/Proj4/pj_phi2.c

 1/* determine latitude angle phi-2 */
 2#ifndef lint
 3static const char SCCSID[]="@(#)pj_phi2.c	4.3	93/06/12	GIE	REL";
 4#endif
 5#include "projects.h"
 6
 7#define HALFPI		1.5707963267948966
 8#define TOL 1.0e-10
 9#define N_ITER 15
10
11	double
12pj_phi2(double ts, double e) {
13	double eccnth, Phi, con, dphi;
14	int i;
15
16	eccnth = .5 * e;
17	Phi = HALFPI - 2. * atan (ts);
18	i = N_ITER;
19	do {
20		con = e * sin (Phi);
21		dphi = HALFPI - 2. * atan (ts * pow((1. - con) /
22		   (1. + con), eccnth)) - Phi;
23		Phi += dphi;
24	} while ( fabs(dphi) > TOL && --i);
25	if (i <= 0)
26		pj_errno = -18;
27	return Phi;
28}