Great Circle Sailing : shortest distance measured along the earth's surface between two places is called the Great Circle Distance. The departure and arrival positions are marked on a Gnomonic Chart and joined by a straight line. Waypoints are transferred from Gnomonic to Mercator chart and it appears as a Curved line on Mercator chart.

Mercator Sailing : Used to find the Rhumb Line course and distance between two positions. It uses the following formula -
tan Course = dLong / DMP
This holds good when the distance is more than 600 NM. For distances lesser than 600 NM, we use Parallel sailing. The formula is given by -
Cos mLat = dep / dLong

Rhumb Line: Rhumb Line is a line drawn on a Mercator Chart which cuts all meridian at same angles.

Composite Sailing: Sometimes, it is not possible to execute a full great circle for a voyage. This could be due to presence of navigational dangers in between OR if the Master decides not to go beyond a certain latitude owing to weather conditions. In this case, a combination of rhumb line and great circle sailing. Together this type of sailing is called as Composite Sailing.