Piloting

Piloting is a form of navigation that relies entirely on landmarks and water depth.  It's only useful in view of the shoreline, because the navigator uses landmarks such as prominent rocks, lighthouses, cliffs, and the depths of the ocean below to determine where the ship is. 

There's a good deal of math, namely geometry, involved with piloting.  The navigator generally needs a compass and a navigational chart to use this method, using bearings of landmarks.  Bearings are the horizontal angles between an object and a reference point.  Bearings are generally measured clockwise from 000° at the reference point through 360° and expressed in three digits.

To determine the ship's position, the navigator uses two bearings.  Both bearings are drawn on a chart, forming two lines of position which intersect at the ship's position.  Various instruments (elaborated on in the "Tools" section) used for sighting the degrees of bearings are the alidade, the pelorus, and the azimuth circle. 

Sometimes only one landmark can be used, and in that case the navigator first must find the landmark's bearing and then must use a range finder, which measures the distance to any clearly defined object, or the stadimeter, which measures distance only if the height of the landmark is known.

Depth is also used to find the ship's position.  In medieval times, the hand lead and line were used.  This was a marked cord with a weight on the end, and the method was sometimes referred to as plumbing the ocean's depths, or simply "plumbing."

For more information:

• Piloting-Dead Reckoning