * sin(lp.phi);
+    }
+    return xy;
+}
+
+
+static LP s_inverse (XY xy, PJ *P) {           /* Spheroidal, inverse */
+    LP lp = {0.0,0.0};
+    double c;
+
+    lp.phi = aasin(P->ctx,xy.y * RC_y);
+    lp.lam = xy.x / (C_x * (1. + (c = cos(lp.phi))));
+    lp.phi = aasin(P->ctx,(lp.phi + sin(lp.phi) * (c + 2.)) * RC_p);
+    return lp;
+}
+
+
+PJ *PROJECTION(eck4) {
+    P->es = 0.0;
+    P->inv = s_inverse;
+    P->fwd = s_forward;
+
+    return P;
+}