What is Dead Reckoning Navigation Technique (Maritime Position Calculation) At Sea?
Before the advent of modern navigation aids such as the Global Positioning System (GPS) and Electronic Chart Display and Information System (ECDIS), maritime positioning relied primarily on celestial and terrestrial methods, especially the reference to radio navigation stations.
Thanks to the 32-bearing magnetic compass, maritime navigation became much more convenient and practical in the 15th century, during the time of Columbus. It has been discovered that Columbus kept voyage logs, recording various measurements taken during his journey. These records suggest that Columbus may have used position calculation.
Position calculation is one of the oldest and most fundamental navigation methods, and its basic principles are still applied in modern navigation today. In fact, Mediterranean navigators had already developed this technology more than two centuries before Columbus’s time. The Carta Pisana, drawn in 1275, is the oldest known chart using position calculation.
Position estimation, often abbreviated as DR, refers to the process of calculating a ship’s current position based on a predetermined location. This location, also known as a reference point, is calculated by applying speed and heading estimates.
Position Estimation
To illustrate more clearly, a reference point is a position derived from an external reference point (such as a line of position). The intersection of two or more lines of position provides relatively accurate position information for a vessel. Sharp bends at the intersection of three lines of position indicate an error. The optimal angle between two lines of position is 90 degrees, and the optimal angle between three lines of position is 120 degrees. Intersections are necessary because a single line of position only provides an estimate of the position, meaning the vessel is located somewhere on that line. Understanding the concepts of reference points and lines of position is fundamental to understanding the DR calculation process used for navigation.
Although the DR calculation method is no longer used in navigation, understanding other methods for calculating a vessel’s position remains useful. To simplify the concept of deviation (DR), the current position is calculated by applying speed and heading to a previously calculated position. This method also allows for the calculation of an estimated future position by applying heading and speed to the current position.
Regarding reliability, the most appropriate and accurate method should be used to obtain precise data. Therefore, the reliability of position calculations is ranked from highest to lowest according to the available methods:
- Static Position
- Current Position
- Estimated Position
- Scale Navigation
When drawing a deviation (DR) calculation map, the following must be considered:
1. If the direction or velocity changes, the deviation calculation map must be redrawn.
2. If the predicted path proves incorrect, the deviation calculation map is redrawn using the corrected data.
3. Do not draw a new path from the estimated position (EP).
4. Always draw a new path from a fixed position.
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Any data obtained through estimation (such as distance calculations) inevitably contains errors. Factors such as ocean current drift, wind, or other external factors are not taken into account when determining future positions, or even hypothetical current positions. This makes the calculated data very close to the precise value; that is, it will be very close to the reached position, but not exactly in place.
It is said that Columbus reached the New World four times solely through distance calculations. He successfully navigated to all the caves and islands and returned to his home port of Spain by the same route.
In the past, navigators measured the distance a ship traveled from a fixed point (such as the port of departure). To do this, they had to keep daily logbooks. Points were marked on nautical charts with pins to indicate the ship’s course, direction, and speed. At that time, magnetic compasses were used to measure direction, and the method for calculating speed was also unique.
Ship speed is measured using the logbook, which involves casting a wooden stick into the sea ahead of the bow. Markers are placed at both the bow and stern. After casting the logbook, the time required for the ship to travel between the two marks is recorded. Speed is calculated by converting the logbook time between the marks into speed. These headings and speed measurements are recorded on nautical charts and in the logbook and are updated regularly.
Despite rapid technological advancements, traditional, proven methods remain effective. As with most modern navigation systems, the human factor remains indispensable in determining the appropriate calculation method. Even with automation, the skills and experience of the crew remain crucial for navigational safety and life at sea.
