PJ_gnom.c
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#ifndef lint
static const char SCCSID[]="@(#)PJ_gnom.c 4.1 94/02/15 GIE REL";
#endif
#define PROJ_PARMS__ \
double sinph0; \
double cosph0; \
int mode;
#define PJ_LIB__
#include "projects.h"
PROJ_HEAD(gnom, "Gnomonic") "\n\tAzi, Sph.";
#define EPS10 1.e-10
#define N_POLE 0
#define S_POLE 1
#define EQUIT 2
#define OBLIQ 3
FORWARD(s_forward); /* spheroid */
double coslam, cosphi, sinphi;
sinphi = sin(lp.phi);
cosphi = cos(lp.phi);
coslam = cos(lp.lam);
switch (P->mode) {
case EQUIT:
xy.y = cosphi * coslam;
break;
case OBLIQ:
xy.y = P->sinph0 * sinphi + P->cosph0 * cosphi * coslam;
break;
case S_POLE:
xy.y = - sinphi;
break;
case N_POLE:
xy.y = sinphi;
break;
}
if (xy.y <= EPS10) F_ERROR;
xy.x = (xy.y = 1. / xy.y) * cosphi * sin(lp.lam);
switch (P->mode) {
case EQUIT:
xy.y *= sinphi;
break;
case OBLIQ:
xy.y *= P->cosph0 * sinphi - P->sinph0 * cosphi * coslam;
break;
case N_POLE:
coslam = - coslam;
case S_POLE:
xy.y *= cosphi * coslam;
break;
}
return (xy);
}
INVERSE(s_inverse); /* spheroid */
double rh, cosz, sinz;
rh = hypot(xy.x, xy.y);
sinz = sin(lp.phi = atan(rh));
cosz = sqrt(1. - sinz * sinz);
if (fabs(rh) <= EPS10) {
lp.phi = P->phi0;
lp.lam = 0.;
} else {
switch (P->mode) {
case OBLIQ:
lp.phi = cosz * P->sinph0 + xy.y * sinz * P->cosph0 / rh;
if (fabs(lp.phi) >= 1.)
lp.phi = lp.phi > 0. ? HALFPI : - HALFPI;
else
lp.phi = asin(lp.phi);
xy.y = (cosz - P->sinph0 * sin(lp.phi)) * rh;
xy.x *= sinz * P->cosph0;
break;
case EQUIT:
lp.phi = xy.y * sinz / rh;
if (fabs(lp.phi) >= 1.)
lp.phi = lp.phi > 0. ? HALFPI : - HALFPI;
else
lp.phi = asin(lp.phi);
xy.y = cosz * rh;
xy.x *= sinz;
break;
case S_POLE:
lp.phi -= HALFPI;
break;
case N_POLE:
lp.phi = HALFPI - lp.phi;
xy.y = -xy.y;
break;
}
lp.lam = atan2(xy.x, xy.y);
}
return (lp);
}
FREEUP; if (P) pj_dalloc(P); }
ENTRY0(gnom)
if (fabs(fabs(P->phi0) - HALFPI) < EPS10)
P->mode = P->phi0 < 0. ? S_POLE : N_POLE;
else if (fabs(P->phi0) < EPS10)
P->mode = EQUIT;
else {
P->mode = OBLIQ;
P->sinph0 = sin(P->phi0);
P->cosph0 = cos(P->phi0);
}
P->inv = s_inverse;
P->fwd = s_forward;
P->es = 0.;
ENDENTRY(P)