We have seen how to calculate estimated navigation and how to estimate the estimated ship point accordingly, but this remains an uncertain parameter.
While in navigation away from the coast, unless using a GPS, we are always unsure of our actual position, in coastal navigation it is easier.
In coastal navigation, in fact, one can determine the ship’s point, which is a certain position, through the use of true bearings.
True bearings and conspicuous spots
To locate your ship’s point during coastal navigation, you need to recognize so-called conspicuous spots. Conspicuous spots are objects that are easily recognizable from the sea at a glance. They can then be traced on the nautical chart.
Portolans often indicate them clearly, and in general they are islands, lighthouses, towers and similar structures.
How to calculate the Ship Point
It’s a step-by-step process:
- You need to use a compass, which allows you to measure the value in degrees of the line joining the conspicuous spot, which you can see on the horizon, with your point of observation;
- This line (a half-line) passes through a given compass graduation: from here it is possible to measure the angle with respect to the North of the conspicuous spot;
- This angle should now be adjusted with the declination value. Then, you can plot it on the chart;
- At this point it is necessary to find a second conspicuous spot;
- Now the two angular values of conspicuous spots must be plotted and at the point where the two lines between the observer and the conspicuous points join.
The concept of ship’s position
It is essential to learn to understand that when you are on a boat you should consider certain factors that terrestrial orientation ignores.
For example, when you are sailing you need to understand the concept of ship’s position: it is a set of points that have a property in common, and that property must be measurable.
Given that the ship point requires there to be at least two spots, let’s consider places of position. They are half-lines that start from the compass bearing line and arrive at a visible point.
We have seen how the intersection of these two half-lines allows us to find the ship point (we will dig deeper in another article).
The ship’s position in this case is a half-line drawn from the conspicuous point, which intersects with a half-line that departs from another conspicuous point.
Also, there are other positions to be considered.
Bathymetric curves
Bathymetric curves or isobaths must also be considered. Such lines indicate sea depth, but they are not enough to determine the ship’s point.
Circle of equal distance
It is nothing but a circle composed of a series of points of equal distance from the center, in which we decide to place the conspicuous point. All the points belong to the same circle and therefore are at the same distance from the conspicuous point.
In this case, the observer knows that he is on one point of the circle, although a second location is still needed to determine the ship point.
Circle of equal difference from Azimuth or capable circle
All points belonging to the same circle having two conspicuous points that always have the same angle and therefore the same difference of AZIMUT between them.
How bearings are plotted
Bearings in nautical charts are plotted in reverse.
Squares are used to draw a half-line from the conspicuous surveyed point, which reaches the observer.
The same is done for the second survey point.
True bearing
To understand what we mean by true detection, we need to introduce the concept of true direction. The observer standing on the boat (no matter what heading) must face True North as indicated by the compass.
At this point at a given angle to the observer there will be the conspicuous spot, which may be as we have said a lighthouse, a turret, a building, or something else.
Assume that the conspicuous point is in the true direction of 45° with respect to the observer. At this point we know that the observer is on a half-line.
Bottom line: the observer is on the true 45-degree bearing of the conspicuous point.
In another guide we will see in more detail how to find the ship point from the bearings.