# spirit posteets tagged function

• 7 years ago
Great circle distance
```Great circle distance
Find the distance in kilometres between two points on the surface of the earth. This is just the sort of problem stored functions were made for. For a first order approximation, ignore deviations of the earth's surface from the perfectly spherical. Then the distance in radians is given by a number of trigonometric formulas. ACOS and COS behave reasonably:

COS(lat1-lat2)*(1+COS(lon1-lon2)) - COS(lat1+lat2)*(1-COS(lon1-lon2))
2

We need to convert degrees latitude and longitude to radians, and we need to know the length in km of one radian on the earth's surface, which is 6378.388. The function:

set log_bin_trust_function_creators=TRUE;

DROP FUNCTION IF EXISTS GeoDistKM;
DELIMITER |
CREATE FUNCTION GeoDistKM( lat1 FLOAT, lon1 FLOAT, lat2 FLOAT, lon2 FLOAT ) RETURNS float
BEGIN
DECLARE pi, q1, q2, q3 FLOAT;
SET pi = PI();
SET lat1 = lat1 * pi / 180;
SET lon1 = lon1 * pi / 180;
SET lat2 = lat2 * pi / 180;
SET lon2 = lon2 * pi / 180;
SET q1 = COS(lon1-lon2);
SET q2 = COS(lat1-lat2);
SET q3 = COS(lat1+lat2);
SET rads = ACOS( 0.5*((1.0+q1)*q2 - (1.0-q1)*q3) );
END;
|
DELIMITER ;

-- toronto to montreal (505km):
select geodistkm(43.6667,-79.4167,45.5000,-73.5833);
+----------------------------------------------+
| geodistkm(43.6667,-79.4167,45.5000,-73.5833) |
+----------------------------------------------+
|                           505.38836669921875 |
+----------------------------------------------+```
• 8 years ago
1. var myfunc = function(optional) {
2.   if (typeof optional == “undefined”) {
3.     optional = “default value”;
4.   }