src/Geocentric.cpp
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1 
11 
12 namespace GeographicLib {
13 
14  using namespace std;
15 
17  : _a(a)
18  , _f(f)
19  , _e2(_f * (2 - _f))
20  , _e2m(Math::sq(1 - _f)) // 1 - _e2
21  , _e2a(abs(_e2))
22  , _e4a(Math::sq(_e2))
23  , _maxrad(2 * _a / numeric_limits<real>::epsilon())
24  {
25  if (!(Math::isfinite(_a) && _a > 0))
26  throw GeographicErr("Equatorial radius is not positive");
27  if (!(Math::isfinite(_f) && _f < 1))
28  throw GeographicErr("Polar semi-axis is not positive");
29  }
30 
32  static const Geocentric wgs84(Constants::WGS84_a(), Constants::WGS84_f());
33  return wgs84;
34  }
35 
37  real& X, real& Y, real& Z,
38  real M[dim2_]) const {
39  real sphi, cphi, slam, clam;
40  Math::sincosd(Math::LatFix(lat), sphi, cphi);
41  Math::sincosd(lon, slam, clam);
42  real n = _a/sqrt(1 - _e2 * Math::sq(sphi));
43  Z = (_e2m * n + h) * sphi;
44  X = (n + h) * cphi;
45  Y = X * slam;
46  X *= clam;
47  if (M)
48  Rotation(sphi, cphi, slam, clam, M);
49  }
50 
52  real& lat, real& lon, real& h,
53  real M[dim2_]) const {
54  real
55  R = Math::hypot(X, Y),
56  slam = R != 0 ? Y / R : 0,
57  clam = R != 0 ? X / R : 1;
58  h = Math::hypot(R, Z); // Distance to center of earth
59  real sphi, cphi;
60  if (h > _maxrad) {
61  // We really far away (> 12 million light years); treat the earth as a
62  // point and h, above, is an acceptable approximation to the height.
63  // This avoids overflow, e.g., in the computation of disc below. It's
64  // possible that h has overflowed to inf; but that's OK.
65  //
66  // Treat the case X, Y finite, but R overflows to +inf by scaling by 2.
67  R = Math::hypot(X/2, Y/2);
68  slam = R != 0 ? (Y/2) / R : 0;
69  clam = R != 0 ? (X/2) / R : 1;
70  real H = Math::hypot(Z/2, R);
71  sphi = (Z/2) / H;
72  cphi = R / H;
73  } else if (_e4a == 0) {
74  // Treat the spherical case. Dealing with underflow in the general case
75  // with _e2 = 0 is difficult. Origin maps to N pole same as with
76  // ellipsoid.
77  real H = Math::hypot(h == 0 ? 1 : Z, R);
78  sphi = (h == 0 ? 1 : Z) / H;
79  cphi = R / H;
80  h -= _a;
81  } else {
82  // Treat prolate spheroids by swapping R and Z here and by switching
83  // the arguments to phi = atan2(...) at the end.
84  real
85  p = Math::sq(R / _a),
86  q = _e2m * Math::sq(Z / _a),
87  r = (p + q - _e4a) / 6;
88  if (_f < 0) swap(p, q);
89  if ( !(_e4a * q == 0 && r <= 0) ) {
90  real
91  // Avoid possible division by zero when r = 0 by multiplying
92  // equations for s and t by r^3 and r, resp.
93  S = _e4a * p * q / 4, // S = r^3 * s
94  r2 = Math::sq(r),
95  r3 = r * r2,
96  disc = S * (2 * r3 + S);
97  real u = r;
98  if (disc >= 0) {
99  real T3 = S + r3;
100  // Pick the sign on the sqrt to maximize abs(T3). This minimizes
101  // loss of precision due to cancellation. The result is unchanged
102  // because of the way the T is used in definition of u.
103  T3 += T3 < 0 ? -sqrt(disc) : sqrt(disc); // T3 = (r * t)^3
104  // N.B. cbrt always returns the real root. cbrt(-8) = -2.
