Dealing with Variation and Deviation
A lot of students get very confused with variation and deviation and how to convert True, Magnetic and Compass bearings. There are many ways to deal with this but over the years have found the method particularly useful and have adopted a fail-safe method that works.
To make things easy for us, its best to adopt some conventions:.
Lets think of Variation and Deviation as small errors that can be added or subtracted (do not think of them as directional at all, just little errors name West and East.
True bearings (°T) are what we plot and read from the charts.
Magnetic Bearings (°M) are what we take when we use the handheld compass and only affected by variation
Compass bearings (°C) is what is read from the vessels steering compass and affected by both variation and deviation.
By adopting and learning a method you will have the core skills to manipulate bearings between the charts, hand held compass and the ships compass. By dealing with variation and deviation in our calculation we are able to reduce the amount of error in our navigational calculations. If we want to sail 60 nautical miles and are 10° out in our calculations, we are going to be around 10 miles out when we think we are at our destination! Our variation and deviation errors may be small but can be compounded and make large navigational mistakes.
Deviation has been introduced into this course but is not part of the syllabus for RYA Day Skipper. I feel it is important that you are aware and are also able to deal with Deviation in calculations. However the test questions will not ask you to deal with deviation in calculations.
Converting bearings between True North, Magnetic North and Compass North.
As discussed above:
Whenever we are working on the chart we are working with ‘True (T)’ bearings,
When using a handheld compass we are working with ‘Magnetic (M)’ bearings
When using the ships compass we are working with ‘Compass (C)’ bearings
We have the error of Variation (v) to apply when converting between True and Magnetic bearings.
We also have the error of Deviation (d) to apply between Magnetic and Compass bearings.
So it makes sense that when we are working on calculations between the chart and the steering compass that we need to apply both variation and deviation. We can use a mnemonic to remember where we have to apply our errors when converting bearings (Fig 4.17).
We don’t always need to work from True towards Compass, we could be steering a course on the ships compass and want to plot this bearing on the chart. In this case it might be easier for us to rewrite the mnemonic “Cadburys Dairy Milk Very Tasty”. They remind us of exactly the same information about what error needs to applied to convert bearings between the steering compass, handheld magnetic compass and chart. (Fig 4.18).
Applying variation and deviation
Now that we know where we need to apply the errors we have another mnemonic that helps us apply the variation and deviation errors.
We can use another mnemonic to apply the little errors of variation and deviation when converting bearings between True-Magnetic-Compass or Compass-Magnetic-True. We use the mnemonic CADET “Compass AdD East True” or Compass Add East to True. This basically tells us that when we are going from Compass (steering compass) towards True (the chart) we add Easterly errors. Once we know this we can work out how we apply any error involved with variation or deviation.
1. Write down CADET which tells us that when moving from a compass bearing towards a True bearing we need to add Easterly errors. The trick is to actually write the arrowhead in clearly with +E beside it so its completely clear to us that when we are converting Compass bearings to True bearings that we add easterly errors.
2. So it makes sense and totally logical that when we are going the opposite way and converting True bearings to Compass bearings, it must be the other way around. So we can draw the bottom arrowhead in and clearly write -E beside it. We now know how to deal with any easterly variation or deviation errors!
3. Its just as simple with westerly errors, our cadet mnemonic sorts everything out for us! When we convert a Compass to a True bearing we know we add East errors. If you think about it, if you add something to the east, its exactly the same as taking something away from the west! So we can write -W beside the +E. They are both the same thing.
If we go less west (-W) its the same as going more east (+E) and if we go less east (-E) its the same as going more west (+W).
We can combine our mnemonics and summarise all the information we need to know to covert bearings and apply errors for deviation and variation (Fig 4.22)
Magnetic to True Bearing Conversion with East Variation
We sight a radio mast in the distance with the hand-bearing compass and is on a bearing of 271°M. What is the True° bearing we use to plot on the chart when variation is 7°E?
The first thing we do is write down T v M d C and see that we need to apply variation to convert a magnetic bearing into a true bearing.
- Fill in the numbers we know, underneath the relevant letter in the mnemonic. Magnetic = 271° and Variation = 7°E (Fig 4.23).
- Using CadET (Fig 4.24) we can calculate what we need to with the errors when working from Compass towards True bearings. In this case, when converting a magnetic to a true bearing we need to add easterly variation errors.
- So the True bearing we need to plot on the chart is 271°M + 7°E variation = 278° True.
- We could now plot a 278° True bearing line onto the chart towards the lighthouse and know we are somewhere along this position line.
Magnetic to True Bearing Conversion with West Variation
We sight a church spire using a hand-bearing compass and is on a bearing of 003°M. What is the True° bearing we use to plot on the chart when variation is 6°W?
- The first thing we do is write down T v M d C and see that we need to apply variation to convert a magnetic bearing into a true bearing (Fig 4.25).
- Fill in the numbers we know below the relevant letter in the mnemonic. Magnetic = 003° and Variation = 6°W
- Using CadET(Fig 4.24) we can calculate what we do with west errors when working from Compass towards True bearings. In this case, when converting a magnetic to a true bearing we subtract westerly variations
- So the True bearing we need to plot on the chart is 003°M – 6°W variation = 357° True.
