/**
 * Calculates the distance between two latitudes and longitudes.
 * This funciton uses forumlae from Ed Williams Aviation Foundry website at http://williams.best.vwh.net/avform.htm.
 * 
 * @param lat1      Latitude of the first point in degrees. (Required)
 * @param lon1      Longitude of the first point in degrees. (Required)
 * @param lat2      Latitude of the second point in degrees. (Required)
 * @param lon2      Longitude of the second point in degrees. (Required)
 * @param units      Unit to return distance in. Options are: km (kilometers), sm (statute miles), nm (nautical miles), or radians.  (Required)
 * @return Returns a number or an error string. 
 * @author Tom Nunamaker (tom@toshop.com) 
 * @version 1, May 14, 2002 
 */
 function LatLonDist(lat1,lon1,lat2,lon2,units)
{
  // Check to make sure latitutdes and longitudes are valid
  if(lat1 GT 90 OR lat1 LT -90 OR
     lon1 GT 180 OR lon1 LT -180 OR
     lat2 GT 90 OR lat2 LT -90 OR
     lon2 GT 280 OR lon2 LT -280) {
    Return ("Incorrect parameters");
  }

  lat1 = lat1 * pi()/180;
  lon1 = lon1 * pi()/180;
  lat2 = lat2 * pi()/180;
  lon2 = lon2 * pi()/180;
  UnitConverter = 1.150779448;  //standard is statute miles
  if(units eq 'nm') {
    UnitConverter = 1.0;
  }
  
  if(units eq 'km') {
    UnitConverter = 1.852;
  }
  
  distance = 2*asin(sqr((sin((lat1-lat2)/2))^2 + cos(lat1)*cos(lat2)*(sin((lon1-lon2)/2))^2));  //radians
  
  if(units neq 'radians'){
    distance = UnitConverter * 60 * distance * 180/pi();
  }
  
  Return (distance) ;
}