# Calculate a Destination Coordinate based on Distance and Bearing in PHP

Today I had to calculate a bunch of geo coordinates. All I had was a starting point and distance and direction of the destination. I googled a bit and soon found a nice description along with some JavaScript code.

Should be easy to port to PHP. Or so I thought. For some reason it simply didn't work. I got no errors but the resulting points were way off. A bit more googling found me a similar solution in python. But again my port didn't work.

When I started to carefully compare all internal variable values in each step between my port and the Python script, I finally found the problem. PHP's standard modulo operator `%`

does only return integer remainders! Replacing the `%`

with a call to fmod fixed everything. Learned a new thing today .

Here's the final code:

/** * Calculate a new coordinate based on start, distance and bearing * * @param $start array - start coordinate as decimal lat/lon pair * @param $dist float - distance in kilometers * @param $brng float - bearing in degrees (compass direction) */ function geo_destination($start,$dist,$brng){ $lat1 = toRad($start[0]); $lon1 = toRad($start[1]); $dist = $dist/6371.01; //Earth's radius in km $brng = toRad($brng); $lat2 = asin( sin($lat1)*cos($dist) + cos($lat1)*sin($dist)*cos($brng) ); $lon2 = $lon1 + atan2(sin($brng)*sin($dist)*cos($lat1), cos($dist)-sin($lat1)*sin($lat2)); $lon2 = fmod(($lon2+3*pi()),(2*pi())) - pi(); return array(toDeg($lat2),toDeg($lon2)); } function toRad($deg){ return $deg * pi() / 180; } function toDeg($rad){ return $rad * 180 / pi(); }

This post was originally published on cosmocode.de