All knowledge about Ship Point, coastal navigation, true bearing, and magnetism is important, but even more important is to keep practicing.
In this short guide we will see how polar bearing works in practice and what the graphometer is.
What is the graphometer
To understand how a graphometer works, we must first talk about polar bearing.
Draw a half-line between a conspicuous point observed in the distance, such as a lighthouse, and yourself (observer).
Polar bearing is defined as the angle between the longitudinal axis of the naval unit, i.e., the bow, and the plane containing the half-line just plotted.
The graphometer is nothing more than a circular or semicircular object used to make polar bearings.
Semicircular graphometer
The semicircular graphometer is mounted on board so that the 0-180 axis of the goniometer corresponds with the longitudinal axis of the hull. Here the zero degree corresponds to the bow direction and the 180 degree to the stern direction. Consequently, the transom of the boat will be at the value of 90 degrees.
When the object is on the starboard side, then the polar bearing is positive.
On the other hand, when the object is on the port side, it is negative.
Circular graphometer
If the graphometer is circular, then detection is also positive at the port side of the vessel, since the gradation is from 0 to 360 degrees
Transverse bearing
This is done by observing a conspicuous point at 90° to the longitude of the boat (caution: so not to the course sailed, but to the heading held).
Ideally, imagine the observer and the conspicuous point within a compass rose. The observer will consequently see the conspicuous point with an angular distance of 95 degrees, and relative to the observer the conspicuous point will be to the southeast.
Consequently, the observer will be northwest of the conspicuous point.
Here are two standard question you should ask yourself, to see if you understood the concept:
1) If the observer detects the conspicuous point at 95°, what position does the observer have relative to the lighthouse?
2) Given the direction of the observer relative to the conspicuous point: for how many degrees of true bearing does The Observer detect the conspicuous point?
Also, you could practice interpreting the visibility sector of a lighthouse; visibility sectors are shown in the eighth column of the list of lighthouses and fog signals with true bearing values taken from the sea.