Visual Position Fixing
When we are in shallow water, getting closer to land, approaching shipping lanes or in failing visibility it is imperative to be able to fix our position. After we fix our position we are able to appreciate, identify and avoid any hazards that may be around the vessel.
The hand-bearing compass is the tool of the trade for navigators and you need to be proficient in its use. As mentioned earlier it is a versatile piece of equipment that will still work even after your GPS is out of battery power or has been soaked by a wave. This handy compass come into its own during pilotage and inshore navigation when it is necessary to discriminate between charted objects or when it becomes impractical to use our GPS and need to be on deck.
We can use the hand-bearing compass to ‘fix’ our vessels position by taking bearings from charted objects and working up bearing lines onto the chart or we can take bearings lines from the chart and discriminate charted objects that we can see from the vessel.
Taking a bearing
It is possible to quickly point the vessel at the object you want to take the bearing and quickly glance at the steering compass, but there are a few reason why this may not be a good idea.
- It is impracticable to plot the progress of the vessel during these small manoeuvres.
- The steering compass needs time to settle before you are able to take an accurate reading
- The divisions on the steering compass are generally in in larger amounts making it harder to take a fix at the 1° accuracy
- Changes of heading may be impracticable due to sea and wind conditions.
- Changes of heading at night may give ambiguous light signals to other vessels.
- The steering compass is subject to deviation, complicating and extending the time need to make a fix.
To take an accurate reading with a hand-bearing compass you must
settle your self on the vessel away from any deviating metallic influences that could produce an erroneous reading.
You must properly identify the object you are taking a bearing from and know where it is on the chart.
Due to motion of the vessel it is often tricky to get a reading down to the nearest whole degree, and have to take a reading from middle of the compass card area as it swings around with the movement of the vessel.
Try to take bearings from objects that are close to you as less prone to error.
Take your first bearing from an object behind or in front of the yacht as it position will change little by the time you take bearings towards the beam of the vessel (Fig 4.33).
Plotting 3 position lines to fix our position
To get a decent fix of our position we need to plot three position lines and triangulate our position on the chart. You should follow this example and plot these bearing lines on RYA Training Chart 3 and find our exact position as we make a passage around the bottom of South Douglas Island. In the example we will use 5°W as our value for variation.
Position Line No. 1
From an area south of South Douglas Island we can see the Lawrence Channel West special mark in the distance and take a bearing of 249°M using the handheld compass (Fig 4.34). Remember when we work on the chart all our bearings must be in True° as everything on the chart is in relation to true north.
- We write down the T-v-M-d-C mnemonic and see that to convert a magnetic bearing to a true bearing we must apply variation, which we have been given as 5°W. (FIg 4.35)
- We know from our ‘CadET’ mnemonic (Fig 4.21) that when we convert from a compass bearing towards a true bearing we add east variation errors, which means conversely that we must subtract west variation errors. So we can convert the bearing 249°M – 5°W variation = 244°T
- We now dial up 244°T on the Portland Plotter, making sure the 244° figure is exactly in line with the large ‘0’ along the centerline of the plotter. (FIg 4.36 ‘A’) Make sure the dial does not shift in the following steps and 244°M stays
- We must remember this bearing is from our position on the yacht looking at the LCW Special Mark, and must point the plotter with the big course arrow towards the object we are taking the bearing of. (Fig 4.36 ‘B’).
- It helps a little if we place and hold the point of the pencil on the LCW Special mark and then swivel the edge of the plotter in a position where the small blue arrows in the plotter dial are pointing northwards. The hatching in the plotter dial should be lined up with parallels of longitude and meridians of latitude. (FIg 4.36 ‘C’)
- While holding the plotter firmly we can now draw a pencil line from the buoy back to where we think our position is. We are somewhere along this position line. We can now define the line we have just drawn by placing an arrow head on the direction of the bearing we have just taken. (Fig 4.37).
Position Line No. 2
Our 2nd bearing was taken to the South Head Lighthouse on South Douglas Island and reads 031°M on the hand bearing compass.
- We write down the T-v-M-d-C (Fig 4.38) mnemonic and see that to convert a magnetic bearing to a true bearing we must apply variation, which we have been given as 5°W.
- We know from our ‘CadET’ mnemonic (4.21) that when we convert from a compass bearing towards a true bearing we add east variation errors, which means conversely that we must subtract west variation errors. So we can convert the bearing 031°M – 5°W variation = 026°T.
- We no dial up 026°T on the Portland Plotter, making sure the 026° figure is exactly in line with the large ‘0’ along the centerline of the plotter (FIg 4.39 ‘A’).
- We must remember this bearing is from our position on the yacht looking towards South Head Lighthouse, and must point the plotter with the big course arrow towards the object we are taking the bearing of (Fig 4.39 ‘B’).
- It helps if we place and hold the point of the pencil on the lighthouse and then swivel the edge of the plotter in a position where the small blue arrows in the plotter dial are pointing northwards and the hatching in the plotter dial are lined up with parallels of longitude and meridians of latitude (FIg 4.39 ‘C’).
