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InitialTrueCourse(lat1, lon1, lat2, lon2)

Last updated May 14, 2002

author

Tom Nunamaker

Version: 1 | Requires: CF5 | Library: MathLib

Description:
Calculates the initial true course between two latitudes and longitudes.

Return Values:
Returns a number or an error string.

Example:

<cfoutput>
Example:<br>
Suppose point 1 is LAX: (33deg 57min N, 118deg 24min W)<br>
Suppose point 2 is JFK: (40deg 38min N,  73deg 47min W)<br>

<cfscript>
  Lat1 = 33.95;
  Lon1 = 118.4;
  Lat2 = 40.63333;
  Lon2 = 73.7833;
</cfscript>

Lat1 = #lat1#<br>
Lon1 = #lon1#<br>
Lat2 = #lat2#<br>
Lon2 = #lon2#<br>

Initial True Course from lat1/lon1 = #Numberformat(InitialTrueCourse(lat1,lon1,lat2,lon2),'0.00')# Degrees True Course
</cfoutput>

Parameters:

Name Description Required
lat1 Latitude of the first point in degrees. Yes
lon1 Longitude of the first point in degrees. Yes
lat2 Latitude of the second point in degrees. Yes
lon2 Longitude of the second point in degrees. Yes

Full UDF Source:

/**
 * Calculates the initial true course between two latitudes and longitudes.
 * 
 * @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)
 * @return Returns a number or an error string. 
 * @author Tom Nunamaker (tom@toshop.com) 
 * @version 1, May 14, 2002 
 */
function InitialTrueCourse(lat1,lon1,lat2,lon2)
{
  // 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");
  }
     
  // Calculate distance betweent the two points in radians
  d = LatLonDist(lat1,lon1,lat2,lon2,'radians');


  // Convert latitudes and longitudes to radians and set truc course to zero
  lat1 = lat1 * pi()/180;
  lon1 = lon1 * pi()/180;
  lat2 = lat2 * pi()/180;
  lon2 = lon2 * pi()/180;
  tc1 = 0;  
  
  // Handle the special cases of starting at the poles 
  if(lat1 IS pi()/2)
       Return ( 180 );    //  starting from noth pole
  if(lat1 IS -1*pi()/2)
       Return ( 360 );  //  starting from south pole

  
  if (sin(lon2 - lon1) LT 0)
    tc1 = acos((sin(lat2)-sin(lat1)*cos(d))/(sin(d)*cos(lat1)));
  else
    tc1 = 2*pi()-acos((sin(lat2)-sin(lat1)*cos(d))/(sin(d)*cos(lat1)));  

  Return ( tc1 * 180/pi() );
}
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