diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/pj_phi2.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/src/pj_phi2.c b/src/pj_phi2.c index b55d9690..a83302e6 100644 --- a/src/pj_phi2.c +++ b/src/pj_phi2.c @@ -7,7 +7,22 @@ static const double TOL = 1.0e-10; static const int N_ITER = 15; +/*****************************************************************************/ double pj_phi2(projCtx ctx, double ts, double e) { +/****************************************************************************** +Determine latitude angle phi-2. +Inputs: + ts = exp(-psi) where psi is the isometric latitude (dimensionless) + e = eccentricity of the ellipsoid (dimensionless) +Output: + phi = geographic latitude (radians) +Here isometric latitude is defined by + psi = log( tan(pi/4 + phi/2) * + ( (1 - e*sin(phi)) / (1 + e*sin(phi)) )^(e/2) ) + = asinh(tan(phi)) - e * atanh(e * sin(phi)) +This routine inverts this relation using the iterative scheme given +by Snyder (1987), Eqs. (7-9) - (7-11) +*******************************************************************************/ double eccnth = .5 * e; double Phi = M_HALFPI - 2. * atan(ts); double con; |