1 files changed, 53 insertions, 0 deletions
diff --git a/src/projections/nell_h.cpp b/src/projections/nell_h.cpp
new file mode 100644
index 00000000..28c3ace7
--- /dev/null
+++ b/src/projections/nell_h.cpp
@@ -0,0 +1,53 @@
+#define PJ_LIB__
+
+#include <math.h>
+
+#include "projects.h"
+
+PROJ_HEAD(nell_h, "Nell-Hammer") "\n\tPCyl, Sph";
+
+#define NITER 9
+#define EPS 1e-7
+
+
+static XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
+    XY xy = {0.0,0.0};
+    (void) P;
+
+    xy.x = 0.5 * lp.lam * (1. + cos(lp.phi));
+    xy.y = 2.0 * (lp.phi - tan(0.5 *lp.phi));
+
+    return xy;
+}
+
+
+static LP s_inverse (XY xy, PJ *P) {           /* Spheroidal, inverse */
+    LP lp = {0.0,0.0};
+    double V, c, p;
+    int i;
+    (void) P;
+
+    p = 0.5 * xy.y;
+    for (i = NITER; i ; --i) {
+        c = cos(0.5 * lp.phi);
+        lp.phi -= V = (lp.phi - tan(lp.phi/2) - p)/(1. - 0.5/(c*c));
+        if (fabs(V) < EPS)
+            break;
+    }
+    if (!i) {
+        lp.phi = p < 0. ? -M_HALFPI : M_HALFPI;
+        lp.lam = 2. * xy.x;
+    } else
+        lp.lam = 2. * xy.x / (1. + cos(lp.phi));
+
+    return lp;
+}
+
+
+PJ *PROJECTION(nell_h) {
+    P->es = 0.;
+    P->inv = s_inverse;
+    P->fwd = s_forward;
+
+    return P;
+}