When I was a **deck cadet** watching on the bridge, the **Chief Officer** was admiring the beautiful afternoon sun. Surely, this was a hint. I already had an idea that he wants me to check for our gyro error.

Since then, I’ve been checking gyro errors and compass errors and logged them in the deck logbook. I also learned many methods on how to get them as I worked with different officers who were gracious enough to teach me their ways.

**What is a Gyro Error?**

A gyro error is the difference between the reading of a **gyro compass** and the **true north**. In simple terms, it is the deviation of the gyro compass reading from the actual geographical north. The true geographic north is the North Pole.

Magnetic compass on the other hand points to the magnetic North Pole which is somewhere around northern Canada.

Because a gyro compass is very accurate, seafarers use it to navigate their ships in the vast oceans. However, they can drift over time, which is why it is important to check for errors.

**Why do we need to check the gyro error regularly?**

Once every watch or every four hours, the Officer of the Watch (OOW) needs to check the gyro error together with the compass error. They log the result in the deck logbook and use it as a reference if they observe that the errors become bigger.

Every **merchant ship** needs to check the gyro error. Here are the reasons why:

**1. Accurate Navigation** – If you are off course (or if you have a gyro error) of 1 degree, you will end up 1 mile away from your destination after traveling 60 miles. Imagine crossing the ocean which is hundreds or thousands of miles away!

**2. Course Correction** – That being said, you can adjust your course to compensate for your gyro error and arrive accurately at your destination.

**3. Prevent Accidents** – The gyro compass is used to navigate the ships either in open or restricted waters. This equipment also connects to other important devices on board. Not being aware of the gyro error could lead to confusion and/ or accidents

**4. Compliance with Regulations** – Because of the potential chaos it brings when the error is unknown, port authorities, coast guards, and **vetting inspectors**, check your gyro compass and its errors during inspection.

**5. Efficient Passage Planning** – Accurate gyro compass readings allow mariners to optimize their voyage. Minimizing course deviations can save time and fuel consumption and costs.

Gyro errors can be caused by various factors, such as mechanical imperfections, electrical disturbances, or the effect of the Earth’s magnetic field on the compass. These errors can accumulate over time and may lead to inaccuracies in the ship’s course.

**8 Methods for Checking Your Gyro Error**

There are 8 ways you can use to check the gyro error of your gyro compass on board. By observing celestial objects like the sun and the stars, you can get their true bearing and compare it with your observed bearing using the gyro compass.

No, you don’t need a **marine sextant** to do that although it’s possible if you’re taking a line of position or LOP. The most common method is by the use of Sight Reduction Tables or Norries Tables. You can even use pure maths with **spherical trigonometry**.

Observing charted landmarks are also useful in taking the error of your gyro compass.

**1. ABC Method**

The ABC Method is one of the most famous and widely used in finding the gyro error of your gyro compass. It got its name from the A, B, & C Azimuth Tables of the book Norie’s Nautical Tables.

Before we dive into that, we will use the detail below for our calculations. This was the observation I made during my heydays at sea.

Gyro Error Using the star **Betelgeuse:**

**Date**: 08 Apr 2014**Time**: 21h 07m 22s**Position**: N 41° 05.0′ Latitude / E 001° 17.3′ Longitude**Gyro Bearing**: 259.5°**Gyro Course**: 298.0°**Place**: Tarragona Anchorage

Using the date, time, and position, we can now find its Local Hour Angle or LHA using the solution below:

After finding the LHA, we can use it together with the latitude and declination to find the true Azimuth (or bearing of the celestial object) using the ABC Method.

Get your Nories nautical tables and turn to page 380 to start looking for the values based on the Latitude, Declination, and SHA.

Refer to the image below:

With the true azimuth or true bearing of the star Betelgeuse at 259.3° and our observed gyro bearing of 259.6°, we now have a gyro error of 0.5°W.

I also made a video with another example of finding the gyro error using ABC Method. Check it out below:

**2. Using Sight Reduction Tables**

A Sight Reduction Table is a reference book with a set of tables used in **celestial navigation**. By using the tables in this book, you can determine the altitude and azimuth of celestial objects. You can also check the gyro errors using this book without applying too much maths.

To proceed, we will be using the data in ABC Method such as the **position**, **declination**, and **LHA**.

And we will also use columns to organize our solution. We won’t be doing any serious maths. A basic calculator and the SRT book are enough.

