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#define PJ_LIB__
#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
FORWARD(s_forward); /* spheroid */
double p, V, s, c;
int i;
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);
}
INVERSE(s_inverse); /* spheroid */
double c;
lp.phi = aasin(xy.y / C_y);
lp.lam = xy.x / (C_x * (1. + (c = cos(lp.phi))));
lp.phi = aasin((lp.phi + sin(lp.phi) * (c + 2.)) / C_p);
return (lp);
}
FREEUP; if (P) pj_dalloc(P); }
ENTRY0(eck4); P->es = 0.; P->inv = s_inverse; P->fwd = s_forward; ENDENTRY(P)
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