UTM.cpp

// UTM.c

// Original Javascript by Chuck Taylor
// Port to C++ by Alex Hajnal
//
// *** THIS CODE USES 32-BIT FLOATS BY DEFAULT ***
// *** For 64-bit double-precision edit UTM.h: undefine FLOAT_32 and define FLOAT_64
// 
// This is a simple port of the code on the Geographic/UTM Coordinate Converter (1) page from Javascript to C++.
// Using this you can easily convert between UTM and WGS84 (latitude and longitude).
// Accuracy seems to be around 50cm (I suspect rounding errors are limiting precision).
// This code is provided as-is and has been minimally tested; enjoy but use at your own risk!
// The license for UTM.cpp and UTM.h is the same as the original Javascript: 
// "The C++ source code in UTM.cpp and UTM.h may be copied and reused without restriction."
// 
// 1) http://home.hiwaay.net/~taylorc/toolbox/geography/geoutm.html

#include "UTM.h"

// DegToRad
// Converts degrees to radians.
FLOAT DegToRad(FLOAT deg) {
  return (deg / 180.0 * pi);
}


// RadToDeg
// Converts radians to degrees.
FLOAT RadToDeg(FLOAT rad) {
  return (rad / pi * 180.0);
}

// ArcLengthOfMeridian
// Computes the ellipsoidal distance from the equator to a point at a
// given latitude.
// 
// Reference: Hoffmann-Wellenhof, B., Lichtenegger, H., and Collins, J.,
// GPS: Theory and Practice, 3rd ed.  New York: Springer-Verlag Wien, 1994.
// 
// Inputs:
//     phi - Latitude of the point, in radians.
// 
// Globals:
//     sm_a - Ellipsoid model major axis.
//     sm_b - Ellipsoid model minor axis.
// 
// Returns:
//     The ellipsoidal distance of the point from the equator, in meters.
FLOAT ArcLengthOfMeridian (FLOAT phi) {
  FLOAT alpha, beta, gamma, delta, epsilon, n;
  FLOAT result;
  
  /* Precalculate n */
  n = (sm_a - sm_b) / (sm_a + sm_b);
  
  /* Precalculate alpha */
  alpha = ((sm_a + sm_b) / 2.0)
        * (1.0 + (POW(n, 2.0) / 4.0) + (POW(n, 4.0) / 64.0));
  
  /* Precalculate beta */
  beta = (-3.0 * n / 2.0) + (9.0 * POW(n, 3.0) / 16.0)
       + (-3.0 * POW(n, 5.0) / 32.0);
  
  /* Precalculate gamma */
  gamma = (15.0 * POW(n, 2.0) / 16.0)
        + (-15.0 * POW(n, 4.0) / 32.0);
  
  /* Precalculate delta */
  delta = (-35.0 * POW(n, 3.0) / 48.0)
      + (105.0 * POW(n, 5.0) / 256.0);
  
  /* Precalculate epsilon */
  epsilon = (315.0 * POW(n, 4.0) / 512.0);
  
  /* Now calculate the sum of the series and return */
  result = alpha
         * (phi + (beta * SIN(2.0 * phi))
         + (gamma * SIN(4.0 * phi))
         + (delta * SIN(6.0 * phi))
         + (epsilon * SIN(8.0 * phi)));
  
  return result;
}



// UTMCentralMeridian
// Determines the central meridian for the given UTM zone.
//
// Inputs:
//     zone - An integer value designating the UTM zone, range [1,60].
//
// Returns:
//   The central meridian for the given UTM zone, in radians
//   Range of the central meridian is the radian equivalent of [-177,+177].
FLOAT UTMCentralMeridian(int zone) {
  FLOAT cmeridian;
  cmeridian = DegToRad(-183.0 + ((FLOAT)zone * 6.0));
  
  return cmeridian;
}



// FootpointLatitude
//
// Computes the footpoint latitude for use in converting transverse
// Mercator coordinates to ellipsoidal coordinates.
//
// Reference: Hoffmann-Wellenhof, B., Lichtenegger, H., and Collins, J.,
//   GPS: Theory and Practice, 3rd ed.  New York: Springer-Verlag Wien, 1994.
//
// Inputs:
//   y - The UTM northing coordinate, in meters.
//
// Returns:
//   The footpoint latitude, in radians.
FLOAT FootpointLatitude(FLOAT y) {
  FLOAT y_, alpha_, beta_, gamma_, delta_, epsilon_, n;
  FLOAT result;
  
