Class: Graticule::Distance::Vincenty

Inherits:
DistanceFormula show all
Defined in:
lib/graticule/distance/vincenty.rb

Overview

The Vincenty Formula uses an ellipsoidal model of the earth, which is very accurate.

Thanks to Chris Veness (www.movable-type.co.uk/scripts/LatLongVincenty.html) for distance formulas.

Class Method Summary collapse

Methods inherited from DistanceFormula

#initialize

Constructor Details

This class inherits a constructor from Graticule::Distance::DistanceFormula

Class Method Details

.distance(from, to, units = :miles) ⇒ Object

Calculate the distance between two Locations using the Vincenty formula

Graticule::Distance::Vincenty.distance(
  Graticule::Location.new(:latitude => 42.7654, :longitude => -86.1085),
  Graticule::Location.new(:latitude => 41.849838, :longitude => -87.648193)
)
#=> 101.070118000159



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# File 'lib/graticule/distance/vincenty.rb', line 20

def self.distance(from, to, units = :miles)
  from_longitude  = from.longitude.to_radians
  from_latitude   = from.latitude.to_radians
  to_longitude    = to.longitude.to_radians
  to_latitude     = to.latitude.to_radians
  
  earth_major_axis_radius = EARTH_MAJOR_AXIS_RADIUS[units.to_sym]
  earth_minor_axis_radius = EARTH_MINOR_AXIS_RADIUS[units.to_sym]

  f = (earth_major_axis_radius - earth_minor_axis_radius) / earth_major_axis_radius

  l = to_longitude - from_longitude
  u1 = atan((1-f) * tan(from_latitude))
  u2 = atan((1-f) * tan(to_latitude))
  sin_u1 = sin(u1)
  cos_u1 = cos(u1)
  sin_u2 = sin(u2)
  cos_u2 = cos(u2)

  lambda = l
  lambda_p = 2 * PI
  iteration_limit = 20
  while (lambda-lambda_p).abs > 1e-12 && (iteration_limit -= 1) > 0
    sin_lambda = sin(lambda)
    cos_lambda = cos(lambda)
    sin_sigma = sqrt((cos_u2*sin_lambda) * (cos_u2*sin_lambda) + 
      (cos_u1*sin_u2-sin_u1*cos_u2*cos_lambda) * (cos_u1*sin_u2-sin_u1*cos_u2*cos_lambda))
    return 0 if sin_sigma == 0  # co-incident points
    cos_sigma = sin_u1*sin_u2 + cos_u1*cos_u2*cos_lambda
    sigma = atan2(sin_sigma, cos_sigma)
    sin_alpha = cos_u1 * cos_u2 * sin_lambda / sin_sigma
    cosSqAlpha = 1 - sin_alpha*sin_alpha
    cos2SigmaM = cos_sigma - 2*sin_u1*sin_u2/cosSqAlpha

    cos2SigmaM = 0 if cos2SigmaM.nan?  # equatorial line: cosSqAlpha=0 (ยง6)

    c = f/16*cosSqAlpha*(4+f*(4-3*cosSqAlpha))
    lambda_p = lambda
    lambda = l + (1-c) * f * sin_alpha *
      (sigma + c*sin_sigma*(cos2SigmaM+c*cos_sigma*(-1+2*cos2SigmaM*cos2SigmaM)))
 end
 # formula failed to converge (happens on antipodal points)
 # We'll call Haversine formula instead.
 return Haversine.distance(from, to, units) if iteration_limit == 0 

 uSq = cosSqAlpha * (earth_major_axis_radius**2 - earth_minor_axis_radius**2) / (earth_minor_axis_radius**2)
 a = 1 + uSq/16384*(4096+uSq*(-768+uSq*(320-175*uSq)))
 b = uSq/1024 * (256+uSq*(-128+uSq*(74-47*uSq)))
 delta_sigma = b*sin_sigma*(cos2SigmaM+b/4*(cos_sigma*(-1+2*cos2SigmaM*cos2SigmaM)-
   b/6*cos2SigmaM*(-3+4*sin_sigma*sin_sigma)*(-3+4*cos2SigmaM*cos2SigmaM)))
   
  earth_minor_axis_radius * a * (sigma-delta_sigma)
end