Departure 

We know that for every 1° of latitude along a meridian a distance of 60 NM is represented. Only at the Equator does 1° of longitude represent 60 NM so, we have a problem calculating distance along a Parallel of Latitude. 

Departure results from the convergence of the meridians at different latitudes, and can be defined as: 

The Earth Distance (ED) between two meridians along the same parallel of latitude (i.e. E/W distance) and is: 

• Normally measured in nm 

• Maximum at the Equator, where 1nm = 1 min Ch long 

• Zero at the poles, where all meridians meet at a single point 

• Departure decreases as the cosine of the Latitude 

The formula for departure is: 

Departure = Ch. Long (in minutes) x cos Lat 

Ch. Long (mins) = Departure ÷ cos Lat 

cos Lat = Departure ÷ Ch. Long (mins) 

Note: It is important to remember that change of longitude is in minutes when used in Departure. 

Example 1 

What is the distance in Nautical Miles between A (60°N 043°E) and 
B (60°N 002°W)? 

Solution 

Departure  = Ch. Long x cos Lat 

= 45° x cos 60 

= 2700’ x 0.5 

= 1350nm 

Example 2 

An aircraft flies from A to B, tracking 090°T for 243nm. The change of Longitude is 4 degrees 45 minutes.  What is the Latitude of A and B? 

Solution 

We know the departure and the Ch Long 

So, we need to calculate the cos Lat, and thence establish the Latitude 

Departure  = Ch Long x cos Lat 

cos Lat  = Departure ÷ Ch long 

= 243nm ÷ 4°45’ 

= 243nm ÷ 285’ 

= 0.8526 

Go to your calculators (shift cos 0.8526 =) 

0.8526 is the cos of 31.5, so the Latitude of A and B is 31°30’N (or S!)