Converting Easting/Northing to Latitude/Longitude

elwappo · November 20th, 2008, 11:42 AM

Does anybody have a pre-written class that can do this in C#? I need to be able to pass it an UK easting/northing (e.g. 262600, 191900) and return a Lat/Long (e.g. 51.3456, -3.17624).

Your help would be most appreciated.
Elwap.

samjudson · November 20th, 2008, 11:52 AM

Here is your answer:

http://blogs.msdn.com/oldnewthing/ar...4/9068064.aspx

/- Sam Judson : Wrox Technical Editor -/

elwappo · November 20th, 2008, 02:12 PM

Why thank you. Your answer was most informative. I'll be sure to seek advice here again.

samjudson · November 20th, 2008, 02:26 PM

:)

It was meant to be a funnier way of saying, "I don't know"

I have written the code in PHP, but I don't know where I put it, so it's not that hard (provided you don't try to understand the math).

/- Sam Judson : Wrox Technical Editor -/

elwappo · November 20th, 2008, 04:31 PM

I certainly don't want to understand the math, as I've seen the formulas. That's why we have encapsulation :)

If you do find the code, I'd really appreciate it. Or if you can point me to a website which has a no frills way of describing how to implement the formula, I'd be most grateful. There must be a converter out there. Here's great online VB.NET to C# converter I use for code snippets all the time:

http://www.developerfusion.com/tools.../vb-to-csharp/

samjudson · November 21st, 2008, 05:20 AM

Code:

<?php

$lon = 0;
$lat = 0;

function convertXToLong($xCoord, $yCoord)
{
    global $lat, $lon;

  $E = $xCoord;
  $N = $yCoord;

  $a = 6377563.396;
  $b = 6356256.910;              // Airy 1830 major & minor semi-axes
  $F0 = 0.9996012717;                             // NatGrid scale factor on central meridian
  $lat0 = 49*pi()/180;
  $lon0 = -2*pi()/180;  // NatGrid true origin
  $N0 = -100000;
  $E0 = 400000;                     // northing & easting of true origin, metres

  $e2 = 1 - ($b*$b)/($a*$a);                          // eccentricity squared
  $n = ($a-$b)/($a+$b);
  $n2 = $n*$n;
  $n3 = $n*$n*$n;

  $lat=$lat0;
  $M=0;
  do {
    $lat = ($N-$N0-$M)/($a*$F0) + $lat;

    $Ma = (1 + $n + (5/4)*$n2 + (5/4)*$n3) * ($lat-$lat0);
    $Mb = (3*$n + 3*$n*$n + (21/8)*$n3) * sin($lat-$lat0) * cos($lat+$lat0);
    $Mc = ((15/8)*$n2 + (15/8)*$n3) * sin(2*($lat-$lat0)) * cos(2*($lat+$lat0));
    $Md = (35/24)*$n3 * sin(3*($lat-$lat0)) * cos(3*($lat+$lat0));
    $M = $b * $F0 * ($Ma - $Mb + $Mc - $Md);             // meridional arc

  } while ($N-$N0-$M >= 0.00001);  // ie until < 0.01mm

  $cosLat = cos($lat);
  $sinLat = sin($lat);
  $nu = $a*$F0/sqrt(1-$e2*$sinLat*$sinLat);              // transverse radius of curvature
  $rho = $a*$F0*(1-$e2)/pow(1-$e2*$sinLat*$sinLat, 1.5);  // meridional radius of curvature
  $eta2 = $nu/$rho-1;

  $tanLat = tan($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);

  $dE = ($E-$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 = $lon0 + $X*$dE - $XI*$dE3 + $XII*$dE5 - $XIIA*$dE7;

  $lat = $lat*180/pi();
  $lon = $lon*180/pi();
}
?>

/- Sam Judson : Wrox Technical Editor -/

elwappo · November 21st, 2008, 06:41 AM

Thanks, that worked a treat. :)