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#ifndef lint
static const char SCCSID[]="@(#)PJ_vandg4.c 4.1 94/02/15 GIE REL";
#endif
#define PJ_LIB__
#include "projects.h"
PROJ_HEAD(vandg4, "van der Grinten IV") "\n\tMisc Sph, no inv.";
#define TOL 1e-10
#define TWORPI 0.63661977236758134308
FORWARD(s_forward); /* spheroid */
double x1, t, bt, ct, ft, bt2, ct2, dt, dt2;
if (fabs(lp.phi) < TOL) {
xy.x = lp.lam;
xy.y = 0.;
} else if (fabs(lp.lam) < TOL || fabs(fabs(lp.phi) - HALFPI) < TOL) {
xy.x = 0.;
xy.y = lp.phi;
} else {
bt = fabs(TWORPI * lp.phi);
bt2 = bt * bt;
ct = 0.5 * (bt * (8. - bt * (2. + bt2)) - 5.)
/ (bt2 * (bt - 1.));
ct2 = ct * ct;
dt = TWORPI * lp.lam;
dt = dt + 1. / dt;
dt = sqrt(dt * dt - 4.);
if ((fabs(lp.lam) - 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 = HALFPI * x1;
xy.y = 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);
}
FREEUP; if (P) pj_dalloc(P); }
ENTRY0(vandg4) P->es = 0.; P->fwd = s_forward; ENDENTRY(P)
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