## InitialTrueCourse(lat1, lon1, lat2, lon2)

##### Last updated May 14, 2002

**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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