Class: OrdnanceSurveyVsTheWorld::OsGridRef

Inherits:
Object
  • Object
show all
Defined in:
lib/ordnance_survey_vs_the_world/os_grid_ref.rb

Constant Summary collapse

A =

Airy 1830 major & minor semi-axes

6377563.396
B = 
6356256.910
E2 =

eccentricity squared

1 - (B*B)/(A*A)
N = 
(A-B)/(A+B)
N2 = 
N*N
N3 = 
N*N*N
F0 =

NatGrid scale factor on central meridian

0.9996012717
LAT_0 =

NatGrid true origin

49*Math::PI/180
LON_0 = 
-2*Math::PI/180
N0 =

Northing & easting of true origin, metres

-100000
E0 = 
400000

Instance Method Summary collapse

Constructor Details

#initialize(easting, northing) ⇒ OsGridRef

Returns a new instance of OsGridRef.



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# File 'lib/ordnance_survey_vs_the_world/os_grid_ref.rb', line 36

def initialize(easting, northing)
  @easting = easting
  @northing = northing
end

Instance Method Details

#airy1830_latitude ⇒ Object 



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# File 'lib/ordnance_survey_vs_the_world/os_grid_ref.rb', line 79

def airy1830_latitude
  lat = LAT_0
  m=0
  begin
    lat = (@northing-N0-m)/(A*F0) + lat

    ma = (1 + N + (5.to_f/4)*N2 + (5.to_f/4)*N3) * (lat-LAT_0)
    mb = (3*N + 3*N*N + (21.to_f/8)*N3) * Math.sin(lat-LAT_0) * Math.cos(lat+LAT_0)
    mc = ((15.to_f/8)*N2 + (15/8)*N3) * Math.sin(2*(lat-LAT_0)) * Math.cos(2*(lat+LAT_0))
    md = (35.to_f/24)*N3 * Math.sin(3*(lat-LAT_0)) * Math.cos(3*(lat+LAT_0))
    m = B * F0 * (ma - mb + mc - md) # meridional arc
  end while (@northing-N0-m >= 0.00001) # until error is < 0.01mm

  lat
end

#to_latlon ⇒ Object 



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# File 'lib/ordnance_survey_vs_the_world/os_grid_ref.rb', line 41

def to_latlon
  lat = airy1830_latitude

  cosLat = Math.cos(lat)
  sinLat = Math.sin(lat)
  nu = A*F0/Math.sqrt(1-E2*sinLat*sinLat)              # transverse radius of curvature
  rho = A*F0*(1-E2)/((1-E2*sinLat*sinLat)**1.5)  # meridional radius of curvature
  eta2 = nu/rho-1

  tanLat = Math.tan(lat)
  tan2lat = tanLat*tanLat
  tan4lat = tan2lat*tan2lat
  tan6lat = tan4lat*tan2lat
  secLat = 1/cosLat
  nu3 = nu*nu*nu
  nu5 = nu3*nu*nu
  nu7 = nu5*nu*nu
  vii = tanLat/(2*rho*nu)
  viii = tanLat/(24*rho*nu3)*(5+3*tan2lat+eta2-9*tan2lat*eta2)
  ix = tanLat/(720*rho*nu5)*(61+90*tan2lat+45*tan4lat)
  x = secLat/nu
  xi = secLat/(6*nu3)*(nu/rho+2*tan2lat)
  xii = secLat/(120*nu5)*(5+28*tan2lat+24*tan4lat)
  xiia = secLat/(5040*nu7)*(61+662*tan2lat+1320*tan4lat+720*tan6lat)

  dE = (@easting-E0)
  dE2 = dE*dE
  dE3 = dE2*dE
  dE4 = dE2*dE2
  dE5 = dE3*dE2
  dE6 = dE4*dE2
  dE7 = dE5*dE2
  lat = lat - vii*dE2 + viii*dE4 - ix*dE6;
  lon = LON_0 + x*dE - xi*dE3 + xii*dE5 - xiia*dE7

  LatLon.new(rad_to_deg(lat), rad_to_deg(lon));
end