  /* Precalculate n (Eq. 10.18) */
  n = (sm_a - sm_b) / (sm_a + sm_b);
    
  /* Precalculate alpha_ (Eq. 10.22) */
  /* (Same as alpha in Eq. 10.17) */
  alpha_ = ((sm_a + sm_b) / 2.0)
         * (1 + (POW(n, 2.0) / 4) + (POW(n, 4.0) / 64));
  
  /* Precalculate y_ (Eq. 10.23) */
  y_ = y / alpha_;
  
  /* Precalculate beta_ (Eq. 10.22) */
  beta_ = (3.0 * n / 2.0) + (-27.0 * POW(n, 3.0) / 32.0)
        + (269.0 * POW(n, 5.0) / 512.0);
  
  /* Precalculate gamma_ (Eq. 10.22) */
  gamma_ = (21.0 * POW(n, 2.0) / 16.0)
         + (-55.0 * POW(n, 4.0) / 32.0);
    
  /* Precalculate delta_ (Eq. 10.22) */
  delta_ = (151.0 * POW(n, 3.0) / 96.0)
         + (-417.0 * POW(n, 5.0) / 128.0);
    
  /* Precalculate epsilon_ (Eq. 10.22) */
  epsilon_ = (1097.0 * POW(n, 4.0) / 512.0);
    
  /* Now calculate the sum of the series (Eq. 10.21) */
  result = y_ + (beta_ * SIN(2.0 * y_))
         + (gamma_ * SIN(4.0 * y_))
         + (delta_ * SIN(6.0 * y_))
         + (epsilon_ * SIN(8.0 * y_));
  
  return result;
}



// MapLatLonToXY
// Converts a latitude/longitude pair to x and y coordinates in the
// Transverse Mercator projection.  Note that Transverse Mercator is not
// the same as UTM; a scale factor is required to convert between them.
//
// Reference: Hoffmann-Wellenhof, B., Lichtenegger, H., and Collins, J.,
// GPS: Theory and Practice, 3rd ed.  New York: Springer-Verlag Wien, 1994.
//
// Inputs:
//    phi - Latitude of the point, in radians.
//    lambda - Longitude of the point, in radians.
//    lambda0 - Longitude of the central meridian to be used, in radians.
//
// Outputs:
//    x - The x coordinate of the computed point.
//    y - The y coordinate of the computed point.
//
// Returns:
//    The function does not return a value.
void MapLatLonToXY (FLOAT phi, FLOAT lambda, FLOAT lambda0, FLOAT &x, FLOAT &y) {
    FLOAT N, nu2, ep2, t, t2, l;
    FLOAT l3coef, l4coef, l5coef, l6coef, l7coef, l8coef;
    //FLOAT tmp; // Unused

    /* Precalculate ep2 */
    ep2 = (POW(sm_a, 2.0) - POW(sm_b, 2.0)) / POW(sm_b, 2.0);

    /* Precalculate nu2 */
    nu2 = ep2 * POW(COS(phi), 2.0);

    /* Precalculate N */
    N = POW(sm_a, 2.0) / (sm_b * SQRT(1 + nu2));

    /* Precalculate t */
    t = TAN(phi);
    t2 = t * t;
    //tmp = (t2 * t2 * t2) - POW(t, 6.0); // Unused

    /* Precalculate l */
    l = lambda - lambda0;

    /* Precalculate coefficients for l**n in the equations below
       so a normal human being can read the expressions for easting
       and northing
       -- l**1 and l**2 have coefficients of 1.0 */
    l3coef = 1.0 - t2 + nu2;

    l4coef = 5.0 - t2 + 9 * nu2 + 4.0 * (nu2 * nu2);

    l5coef = 5.0 - 18.0 * t2 + (t2 * t2) + 14.0 * nu2
        - 58.0 * t2 * nu2;

    l6coef = 61.0 - 58.0 * t2 + (t2 * t2) + 270.0 * nu2
        - 330.0 * t2 * nu2;

    l7coef = 61.0 - 479.0 * t2 + 179.0 * (t2 * t2) - (t2 * t2 * t2);

    l8coef = 1385.0 - 3111.0 * t2 + 543.0 * (t2 * t2) - (t2 * t2 * t2);

