1 files changed, 55 insertions, 0 deletions
diff --git a/src/projections/vandg4.cpp b/src/projections/vandg4.cpp
new file mode 100644
index 00000000..d9a53c87
--- /dev/null
+++ b/src/projections/vandg4.cpp
@@ -0,0 +1,55 @@
+#define PJ_LIB__
+
+#include <math.h>
+
+#include "projects.h"
+
+PROJ_HEAD(vandg4, "van der Grinten IV") "\n\tMisc Sph, no inv";
+
+#define TOL 1e-10
+
+
+static XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
+    XY xy = {0.0,0.0};
+    double x1, t, bt, ct, ft, bt2, ct2, dt, dt2;
+    (void) P;
+
+    if (fabs(lp.phi) < TOL) {
+        xy.x = lp.lam;
+        xy.y = 0.;
+    } else if (fabs(lp.lam) < TOL || fabs(fabs(lp.phi) - M_HALFPI) < TOL) {
+        xy.x = 0.;
+        xy.y = lp.phi;
+    } else {
+        bt = fabs(M_TWO_D_PI * lp.phi);
+        bt2 = bt * bt;
+        ct = 0.5 * (bt * (8. - bt * (2. + bt2)) - 5.)
+            / (bt2 * (bt - 1.));
+        ct2 = ct * ct;
+        dt = M_TWO_D_PI * lp.lam;
+        dt = dt + 1. / dt;
+        dt = sqrt(dt * dt - 4.);
+        if ((fabs(lp.lam) - M_HALFPI) < 0.) dt = -dt;
+        dt2 = dt * dt;
+        x1 = bt + ct; x1 *= x1;
+        t = bt + 3.*ct;
+        ft = x1 * (bt2 + ct2 * dt2 - 1.) + (1.-bt2) * (
+            bt2 * (t * t + 4. * ct2) +
+            ct2 * (12. * bt * ct + 4. * ct2) );
+        x1 = (dt*(x1 + ct2 - 1.) + 2.*sqrt(ft)) /
+            (4.* x1 + dt2);
+        xy.x = M_HALFPI * x1;
+        xy.y = M_HALFPI * sqrt(1. + dt * fabs(x1) - x1 * x1);
+        if (lp.lam < 0.) xy.x = -xy.x;
+        if (lp.phi < 0.) xy.y = -xy.y;
+    }
+    return xy;
+}
+
+
+PJ *PROJECTION(vandg4) {
+    P->es = 0.;
+    P->fwd = s_forward;
+
+    return P;
+}