8/10 cocked hat navigation running fix method position fixing methods radar fix methods of obtaining a fix position
When plotting the estimated position or EP on a passage, errors do occur, making the position doubtful. To corroborate a position, periodically the navigator fixes the position of the yacht relative to known landmarks or features. When close to a charted sea mark or landmark the position is determined by eye, where in other circumstances fixes are obtained with a hand bearing compass or a depth sounder.
If alongside a charted mark the navigator can confidently plot a fix on the chart but at other times, obtaining a fix is done by producing a minimum of two position lines. This is done by measuring either the bearing or the distance of a known object then plotting the line on the chart. Measuring the bearing of an object produces a straight line, while measuring the distance of an object produces a curved one.
A single position line does not give an accurate fix and needs to be crossed with a second, and preferably a third to fully confirm the fix. A single line indicates that the boat is situated somewhere
along the line. If the position line is at 90 degrees to the boat’s track, it confirms what position the boat is along the track. Single position lines formed by a transit are used when navigating harbours, or as a clearing line.
Before plotting a visual fix, identify two or three seamarks or landmarks on the chart that are within sight. Use three marks if possible, as any error becomes more obvious. If using only two objects, pick marks that are about 90° apart as this affords a more accurate result.
When using three marks, choose ones that are about 60° apart from each other. Marks that are chosen close to the boat will increase accuracy. If the mark is of some distance, an error of 3° in the bearing produces a considerable positional error. It is critical that the correct marks on the chart are identified, or the fix will be useless.
Use the [ hand bearing compass ] to take compass bearings from a position on the boat known to be free of magnetic deviation. Brace the body firmly against the boat structure and take three compass bearings on each mark and have another write down the bearings as they are taken.
When taking compass bearings, the possibility of an error increases because it is difficult to hold a compass steady on a moving deck.
Before plotting the position lines, calculate the average compass bearing from the three readings. If plotting in °T on the chart, correct the compass bearing by applying variation. Remember that the bearing from the object to the yacht is the reciprocal of the bearing measured. Add or subtract 180° from a bearing to calculate the reciprocal and then draw a line on the chart through the object on the reciprocated bearing.
When compass bearings are taken of three marks, it is unlikely that the three position lines will meet at one point, instead forming a triangle known as a [ cocked hat ] . A good guide to accuracy is the size of the cocked hat, the smaller the more accurate the plot. A circle is drawn around the triangle and labelled with the time and log reading. Assume that the position is at the corner of the cocked hat triangle closest to any hazard along the proposed track for safety reasons and shape the next course from that point.
There are occasions when sailing along a coastline there is only one identifiable seamark or landmark in sight. In this case it is possible to obtain a useful fix using two position lines taken at different times on the same mark, known as a running fix or a transferred position line.
The first bearing is taken approaching the mark and the second one after passing it, with its bearing to 90° of the first reading.
GPS navigation to fix the position has reduced the need for this type of fix.
On the horizon, a light rising or dipping is used at night to obtain a position line or a fix. The distance you can see a major light is influenced by the height of the light and the height of eye over the earths [ curvature ]. When first seeing the light above the horizon, take a note of the time and take a bearing.
Calculate your eye height above the water taking in tide height. Obtain the height of light above sea level from the chart. A Rising and Dipping light table in nautical almanacs allows for the distance off from a major light to be calculated by using these two reference points.
Distance off is obtained from a radar range or by the use of a visual range finder or a [ vertical sextant angle. ] At night, obtain distance off by observing the light house light from when it appears or disappears over the horizon when sailing towards or away from it. Having obtained a distance off an object, set the drawing compass to the distance from the chart scale and with one point on the object, draw the curved position line.
Heights of lighthouses and other landmarks on charts are given as height above Mean High Water Spring (MHWS) tide. On occasions when trying to find the height of a landmark, the tide will be lower than the MHWS level.
A depth sounder is a handy device for confirming a fix construed from other sources. Sometimes, it is also used to obtain a position line. To use a depth finder this way it is set to show the [ water's depth ] from the surface so adjust it if necessary if it measures the depth from the keel bottom or the transducer level.
Using the depth sounder to check fixes requires the depth observed to be converted to a sounding at chart datum and known as a 'reduction to soundings'.
Depth soundings can be used to create a fix and although not very accurate. It is helpful when in fog or when there are no fixing aids available. This fix method works when sailing into shallower or deeper water across depth contours marked on a chart. The time, course, and log reading are noted when taking a depth sounding. While holding a steady course take regular depth soundings, noting the time and log reading.
A series of depth readings is reduced to soundings as done for checking fixes and the results are plotted.
Readings from a standard GPS set offers the most exact fix available, accurate to 100m or 330ft for 95 per cent of the time for, or about 10m or 33ft with a DGPS unit. From at least four satellites, a GPS set calculates its [ precise position ] from the intersection of the curved position lines. Increased accuracy is offered by a DGPS set by the utilization of radio signals from a base station that refines the satellite signals and reduces built-in errors. A GPS fix is always available tempting the navigator to rely on the system. A practical navigator enjoys the GPS’s simplicity and accuracy but double-checks the reading against an EP or a fix from other sources.
A GPS set displays the boat's position in degrees, minutes, and tenths of a minute of latitude and longitude. For the small-boat navigator, plotting a latitude and longitude position on the chart may be awkward and leads to errors.
An improved system is to find a suitable seamark or landmark near the track, then programme its latitude and longitude into the GPS as a waypoint. The GPS is then set to display the boat's position as a bearing and distance from the waypoint making it easier to plot on the chart. Having plotted the fix by GPS, mark it with the time and log reading and note the depth as it is a rough check of position.
Compare the fix with the EP and if there is a significant discrepancy, double-check by using another source of information.
A unique skill and experience is required to effectively use radar, as radar reflections from coastlines can be confusing. Accurate identification of objects on the screen can be difficult when separating [ clutter ] created from rough sea making reflections from buoys hard to identify.
The best radar fixes use the distance of two or preferably three objects which are then [ plotted ] as curved position lines using a pair of compasses. Where the position lines intersect is the boat's fixed position. The cocked hat size, signals the accuracy of the radar fixes.