Charts are generally drawn on the true meridian; if otherwise, a true meridian is given on the chart. The datum to which the soundings are reduced is mean low water spring tides, unless otherwise stated in the title of the chart. Soundings are expressed in feet or fathoms, as stated in the title of the chart. I fathom = 6 feet = 1.828766 mètres. Underlined figures, on rocks and banks which uncover, express the heights in feet above low water of ordinary springs, unless otherwise stated. All heights (except those expressed in underlined figures) are given in feet above high water springs, or, in places where there is no tide, above the level of the sea. The natural scale is the proportion which the scale of the chart bears to the actual distance represented, and is shown thus- 12,150 1 A sea mile is the length of a minute of latitude at the place, and a cable is assumed to be a tenth part of a sea mile. The figures in brackets in the bottom right-hand corner of a chart, thus-(38.43 × 25°49) are the dimensions of the plate in inches between the innermost graduation or border lines. All longitudes are referred to the meridian of Greenwich. THE STATION POINTER The theory of the Station Pointer is founded on the twenty-first proposition of the Third Book of Euclid, which proves that the angle subtended by any chord will be the same from any part of the same segment of a circle. In Fig. 1, if the line A B subtend an angle of, say, 30° at C, it will subtend the same angle at D or anywhere on the arc A D C B. Therefore it follows that the position of observer can only be found by taking a bearing of one of the points. The Station Pointer is useful in fixing a point by means of the "three-point problem" or "two points "in transit and one angle. A Fig. 1. The instrument (see Fig. 2) consists of a disc about 6 inches in diameter. From a central ring proceed three arms about 12 inches long. The central arm is fixed and its bevelled edge coincides with the zero of the graduations. The other two arms can be moved to any required angle and can be fixed by a clamping screw. The instrument is graduated in degrees to the right and left of the fixed arm, which is the zero of the instrument. Method of using.-The legs of the instrument are clamped at the observed angles and the bevelled edge of the central leg is placed over the middle object, then the instrument is moved about, still keeping the central leg on the centre object, until the bevelled edges of the other two legs coincide with the other two objects or points observed. When the three legs lie over the three points (as shown in Fig. 2), the pricking point at the centre of the instrument being pressed down will mark the position of the observer on the chart. The fix by station pointer is good If the three points are in the same straight line. If two of the points are in transit and the observed angle is not small. If the "central point" is the nearest to the observer and the angles not too small. If the observer's position is within the triangle formed by the points. If the points are nearly equidistant from the observer and the angles not less than 70°. If one angle is large and the other small, and the small angle is made with an outer object far behind the central object. The angles should be sensitive on a change of position of observer. If the centre of the instrument can be moved without displacing one of the objects the "fix" is bad. When two points are in line they are said to be in transit. THE INDETERMINATE CASE When the sum of the observed angles is equal to the supplement of the angle at the central object, B, the position of the observer is indeterminate. It can, however, be determined by taking a bearing of one of the objects. If no station pointer is at hand the centres of the circles on which the observer is situated can be found in the following manner : A B subtend an angle of 60°; from each end of the line A B lay off an angle of 30°, and at C, their point of intersection, is the centre of the required circle. The angle at D being less than a right angle, the segment A D B is greater than a semicircle (Euc. III. 31), therefore the centre of the circle and the observer are on the same side of the line A B. When the angle is obtuse.-In Fig. 5 let A B subtend an angle of 120° at D. Lay off from each end of A B an angle equal to the excess of D above 90°, and at C, their point of intersection, is the centre of the required circle. E 60° 120° Also D being obtuse the segment A D B is less than a semicircle (Euc. III. 31), therefore the centre of the circle is on the side of AB remote from D, and the arc on which the observer is situated is the lesser segment, A DB. |