- We could now plot a 357° True bearing onto the chart towards the the church spire and assume we are somewhere along this bearing line.
True to Magnetic Bearing Conversion with West Variation
Using the Portland Plotter on the chartwe have measured a bearing to a lighthouse to be 276°T. What bearing should this be when we try to sight the lighthouse with the hand-bearing compass when variation is 5°W?
- The first thing to do is write down T v M d C and see that we need to apply variation when converting a True bearing from the chart in to a Magnetic bearing for use with the hand-bearing compass. (Fig 4.26)
- Fill in the numbers below the relevant letters in the mnemonic. True° = 276°T and Variation = 5°W
- Using CadET (Fig 4.24) we can determine what we need to with west errors when working from True bearings towards Compass bearings. In this case, when converting from True to a Magnetic bearing we add westerly variation
- So the True bearing we need to plot on the chart is 275°T + 5°W variation = 280° Magnetic.
- We could view 280°M through the handheld compass we should be looking directly towards the lighthouse.
True to Magnetic Bearing Conversion – East Variation
We examine the chart and see that a Safe Water Mark is on a bearing of 189°T. We want to sight the rock with the handheld compass. What bearing will the rock be on when we sight it with the handheld compass. Variation is 4°E?
- The first thing to do is write down T v M d C and see that we need to apply variation when converting a True bearing from the chart to a Magnetic bearing for use with the hand-bearing compass (Fig 4.27).
- Fill in the numbers below the relevant letters in the mnemonic. True° = 189°T and Variation = 4°E
- Using CadET (Fig 4.24) we can calculate what we need to with the errors when working from True bearings towards Compass bearings. In this case, when converting from True to a Magnetic bearing we subtract easterly variation
- So the True bearing we need to plot on the chart is 189°T – 4°E variation = 185° Magnetic.
- When could view 185°M through the handheld compass we should be looking towards the Safe Water Mark.
Converting to Compass Bearings
Converting to compass bearings is not part of the Day Skipper course but have included a couple examples below so that you can see how it is performed. We use exactly the same process as before and complete it in a stepwise fashion adding or subtracting variation and deviation errors, which depends if working from a True Bearing to a Compass Bearing or the other way around. In some examples you may have both an easterly variation and deviation, sometimes there may be may be a westerly variation and an east deviation. It does not matter what kind of errors we have or if we are working from compass to true or back again, its exactly the same process as before except we have to look up our value of deviation from the deviation table in the back page of the RYA Training Almanac (Fig 4.11).
True to Compass Bearing
A navigator has plotted a bearing line 174°T on a chart to steer the boat safely between a gap in the reef. What course will he tell the helmsman to steer. The Variation on the map compass rose = 7°W?
Fill in the numbers below the relevant letters in the mnemonic. True° = 174°T and Variation = 7°W
- Using CadET (Fig 4.24) we can calculate what we need to with the errors when working from True bearings towards Compass bearings.
- We first need to convert our True bearing to a Magnetic bearing, and using CadET we know we must add westerly variation.
So the magnetic bearing will be 174°T + 7°W variation = 181° Magnetic.
- Now we have the Magnetic bearing 181°M we can see from the T v M d C mnemonic that we now need to apply deviation to get a compass bearing.
- We enter the deviation card on page 97 of RYA Training Almanac and can see that for a bearing of around 181° the deviation is 4°E (Fig 4.29)
- Again using the CadET Mnemonic we can see that when converting from True to Compass bearings we subtract easterly deviation errors.
- The Compass bearing is the Magnetic bearing 181°M – 4°E deviation = 177° Compass.
- The helmsman could now steer 177° on his steering compass to navigate through the reef safely.
You may have noticed in the previous example that variation and deviation have cancelled themselves out a little. This is not always the case and imperative that you work through this type of calculation in a stepwise fashion or may end up with a cumulative and potentially serious error.
Compass to True Bearing
A helmsman is steering a course of 270°C in the fog and navigator wishes to plot this bearing line on the chart. What will be the true bearing that the navigator will plot on the chart when variation = 7°W?
The first thing to do is write down T v M d C (Fig 4.30) and see that we need to apply deviation then variation in a stepwise manner when converting a Compass bearing from the helms steering compass to a True bearing to plot on the chart.
- We therefore need to determine the value of deviation from the deviation card on page 97 of the RYA Training Almanac, so we can firstly convert our compass bearing to a magnetic bearing. We can see that on a course of 270°C the deviation error is 4°W.
- Using the CadET(Fig 4.24) Mnemonic we can see that when converting from Compass towards True bearings we subtract west deviation errors. 270°Compass – 4°W Variation = 166°Magnetic.
- Following our stepwise process we can see that from our T-v-M-d-C that all we need to do now is apply our 7°W error to the magnetic bearing to convert it to a True bearing.
- Our CadET mnemonic tells us that again we must subtract west deviation errors when converting compass bearings towards true bearings. 166°Magnetic – 7°W variation = 159° True.
- The navigator can now plot the 159° True bearing line on his chart, which will help safely navigate the boat in the fog.
In the above example both variation and deviation were west errors and had a cumulative effect. It is important to follow the process when converting bearings in a stepwise process so that we add or subtract our errors in the correct order. In the RYA Coastal / Yachtmaster level we look at deviation in more detail and take other factors like the vessels leeway into consideration.