Taking transit bearings involves observing two specific objects that appear in perfect alignment from your vessel to verify the accuracy of your ship's compass or gyro. These specialized bearings are specifically drawn on charts of a couple of objects in line with the true bearing written, serving as a reliable way of checking the compass or Gyro error on your vessel.
Understanding Transit Bearings
A transit bearing is a pre-established line of position formed when two distinct fixed objects ashore (such as lighthouses, prominent peaks, beacons, or other navigational aids) are observed to be in exact alignment. Mariners do not "create" these bearings; rather, they identify and utilize those that are already marked on nautical charts. The chart explicitly states the true bearing of these aligned objects.
The primary purpose of using transit bearings is to obtain a quick and accurate check of the compass or Gyro error without the need for celestial observations. When your vessel is precisely on this charted line, the true bearing to the objects is known, allowing for a direct comparison with your ship's observed magnetic or gyro bearing.
The Process of Taking (Utilizing) Transit Bearings
While transit bearings are pre-defined on charts, "taking" them refers to the observational process and subsequent comparison to determine your vessel's compass or gyro error.
Step-by-Step Guide
To effectively take or utilize a transit bearing, follow these steps:
- Identify a Charted Transit: On your nautical chart, locate transit bearings marked by a dashed line connecting two prominent objects, with the true bearing clearly indicated alongside. Choose one that you anticipate passing near.
- Navigate Towards the Transit Line: Maneuver your vessel so that you are approaching the line of transit.
- Observe the Alignment: As you navigate, continuously observe the two charted objects. The moment they appear to be in a perfect straight line, one directly behind the other, you are on the transit line.
- Record Your Bearing: At the exact instant the objects are in transit (perfectly aligned), take a precise bearing of the objects using your ship's compass (magnetic or gyro). Ensure this reading is taken carefully, noting any deviation or oscillation.
- Calculate Compass Error:
- Compare your observed bearing from the ship's compass with the true bearing written on the chart.
- The difference between the two is your compass or gyro error.
- Example: If the charted true bearing is 090°T and your observed gyro bearing is 092°G, your gyro error is 2° Easterly (Gyro Best, Error East). If your observed magnetic bearing is 088°M, your magnetic compass error is 2° Westerly (Compass Least, Error West).
Key Benefits
- Accuracy: Transit bearings offer a highly accurate method for determining compass error because they are based on fixed, charted objects with known true bearings, unaffected by vessel motion or environmental factors like swell.
- Simplicity: The method is straightforward and requires minimal calculations, making it a quick and practical check.
- Reliability: Unlike some celestial methods, transit bearings can be used during daylight hours and in varying weather conditions, provided visibility allows for clear observation of the objects.
- Safety: By regularly checking compass error, mariners ensure their navigational instruments are accurate, contributing directly to safe navigation.
Example Scenario
Imagine you are sailing along a coastline, and your chart shows a transit bearing where a prominent lighthouse aligns with a distant church spire on a true bearing of 275°T. As you approach this line, you constantly monitor the alignment of the lighthouse and the spire. The moment they appear perfectly stacked, you quickly take a bearing with your vessel's gyro compass, which reads 272°G.
Feature | Value |
---|---|
Charted True Bearing | 275°T |
Observed Gyro Bearing | 272°G |
Gyro Error Calculation | 275°T - 272°G = 3° |
Resulting Error | 3° Westerly (Gyro is reading 3° low) |
This calculation immediately tells you that your gyro compass has a 3° westerly error, which you can then apply to subsequent bearings for accurate course plotting.
For more information on chartwork and tides, you can explore resources on maritime navigation and chart interpretation.