- While holding the plotter firmly we can now draw a pencil line from the lighthouse to where we think our position is. We can be confident that we are somewhere along this position line where it intersects out first position line. We can now define this line we have just drawn by placing an arrow head on the direction of the bearing we have just taken (Fig 4.40).
Position Line No. 3
Our 3rd bearing was taken to the lighthouse on Guillemot Island and reads 311°M on the hand bearing compass.
- We write down the T-v-M-d-C mnemonic and see that to convert a magnetic bearing to a true bearing we must apply variation, which we have been given as 5°W. (FIg 4.41)
- We know from our ‘CadET’ mnemonic that when we convert from a compass bearing towards a true bearing we add east variation errors, which means conversely that we must subtract west variation errors. So we can convert the bearing 317°M – 5°W variation = 312°T
- We no dial up 312°T on the Portland Plotter, making sure the 312° mark is exactly in line with the large ‘0’ along the centerline of the plotter. (FIg 4.42 ‘A’)
- We must remember this bearing is from our position on the yacht looking towards Guillemot Island lighthouse, and must point the plotter with the big course arrow towards the object we are taking the bearing of. (Fig 4.42 ‘B’)
- It helps if we place and hold the point of the pencil on the lighthouse and then swivel the edge of the plotter in a position where the small blue arrows in the plotter dial are pointing northwards and the hatching in the plotter dial are lined up with parallels of longitude and meridians of latitude. (FIg 4.42 ‘C’)
- While holding the plotter firmly we can now draw a pencil line from the lighthouse to where we think our position is. We can be really confident that we are where this line intersects our two original position lines. We can now define this line we have just drawn by placing an arrow head on the direction of the bearing we have just taken. (Fig 4.43)
Finishing The 3 Point fix
If we only used 2 position lines ( 2 point fix) there would be nothing to warn us if there was something wrong with our position fixing. By taking a 3rd bearing and plotting a position line we gain considerable confidence that we are at the position marked on the chart.
We can say with a high level of accuracy that our vessel is at the position where these position lines intersect 46°05’.10N 005°50’.70W.
When at sea we always finish plotting out fix by noting the time and log readings on the chart and/or transferring the information to the ships log book . An accurate 3 point fix is a confirmed position and as such we draw a circle around the fix (Fig 4.40).
Are the features you taking a bearing of actually on the chart
When you are voyaging through the seascape, use the chart to identify individual features and landmarks until you have developed a good local knowledge of your landscape. This means you wont have big problems trying to identify charted objects or other features to fix your position from. Its also a good idea for the same individual to take the fix and plot it on the chart, unless the crew are familiar with specific landmarks and where you are taking a fix from. By continually checking the chart as you sail along you will can identify features that are charted objects to take your fixes from and less likely to get lost and lose your bearings. When we take bearing with the handheld compass we are always keeping in mind and trying to minimize any cumulative errors that may be occurring.
Angle of Cut and Separation of Landmarks
The example in fig 4.41 is the 3 point fix we have just plotted. You can see that there is good separation between the position lines. This pinpoints the exact area where the position lines cross. If we are conducting a 3 point fix the optimum angle between the position lines is 60° although not always practicable.
We can see that in Fig 4.42 the position lines are very close together and produces uncertainty in our fix and a result of trying to take fixes from landmarks that are to close together. If there was even a small movement or error when reading the hand-bearing compass, it would be very hard to rely on a position fix like this. The best thing to do, and should be done when a fix in shallow water is to corroborate it against the depth reading.
The Cocked Hat
In fig 4.43 you can see that errors in the fix has resulted in a large triangular area which is what we call a ‘cocked hat’. This is the result or errors creeping into the bearing acquisition and plotting process. It is quite normal to have a small cocked hat and if there are no dangers around it is normal to put your position in the centre of the cocked hat. However if there is a larger cocked hat or you are near some dangers, you put your position within the cocked hat but nearest the danger and navigate from that position. In other words, you put yourself in the worst case scenario and then navigate away form the danger. In the example of fig 4.43 we have placed our position closest to land. We have finished the plot off with a pencil point surrounded by a small circle which is the symbol for a ‘confirmed position’. As described above we always place a time and log reading beside a fix but in position where it does not clutter the fix.
Order to take bearings
It most accurate to have the boat stopped in the water to take a bearing but impracticable most of the time. We can ameliorate the error in our fixes as we move through the water. Our first bearing should ideally be ahead or astern of the boat as this bearing would be expected to change little as the vessel moves onwards and you take the other bearings to the side of the vessel. In other words, the object that your taking the bearing from that is moving most on your horizon should be the last bearing you take. This generally amounts to taking the bearing of objects ahead or astern first then those towards the beam of the vessel (Fig 4.33).
Reducing the Errors
There are lots of little errors that can creep in when we are using a hand-bearing compass which can cumulatively reduce the accuracy of our position fixes and result in what is known as a ‘cocked hat’ fix.
