Methods of Position Fixing
Knowing where you are at all times is crucial to marine navigation. In this age of modern electronics, we rely heavily on GPS. However, it is foolhardy to rely totally on this singular method and a good strategy to look at alternatives to back up your electronic position.
Here are a few methods to help fix your position;
GPS
Latitude and Longitude
This is the obvious one! Switch it on, and in a few seconds your position will be displayed as a Latitude and Longitude. Remember the default datum is WGS84, check the datum menu if your not sure.
Range and Bearing to a Waypoint
Another method of using your GPS to give a position. Enter a waypoint (as latitude and longitude) into your GPS. Then select the navigate option and select your waypoint. The GPS will now display the bearing and distance to the waypoint. This can be plotted on your chart to give your position. This technique is particularly useful if you have limited chart table space and you have to keep folding the chart over, therefore, losing the latitude and longitude scales.
Bearing and Transits
Compass Bearing
Traditionally, we use a hand bearing compass to take bearings on objects we can see on the land. We take the bearing, correct it for the current variation marked on the chart and plot it with some form of plotting instrument (Portland plotter / Parallel rules etc.). Our position must lie somewhere along this line. (Remember the rules for variation, subtract if westerly, add if easterly.)
Transit
If 2 objects line up, we have a transit. Typically this may be two dedicated markers or towers built on the land. Often unofficial transits are used like a church spire in line with a conspicuous building, or if in France, maybe the blue shutters of the Boulangerie!. This is very precise and gives an accurate position line. Transits are often used as leading lines to safely bring us into a harbour.
Three Point Fix
This is our mainstay of traditional position fixing. 3 compass bearings on 3 wide spaced objects. By taking 3 bearings, we increase the accuracy over 2 objects. Invariably, the 3 bearings will not cross perfectly, but give us a ‘cocked hat’. As long as the cocked hat is not too large, we know where we are.
Bearing and Transit
A good option and more accurate than a 3 point fix. When the transit comes into view and lines up, take a compass bearing on another object. Plot and you have a fairly precise position.
Depth
Depth can be very handy to confirm your position, especially in areas with differing depth contours. If we have a deep channel, we can clearly see how the depth will change as we move in and out of the channel. This is also a very useful technique when visibility is poor and normal visual references are hidden.
Trigonometry
Rising and Dipping
Rising and dipping distances allow you to find your distance from a lighthouse due to the curvature effect of the earth. By noting when the top of a lighthouse (normally the light itself), appear or disappears we can work out how far we are from the light. The trigonometry is done for us in the guise of a table, which you will find in many navigational books, and more popularly, Reeds Nautical Almanac (Rising and Dipping distances). By entering the table with the height of the lighthouse (printed on the chart, and measured from MHWS) and also by your height of eye (typically 2m on an average cruising boat), you can find your distance off. Combine this with a compass bearing on the light, and we have a fix.
Vertical Sextant angles
Using similar trigonometry as above, we can use a sextant to measure the angle between the horizon and the top of a charted object. Using the tangent rule, we can calculate distance off. Like above, trigonometry is not necessary, as the tables are printed in many Navigational books like Reeds (Distance off by vertical sextant angle)
Running Fix
The running fix (or transferred position line) is a method to determine your position if you only have 1 visual reference point (like a single lighthouse). The methodology is as follows;
1. As you approach the single point, take a bearing on it with your hand held compass. Make a note of the log, and course being steered.
2. Once there is an appreciable change in the angle to the object, take another bearing and note the new log reading.
3. Plot the 1st bearing, and anywhere along this position line, make a mark and from this point, plot the course steered. Measure down this track the distance travelled (the difference between the 2 log reading). and make a second mark.
4. Transfer your first position line, keeping it parallel, down to your second mark.
5. Where this transferred line crosses the second compass bearing is your position.
Consider the accuracy of this method? How well was the course steered? How accurate is the log? Was there any tidal streams of leeway?
These can all be factored in to improve the fix, but generally, if the distance between the 2 bearings is not too large, it will be accurate enough.
Doubling the Angle on the bow
This technique uses basic trigonometry to give a distance off. Like a running fix, it is designed to be use when there is only 1 visual object.
The methodology is as follows;
Take a bearing on the visual object, make a note of this bearing (make life easier by using an easy bearing like 20°,30,40° etc.). Take the log reading. Keep monitoring this bearing until the original bearing has been doubled (e.g. 30° becomes 60°). The distance to the object is the same as the distance run. If we now take a compass bearing on the object, we can plot a fix (Range and Bearing).
Radar
Many leisure vessels are fitted with Radar. It is an invaluable piece of equipment when travelling in poor visibility. It is also a very useful way of position fixing. The Radar detects solid objects like other vessels and land. The display shows your relative position (in the centre of the display), and can give you a range and bearing to a distinctly identifiable point of land that is visible on the screen.
Lights
Sectored Light
Normally, lighthouses show a white light. However, some use sectored lights where the glass is coloured red and green as well as clear glass.
The sectors allow you to determine whether you are on the right track. In it’s simplest form, if the light appears red, you are too far to port of your safe track. If green, too far to starboard. The idea is to keep the lighthouse light white, in which case, you must be on the correct course. If you notice the light changing from white to red (or white to green) you can use the sectored light information printed on the chart to determine the position line you must be on. Sometime the sectored light marks a danger area, for example, St Anthony’s lighthouse at Falmouth shows a red sector to cover the danger of the manacle rocks.
Moiré Effect Light
A moiré effect light is a directional light. This means you can only see it if you are looking directly at it. It is used in the same manner as a transit or leading line. You head towards the light, and once it becomes visible, you head directly towards it. If you deviate from the course, you will lose the light. Once you are on track, you can use it as a method of plotting a position line on your chart. The light is visible both day and night.