def ellipsoidal_distance(point, a = 6378137.0, b = 6356752.3142)
deg_to_rad = 0.0174532925199433
f = (a-b) / a
l = (point.lon - lon) * deg_to_rad
u1 = Math.atan((1-f) * Math.tan(lat * deg_to_rad ))
u2 = Math.atan((1-f) * Math.tan(point.lat * deg_to_rad))
sinU1 = Math.sin(u1)
cosU1 = Math.cos(u1)
sinU2 = Math.sin(u2)
cosU2 = Math.cos(u2)
lambda = l
lambdaP = 2 * Math::PI
iterLimit = 20
while (lambda-lambdaP).abs > 1e-12 && --iterLimit>0
sinLambda = Math.sin(lambda)
cosLambda = Math.cos(lambda)
sinSigma = Math.sqrt((cosU2*sinLambda) * (cosU2*sinLambda) + (cosU1*sinU2-sinU1*cosU2*cosLambda) * (cosU1*sinU2-sinU1*cosU2*cosLambda))
return 0 if sinSigma == 0
cosSigma = sinU1*sinU2 + cosU1*cosU2*cosLambda
sigma = Math.atan2(sinSigma, cosSigma)
sinAlpha = cosU1 * cosU2 * sinLambda / sinSigma
cosSqAlpha = 1 - sinAlpha*sinAlpha
cos2SigmaM = cosSigma - 2*sinU1*sinU2/cosSqAlpha
cos2SigmaM = 0 if (cos2SigmaM.nan?)
c = f/16*cosSqAlpha*(4+f*(4-3*cosSqAlpha))
lambdaP = lambda
lambda = l + (1-c) * f * sinAlpha * (sigma + c * sinSigma * (cos2SigmaM + c * cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)))
end
return NaN if iterLimit==0
uSq = cosSqAlpha * (a*a - b*b) / (b*b)
a_bis = 1 + uSq/16384*(4096+uSq*(-768+uSq*(320-175*uSq)))
b_bis = uSq/1024 * (256+uSq*(-128+uSq*(74-47*uSq)))
deltaSigma = b_bis * sinSigma*(cos2SigmaM + b_bis/4*(cosSigma*(-1+2*cos2SigmaM*cos2SigmaM)- b_bis/6*cos2SigmaM*(-3+4*sinSigma*sinSigma)*(-3+4*cos2SigmaM*cos2SigmaM)))
b*a_bis*(sigma-deltaSigma)
end