Problem: Your vessel is anchored approximately 2 miles west of Cape Light. A horizontal sextant angle between the following points was taken: Cape Light and the water tower south of the light = 65° At the same time a vertical sextant angle was taken of Cape Light. The sextant reading was 0° 40.3’. The height…
Vertical danger angle. A vertical sextant angle can be used to ensure the safety of a ship in the vicinity of dangers. If the danger is close to high land or a lighthouse, the height of the land or lighthouse can be used as a reference for determining the sextant angle. Decide what distance from…
A position circle can be obtained from the rising or dipping of navigation lights over the horizon. The South African List of Lights and Radio Signals has a table of geographical ranges of lights. By entering the table with the height of the observer’s eye above water level and the height of the light above…
Leading lights and markers are usually used in the approaches to harbours or in channels where maneuvering room is very restricted. Their purpose is to guide vessels clear of dangers or down the best channel. They are shown on the chart by a line drawn through them called a leading line. The line is full…
The GPS will provide a position expressed in latitude and longitude. This will be plotted as a small cross, the centre of which is encircled, with GPS written alongside it.
If an observer sees two object in line ashore, ie beacons, then he must lie somewhere on the extended line joining them. This provides an excellent position line, provided the distance between the observer and the nearer of the two objects is no more than three times the distance between the objects themselves. A transit…
Using a sextant, an angle can be measured between two objects ashore, the position of which is known. By plotting the intersection of two lines forming the horizontal sextant angles between the two objects, a circular position line can be drawn. Remember that all angles subtended by a chord in the same segment of a…
Using a sextant, a distance may be obtained of a vertical object of known height, by measuring the angle subtended by the object at the eye of the observer. The determination of the distance is then merely a problem of solving a right angled triangle. One can either do this by calculation of by looking…
A fix may be obtained by using the range of objects ashore, the position of which is known from the ship. The range may be obtained from various sources, ie vertical sextant angle of an object, the height of which is known, range-finder or distance meter, a horizontal sextant angle range or most commonly a…
Radio direction finding (DF) bearings. Use may be made of radio transmissions to obtain a position line. It would necessitate either the ship having a radio direction finding capability or asking a shore radio station that is equipped to take DF bearings to take a bearing of the ship’s transmissions. Since radio waves follow great…