    /* Calculate easting (x) */
    x = N * COS(phi) * l
        + (N / 6.0 * POW(COS(phi), 3.0) * l3coef * POW(l, 3.0))
        + (N / 120.0 * POW(COS(phi), 5.0) * l5coef * POW(l, 5.0))
        + (N / 5040.0 * POW(COS(phi), 7.0) * l7coef * POW(l, 7.0));

    /* Calculate northing (y) */
    y = ArcLengthOfMeridian (phi)
        + (t / 2.0 * N * POW(COS(phi), 2.0) * POW(l, 2.0))
        + (t / 24.0 * N * POW(COS(phi), 4.0) * l4coef * POW(l, 4.0))
        + (t / 720.0 * N * POW(COS(phi), 6.0) * l6coef * POW(l, 6.0))
        + (t / 40320.0 * N * POW(COS(phi), 8.0) * l8coef * POW(l, 8.0));

    return;
}



// MapXYToLatLon
// Converts x and y coordinates in the Transverse Mercator projection to
// a latitude/longitude pair.  Note that Transverse Mercator is not
// the same as UTM; a scale factor is required to convert between them.
//
// Reference: Hoffmann-Wellenhof, B., Lichtenegger, H., and Collins, J.,
//   GPS: Theory and Practice, 3rd ed.  New York: Springer-Verlag Wien, 1994.
//
// Inputs:
//   x - The easting of the point, in meters.
//   y - The northing of the point, in meters.
//   lambda0 - Longitude of the central meridian to be used, in radians.
//
// Outputs:
//   phi    - Latitude in radians.
//   lambda - Longitude in radians.
//
// Returns:
//   The function does not return a value.
//
// Remarks:
//   The local variables Nf, nuf2, tf, and tf2 serve the same purpose as
//   N, nu2, t, and t2 in MapLatLonToXY, but they are computed with respect
//   to the footpoint latitude phif.
//
//   x1frac, x2frac, x2poly, x3poly, etc. are to enhance readability and
//   to optimize computations.
void MapXYToLatLon (FLOAT x, FLOAT y, FLOAT lambda0, FLOAT& phi, FLOAT& lambda)
{
  FLOAT phif, Nf, Nfpow, nuf2, ep2, tf, tf2, tf4, cf;
  FLOAT x1frac, x2frac, x3frac, x4frac, x5frac, x6frac, x7frac, x8frac;
  FLOAT x2poly, x3poly, x4poly, x5poly, x6poly, x7poly, x8poly;
  
  /* Get the value of phif, the footpoint latitude. */
  phif = FootpointLatitude (y);
    
  /* Precalculate ep2 */
  ep2 = (POW(sm_a, 2.0) - POW(sm_b, 2.0))
      / POW(sm_b, 2.0);
    
  /* Precalculate cos (phif) */
  cf = COS(phif);
    
  /* Precalculate nuf2 */
  nuf2 = ep2 * POW(cf, 2.0);
    
  /* Precalculate Nf and initialize Nfpow */
  Nf = POW(sm_a, 2.0) / (sm_b * SQRT(1 + nuf2));
  Nfpow = Nf;
    
  /* Precalculate tf */
  tf = TAN(phif);
  tf2 = tf * tf;
  tf4 = tf2 * tf2;
  
  /* Precalculate fractional coefficients for x**n in the equations
     below to simplify the expressions for latitude and longitude. */
  x1frac = 1.0 / (Nfpow * cf);
  
  Nfpow *= Nf;   /* now equals Nf**2) */
  x2frac = tf / (2.0 * Nfpow);
  
  Nfpow *= Nf;   /* now equals Nf**3) */
  x3frac = 1.0 / (6.0 * Nfpow * cf);
  
  Nfpow *= Nf;   /* now equals Nf**4) */
  x4frac = tf / (24.0 * Nfpow);
  
  Nfpow *= Nf;   /* now equals Nf**5) */
  x5frac = 1.0 / (120.0 * Nfpow * cf);
  
  Nfpow *= Nf;   /* now equals Nf**6) */
  x6frac = tf / (720.0 * Nfpow);
  
