src/pj_phi2.c


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/* determine latitude angle phi-2 */
#include <projects.h>

#define TOL 1.0e-10
#define N_ITER 15

	double
pj_phi2(projCtx ctx, double ts, double e) {
	double eccnth, Phi, con;
	int i;

	eccnth = .5 * e;
	Phi = M_HALFPI - 2. * atan (ts);
	i = N_ITER;
	for(;;) {
		double dphi;
		con = e * sin (Phi);
		dphi = M_HALFPI - 2. * atan (ts * pow((1. - con) /
		   (1. + con), eccnth)) - Phi;
		Phi += dphi;
		if( fabs(dphi) > TOL && --i )
			continue;
                break;
	}
	if (i <= 0)
		pj_ctx_set_errno( ctx, -18 );
	return Phi;
}