In lumpy seas the compass card inside the compass can swing about making it hard to read the bearing accurately.
There could be deviating or metallic influences affecting the compass, including metal rimmed glasses or standing too close to metal work on the vessel.
The thickness of your plotting pencil can reduce the accuracy of your chartwork.
Improper sighting of landmarks can completely throw the fix off position.
Too narrow an angle of cut or taking fixes from objects in the far distance.
On a moving vessel, by the time we take the bearing and plot it on the chart, we could be quite far from where we want really are.
Not wearing prescription glasses.
You should practice taking fixes and using a hand bearing compass, as you do so, you will be gain confidence in its potential accuracy. Its an essential piece of navigational equipment.
Whenever you take a fix from any source, you should always try and corroborate the information from an independent source. For instance as soon as you have plotted the fix on the chart, compare the charted depth with the depth sounder display. If you have lost your bearings it is un-seamanlike to press on into a possible dangerous situation. You should try the position fixing process again, compare it with GPS if fitted, or proceed with extreme caution until you know where you are.
Natural Sources of position lines
There are many things that you can take position lines from that may not be initially very obvious. Distinctive cliffs (FIg 4.44) and edges of islands are good especially when coastal cruising and out of sight of lighthouse and other charted objects. We must be careful to select an edge that is distinct and doesn’t run into the sea or disappear under the horizon. Often there may be pylons or masts on top of hills or mountains that are excellent charted objects but can be overlooked as they a bit out the line of sight. In some areas such as Greek islands, most of the time all you have to fix positions is the edges of islands or the high points of hills.
Bearing and depth contour
If you cannot obtain a 3 point fix from charted objects, it is possible to take a fix from one feature or charted object and use a depth contour to find the vessels position (Fig 4.45). Your nautical charts supply you with a vast amount of information regarding depth. By examining the chart you can get an idea of the depth profiles and what the general morphology of the sea bead looks like. The echo sounder on board the vessel gives an accurate depth reading. Using a bearing and contour to fix position is very common way to gain position information especially where the contour lines are close together and the sea bed shelves steeply.
These kind of fixes should be corroborated at earliest opportunity and would not like to cruise along an unfamiliar coastline using only single position lines and contours to fix a vessels position. The accuracy of this fix comes down to the identification of the charted object and good depth contours as will have little confidence in our fix if we have a completely flat seabed. We also have tidal height to consider as the depth on the chart is taken from chart datum. We will consider tidal heights in detail in Module 5
Transit and contour
When an object in the foreground and one in the back ground line-up they are said to be in ‘transit’. Transit lines are very accurate position lines because they reduce error to a minimum. Thats is there is no need to use a compass, so no calculations, deviation or variation to deal with. A transit is a visual observation and as long as the features are printed on the chart we have a very good source of accurate position lines.
As we illustrated in last section a lot of lights into harbours use transit lines formed by lights to guide vessels along channel avoiding obstacles into harbours etc. Transits are very useful and a good navigator is always on the lookout for them as can be formed by the sides of islands, or distant masts, church spires, buoys…. any object or feature that is charted can form part of a transit.
Charted Transit Lines
Fig 4.46 is an example of a charted transit line for Dawson Harbour (RYA Training Chart 4A). The radio mast on the hill (‘A’) and the tower (‘B’) form an excellent transit line at 152°T that can lead vessels safely though the Flanker Channel on the approach to Dawson Harbour (‘C’). The depths in this area are very shallow and would need to be extremely cautious when trying to fix our position and make this approach without a transit to guide us along the approach between the two islands.
Uncharted Transit Lines
Charted transit lines are commonly found on the approaches to harbours and anchorages. However, there are many naturally occurring and uncharted transit lines we can get position lines from. As you move through the landscape you should always be on the look out for transit lines as they provide an excellent source of positional information and can give you an indication of how tidal currents or leeway are affecting your vessel. Fig 4.47 shows two separate transit lines used to fix the vessels position. Suitable charted objects can be lined up to form a transit, here we have Port Fraser SWM (‘A”) in transit with Hill Shoal S. Cardinal. At the same time we can see a transit formed with Holm Point lighthouse (‘C’) being in lines with the eastern edge of Webb Ellis Island. Fixing position using two transits is remarkably accurate but not always available to us.
Transit and a Bearing
In fig 4.48 we have found our own transit by using the isolated danger mark and a house in Walton Bay marked by a transit on 056°T. Our depth sounder reads around 50 metres. To corroborate our position by transit and depth contour we have taken another sight using hand-bearing compass to the lighthouse near Evans Head and reads 018°M ( 013°T with 5°W variation).
Steering along a transit
Steering along a transit is a very useful skill . Fog 4.49 show a lighthouse in the background and a red buoy in the foreground. . When the two are lined up they are said to be ‘in-transit’ and you can steer down this position line keeping the lighthouse and buoy in transit.
Tidal currents may affect the boat as you steer along a transit but immediately noticeable and easily corrected for.