105  real T = Math::cbrt(T3); // T = r * t
106  // T can be zero; but then r2 / T -> 0.
107  u += T + (T != 0 ? r2 / T : 0);
108  } else {
109  // T is complex, but the way u is defined the result is real.
110  real ang = atan2(sqrt(-disc), -(S + r3));
111  // There are three possible cube roots. We choose the root which
112  // avoids cancellation. Note that disc < 0 implies that r < 0.
113  u += 2 * r * cos(ang / 3);
114  }
115  real
116  v = sqrt(Math::sq(u) + _e4a * q), // guaranteed positive
117  // Avoid loss of accuracy when u < 0. Underflow doesn't occur in
118  // e4 * q / (v - u) because u ~ e^4 when q is small and u < 0.
119  uv = u < 0 ? _e4a * q / (v - u) : u + v, // u+v, guaranteed positive
120  // Need to guard against w going negative due to roundoff in uv - q.
121  w = max(real(0), _e2a * (uv - q) / (2 * v)),
122  // Rearrange expression for k to avoid loss of accuracy due to
123  // subtraction. Division by 0 not possible because uv > 0, w >= 0.
124  k = uv / (sqrt(uv + Math::sq(w)) + w),
125  k1 = _f >= 0 ? k : k - _e2,
126  k2 = _f >= 0 ? k + _e2 : k,
127  d = k1 * R / k2,
128  H = Math::hypot(Z/k1, R/k2);
129  sphi = (Z/k1) / H;
130  cphi = (R/k2) / H;
131  h = (1 - _e2m/k1) * Math::hypot(d, Z);
132  } else { // e4 * q == 0 && r <= 0
133  // This leads to k = 0 (oblate, equatorial plane) and k + e^2 = 0
134  // (prolate, rotation axis) and the generation of 0/0 in the general
135  // formulas for phi and h. using the general formula and division by 0
136  // in formula for h. So handle this case by taking the limits:
137  // f > 0: z -> 0, k -> e2 * sqrt(q)/sqrt(e4 - p)
138  // f < 0: R -> 0, k + e2 -> - e2 * sqrt(q)/sqrt(e4 - p)
139  real
140  zz = sqrt((_f >= 0 ? _e4a - p : p) / _e2m),
141  xx = sqrt( _f < 0 ? _e4a - p : p ),
142  H = Math::hypot(zz, xx);
143  sphi = zz / H;
144  cphi = xx / H;
145  if (Z < 0) sphi = -sphi; // for tiny negative Z (not for prolate)
146  h = - _a * (_f >= 0 ? _e2m : 1) * H / _e2a;
147  }
148  }
149  lat = Math::atan2d(sphi, cphi);
150  lon = Math::atan2d(slam, clam);
151  if (M)
152  Rotation(sphi, cphi, slam, clam, M);
153  }
154 
155  void Geocentric::Rotation(real sphi, real cphi, real slam, real clam,
156  real M[dim2_]) {
157  // This rotation matrix is given by the following quaternion operations
158  // qrot(lam, [0,0,1]) * qrot(phi, [0,-1,0]) * [1,1,1,1]/2
159  // or
160  // qrot(pi/2 + lam, [0,0,1]) * qrot(-pi/2 + phi , [-1,0,0])
161  // where
162  // qrot(t,v) = [cos(t/2), sin(t/2)*v[1], sin(t/2)*v[2], sin(t/2)*v[3]]
163 
164  // Local X axis (east) in geocentric coords
165  M[0] = -slam; M[3] = clam; M[6] = 0;
166  // Local Y axis (north) in geocentric coords
167  M[1] = -clam * sphi; M[4] = -slam * sphi; M[7] = cphi;
168  // Local Z axis (up) in geocentric coords
169  M[2] = clam * cphi; M[5] = slam * cphi; M[8] = sphi;
170  }
171 
172 } // namespace GeographicLib
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GeographicLib::Geocentric::_e2
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