Actual Values | Base Arguments | Base Azimuth | Tabulated Azimuth | Z Difference | Increment | Correction (inc*ZDiff/60) |

Dec N 7° 24.3′ | 7° | 100.8 | 100.0 | -0.8 | 24.3 | -0.3 |

Latitude N 41° 05.0′ | 41° | 100.8 | 101.0 | +0.4 | 05.0 | 0.0 |

LHA W 066° 11.3′ | 066° | 100.8 | 100.1 | -0.7 | 11.3 | -0.1 |

Total Correction | -0.4 |

Based on this solution, we now have a gyro error of **0.1°E**. It appears to be different than the ABC method. This is because of the increments that we didn’t apply.

And if you can’t wrap this around your head, don’t worry. I’ll make a separate article detailing every nook and cranny of this solution so you will understand it better!

For now, just know that you can get the gyro error using Sight Reduction Tables for Marine Navigation.

**3. ABC Method with Algebra**

If you think algebra is difficult here, it can actually offer a quick solution. We will be using a scientific calculator, and some formulas while ditching Norie’s tables for now.

Here’s the formula for finding the A, B, and C.

- A = TanLat / TanLHA
- B = TanDec / SinLHA
- C = A +- B
- True Bearing = [(C * CosLat) Inv. Tan] – 90°

Again, we will be using the given values of latitude, declination, and LHA on ABC Method. Here’s what the solution looks like

**4. Computed Altitude**

When you solve for your LOP using a sextant, one of the values that you must acquire is the true bearing of the body.

But before you can get that, you must have to solve for its **computed altitude or Hc**. So here’s another method for getting the true bearing of a body using computed altitude.

Again, this also involves the use of algebra and a scientific calculator. Here is the formula:

- SinHc = (SinLat * SinDec) + (CosLat * CosDec * CosLHA)
- SinZn = (SinLHA * CosDec) / CosHc

Let’s solve for the gyro error using the formula above and the values given in method 1.

**5. Amplitude by Algebra**

The amplitude method is another way of getting the true gyro bearing and this also involves the use of algebra and a scientific calculator. Here, the process is not so tedious and the formula is very easy to memorize.

However, amplitude method for solving the gyro error is only applicable only to the sun during sunrise or sunset. Specifically, it should be made when its altitude is about two-thirds of its diameter.

This is because the actual sun is already touching the horizon. It only appears as though it is above the horizon due to the refraction of light.

So here’s the formula when using the amplitude method for your gyro compass error. We won’t be making calculations but given this formula, it should be enough.

**SinAmplitude = SinDeclination / CosLatitude**

**6. Amplitude by Norie’s Nautical Tables**

Aside from algebra, we can also check for the gyro error using our friend Norie’s Tables. Like in method 5, it can only be applied to the sun during sunset or sunrise.

The amplitude method always uses your latitude and declination to find the true bearing of the sun when it is on the horizon. Note that visibly, its altitude should be two-thirds the distance of its diameter to count for refraction.

For this method, you have to use interpolation to get the correct value since the latitude and declination only provide whole numbers.

**7. Leading Lights**

Leading lights, also known as range lights, are navigational aids used in nautical charts and coastal areas to guide vessels along a specific safe route.

These two lights are vertically aligned. They appear as a single light in perfect alignment when a vessel is on the correct course following the designated safe channel. If the vessel veers off course, the lights will no longer appear aligned.

Taking a bearing of leading lights with your gyro compass is one of the easiest and quickest methods of finding the gyro error. Once you observe the bearing of the leading lights, compare it to its charted bearing. The difference is your gyro error.

Take the image below for example. If these two lights bear 270.2° in your gyro compass, and their charted bearing is 270° T, you now have a gyro error of 0.2°W.

**8. While berthed or in locks**

When we were inside a lock somewhere in Europe, the pilot informed us that we have a gyro error of 0.3° W. Turns out that any vessel when alongside that lock gives a heading of 92.5° and ours gave 92.8°.

To verify his observation, we check the lock in the nautical chart and plotted its direction. Lo, and behold, the pilot was right all along.

However, this method is not widely used due to available leading lights when transiting canals or channels. Moreover, you have to be very careful when using this since some berths and locks may have deformed fenders.

Lastly, your mooring lines may not be equally tight thus giving an error.

Whatever **types of marine navigation** you use, be it terrestrial, celestial, dead-reckoning, or electronic navigation, you will surely be required to check for gyro error.

What’s your favorite method for getting one? Share with us in the comments below.

May the winds be in your favor.

Featured image: **Chief Officer Domingo Antonio Cargiulo**. You can follow him on Instagram.