# How to compute distance between 2 GPS points?

In one of my projects I was looking for simple way how to compute direct distance between 2 arbitrary airports in the world. Simply, I was looking for a way to measure distance between 2 GPS points. Naturally there exist several ways how to compute such thing. After research I found that many people use algorithms derived from Great circle distance method [wiki].

Here is a PHP code:

```    /**
* Great Circle distance algorithm
*
* Notes:
* - South and West locations are negative, North and East are positive.
* - Automatically transfer degrees to radians
* - https://en.wikipedia.org/wiki/Great-circle_distance
*
* @param lat1 - latitude of first point on sphere
* @param lon1 - longitude of first point on sphere
* @param lat2 - latitude of second point on sphere
* @param lon2 - longitude of second point on sphere
* @param unit - units for output (miles, kilometers, nautical miles)
* @return - distance in desired distance units
*
*/
function GCdistance(\$lat1, \$lon1, \$lat2, \$lon2, \$unit) {
\$theta = \$lon1 - \$lon2;
\$dist = acos(\$dist);
\$miles = \$dist * 60 * 1.1515;
\$unit = strtoupper(\$unit);

switch (\$unit) {
case 'MI':
return \$miles;
break;
case 'KM':
return (\$miles * 1.609344);
break;
case 'NM':
return (\$miles * 0.8684);
break;
}
}```

There are 2 simple corrections how to simplifie this algorithm more. First by padding all the sub-results into one big equation. And second, by ensuring the the input units will be always the same (using numbers instead of Strings) strtoupper() function can be removed.

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