Re: Re: Find midpoint between two points

Thanks for pointing out the PHP's deg2rad requirement. That was the problem. ...Rene On 7-Feb-07, at 7:30 PM, Gregory Beaver wrote:

M5 wrote:

I found a nice javascript function that takes two points of latitude and longitude and returns a midpoint. I'm now trying to rewrite in PHP, but having some problems. Here's the original javascript function, taken from http://www.movable-type.co.uk/scripts/LatLong.html : LatLong.midPoint = function(p1, p2) { var dLon = p2.lon - p1.lon; var Bx = Math.cos(p2.lat) * Math.cos(dLon); var By = Math.cos(p2.lat) * Math.sin(dLon); lat3 = Math.atan2(Math.sin(p1.lat)+Math.sin(p2.lat), Math.sqrt((Math.cos(p1.lat)+Bx)*(Math.cos(p1.lat)+Bx) + By*By ) ); lon3 = p1.lon + Math.atan2(By, Math.cos(p1.lat) + Bx); if (isNaN(lat3) || isNaN(lon3)) return null; return new LatLong(lat3*180/Math.PI, lon3*180/Math.PI); } And here's my PHP variant, which isn't working: function midpoint ($lat1, $lng1, $lat2, $lng2) {

    $dlng = $lng2 - $lng1;
    $Bx = cos($lat2) * cos($dlng);
    $By = cos($lat2) * sin($dlng);
    $lat3 = atan2(    sin($lat1)+sin($lat2),

sqrt((cos($lat1)+$Bx)*(cos($lat1)+$Bx) + $By*$By ));

    $lng3 = $lng1 + atan2($By, (cos($lat1) + $Bx));
    $pi = pi();
    return ($lat3*180)/$pi .' '. ($lng3*180)/$pi;
    }

Any ideas why it's returning wrong values?

Are you converting from degrees to radians? With identical input, the javascript function is identical to the PHP function (I tested to verify) I got this by reading at the bottom of the page:

"    *  Notes: trig functions take arguments in radians, so latitude,

longitude, and bearings in degrees (either decimal or degrees/minutes/seconds) need to be converted to radians, rad = π.deg/180. When converting radians back to degrees (deg = 180.rad/π), West is negative if using signed decimal degrees. For bearings, values in the range -π to +π (-180° to +180°) need to be converted to 0 to +2π (0°–360°); this can be done by (brng+2.π)%2.π where % is the modulo operator. View page source to see JavaScript functions to handle these conversions.

    * The atan2() function widely used here takes two arguments,

atan2(y, x), and computes the arc tangent of the ratio y/x. It is more flexible than atan(y/x), since it handles x=0, and it also returns values in all 4 quadrants -π to +π (the atan function returns values in the range -π/2 to +π/2).

    * If you implement any formula involving atan2 in Microsoft Excel,

you will need to reverse the arguments, as Excel has them the opposite way around from JavaScript – conventional order is atan2(y, x), but Excel uses atan2(x, y)

    * For miles, divide km by 1.609344
    * For nautical miles, divide km by 1.852
    * Thanks to Ed Williams’ Aviation Formulary for many of the formulae

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