  Nfpow *= Nf;   /* now equals Nf**7) */
  x7frac = 1.0 / (5040.0 * Nfpow * cf);
  
  Nfpow *= Nf;   /* now equals Nf**8) */
  x8frac = tf / (40320.0 * Nfpow);
  
  /* Precalculate polynomial coefficients for x**n.
     -- x**1 does not have a polynomial coefficient. */
  x2poly = -1.0 - nuf2;
  
  x3poly = -1.0 - 2 * tf2 - nuf2;
  
  x4poly = 5.0 + 3.0 * tf2 + 6.0 * nuf2 - 6.0 * tf2 * nuf2
         - 3.0 * (nuf2 *nuf2) - 9.0 * tf2 * (nuf2 * nuf2);
  
  x5poly = 5.0 + 28.0 * tf2 + 24.0 * tf4 + 6.0 * nuf2 + 8.0 * tf2 * nuf2;
  
  x6poly = -61.0 - 90.0 * tf2 - 45.0 * tf4 - 107.0 * nuf2
         + 162.0 * tf2 * nuf2;
  
  x7poly = -61.0 - 662.0 * tf2 - 1320.0 * tf4 - 720.0 * (tf4 * tf2);
  
  x8poly = 1385.0 + 3633.0 * tf2 + 4095.0 * tf4 + 1575 * (tf4 * tf2);
    
  /* Calculate latitude */
  phi = phif + x2frac * x2poly * (x * x)
      + x4frac * x4poly * POW(x, 4.0)
      + x6frac * x6poly * POW(x, 6.0)
      + x8frac * x8poly * POW(x, 8.0);
    
  /* Calculate longitude */
  lambda = lambda0 + x1frac * x
         + x3frac * x3poly * POW(x, 3.0)
         + x5frac * x5poly * POW(x, 5.0)
         + x7frac * x7poly * POW(x, 7.0);
    
  return;
}




// LatLonToUTMXY
// Converts a latitude/longitude pair to x and y coordinates in the
// Universal Transverse Mercator projection.
//
// Inputs:
//   lat - Latitude of the point, in radians.
//   lon - Longitude of the point, in radians.
//   zone - UTM zone to be used for calculating values for x and y.
//          If zone is less than 1 or greater than 60, the routine
//          will determine the appropriate zone from the value of lon.
//
// Outputs:
//   x - The x coordinate (easting) of the computed point. (in meters)
//   y - The y coordinate (northing) of the computed point. (in meters)
//
// Returns:
//   The UTM zone used for calculating the values of x and y.
int LatLonToUTMXY (FLOAT lat, FLOAT lon, int zone, FLOAT& x, FLOAT& y) {
  if ( (zone < 1) || (zone > 60) )
    zone = FLOOR((lon + 180.0) / 6) + 1;
  
  MapLatLonToXY (DegToRad(lat), DegToRad(lon), UTMCentralMeridian(zone), x, y);
  
  /* Adjust easting and northing for UTM system. */
  x = x * UTMScaleFactor + 500000.0;
  y = y * UTMScaleFactor;
  if (y < 0.0)
    y = y + 10000000.0;
  
  return zone;
}



// UTMXYToLatLon
//
// Converts x and y coordinates in the Universal Transverse Mercator
// projection to a latitude/longitude pair.
//
// Inputs:
// x - The easting of the point, in meters.
// y - The northing of the point, in meters.
// zone - The UTM zone in which the point lies.
// southhemi - True if the point is in the southern hemisphere;
//               false otherwise.
//
// Outputs:
// lat - The latitude of the point, in radians.
// lon - The longitude of the point, in radians.
// 
// Returns:
// The function does not return a value.
void UTMXYToLatLon (FLOAT x, FLOAT y, int zone, bool southhemi, FLOAT& lat, FLOAT& lon) {
  FLOAT cmeridian;
    
  x -= 500000.0;
  x /= UTMScaleFactor;
    
  /* If in southern hemisphere, adjust y accordingly. */
  if (southhemi)
    y -= 10000000.0;
      
  y /= UTMScaleFactor;
  
  cmeridian = UTMCentralMeridian (zone);
  MapXYToLatLon (x, y, cmeridian, lat, lon);
    
  return;
}