Module: OsgbConvert::Converter

Included in:
Coordinate
Defined in:
lib/osgb_convert.rb

Constant Summary collapse

ELLIPSE =

ellipse parameters

{ 
  :wgs84 =>    { :a=> 6378137,     :b=> 6356752.3142, :f=> 1 / 298.257223563 },
  :airy1830 => { :a=> 6377563.396, :b=> 6356256.910,  :f=> 1 / 299.3249646   } 
}
HELMERT =

helmert transform parameters

{ 
  :wgs84toOSGB36 => { 
    :tx=> -446.448,  :ty=>  125.157,  :tz=> -542.060,  # m
    :rx=>  -0.1502,  :ry=>   -0.2470, :rz=>  -0.8421,  # sec
    :s=>   20.4894                                     # ppm
  },
  :osgb36toWGS84 => { 
    :tx=>  446.448,  :ty=> -125.157,  :tz=>  542.060,
    :rx=>    0.1502, :ry=>    0.2470, :rz=>    0.8421,
    :s=>   -20.4894
  }
}

Instance Method Summary collapse

Instance Method Details

#convert(p1lat, p1lon, p1height, e1, t, e2) ⇒ Object 



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# File 'lib/osgb_convert.rb', line 31

def convert(p1lat, p1lon, p1height, e1, t, e2)
   # -- convert polar to cartesian coordinates (using ellipse 1)

   p1lat = OsgbConvert.degrees_to_rads(p1lat); p1lon = OsgbConvert.degrees_to_rads(p1lon); 

   a = e1[:a]; b = e1[:b];

   sinPhi = Math.sin(p1lat); cosPhi = Math.cos(p1lat);
   sinLambda = Math.sin(p1lon); cosLambda = Math.cos(p1lon);
   h = p1height;

   eSq = (a*a - b*b) / (a*a);
   nu = a / Math.sqrt(1 - eSq*sinPhi*sinPhi);

   x1 = (nu+h) * cosPhi * cosLambda;
   y1 = (nu+h) * cosPhi * sinLambda;
   z1 = ((1-eSq)*nu + h) * sinPhi;

   # -- apply helmert transform using appropriate params

   tx = t[:tx]; ty = t[:ty]; tz = t[:tz];
   rx = t[:rx] / 3600 * Math::PI/180;  #normalise seconds to radians
   ry = t[:ry] / 3600 * Math::PI/180;
   rz = t[:rz] / 3600 * Math::PI/180;
   s1 = t[:s] / 1e6 + 1;              #normalise ppm to (s+1)

   #apply transform
   x2 = tx + x1*s1 - y1*rz + z1*ry;
   y2 = ty + x1*rz + y1*s1 - z1*rx;
   z2 = tz - x1*ry + y1*rx + z1*s1;

   # -- convert cartesian to polar coordinates (using ellipse 2)

   a = e2[:a]; b = e2[:b];
   precision = 4 / a;  # results accurate to around 4 metres

   eSq = (a*a - b*b) / (a*a);
   p = Math.sqrt(x2*x2 + y2*y2);
   phi = Math.atan2(z2, p*(1-eSq)); phiP = 2 * Math::PI;
   while ( (phi-phiP).abs > precision) do
      nu = a / Math.sqrt(1 - eSq*Math.sin(phi)*Math.sin(phi));
      phiP = phi;
      phi = Math.atan2(z2 + eSq*nu*Math.sin(phi), p);
   end
   lambda = Math.atan2(y2, x2);
   h = p/Math.cos(phi) - nu;

   #return array [lat,lon,height]
   return [ OsgbConvert.rads_to_degrees(phi), OsgbConvert.rads_to_degrees(lambda), h ]; 
end