Fixing Position. - The most accurate method of fixing a position relative to the shore is by angles between well defined objects on the
chart. All ships are provided with a station pointer, and this method should be used whenever possible.
Two things are, however, necessary to its successful employment: First, that the objects be well chosen; and, second, that the observer
is skilful and rapid in his use of the sextant and station pointer.
For the former, reference can be made to the pamphlet on the use of the station pointer, which is in every chart box; the latter is to be obtained
only by practice.
It will readily be seen that in war time, when the compass may be knocked away, or gun fire may make it undesireable to expose the person
more than necessary, a sextant offers great advantages, as angles can be obtained from any position whence the objectives are visible. It is this
contingency that makes it especially desireable that all navigating officers should become expert in this method of fixing a ship's position.
In many narrow waters also, where the objects may yet be at some distance, as in coral harbours or narrow passages among mud banks,
navigation by sextant and station pointer is invaluable, as a true position can only be obtained by its means. A small error in either
taking or plotting a bearing under such circumstances may put the ship ashore.
It is not intended that the use of the compass to fix the spip should be given up; there are many circumstances in which it may be
usefully employed, but errors more readily creep into a position so fixed. In all cases where great accuracy of position is desired, angles
should invariably be used, such as the fixing of a rock or shoal, or of additions to a chart of fresh soundings or new buildings. In all such
cases angles should be taken to several objects, the more the better; but five objects is a good number, as the four angles thus obtained not
only prevent any errors, but they at once furnish a means of checking the accuracy of the chart itself. In the case of ordinary soundings, it
is only necessary to make a third angle now and then; firstly to check the general accuracy of the chart, as above stated; secondly, to make
certain that the more important soundings, as at the end of a line, are correctly placed.
Sometimes, when only two objects are visible, a compass bearing and sextant angle may be used with advantage.
In passing near a point of land, or an island, the method of fixing by doubling the angle on the bow is invaluable. The ordinary form of it,
the so-called 'four point bearing', when the bearing is taken four points on the bow and on the beam, the distance of the object from the
latter position being the distance run between the times of taking the two bearings, allowing for current, gives an excellent fix for a
departure but does not ensure safety, as the point and probably the rocks off it are abeam before the position is obtained.
This is, however, strictly only true if the current is directly with or against the course of the ship. If a cross current has to be allowed
for, the results by this method may be altogether erroneous and misleading.
A table of factors, by which to multiply the distance run, to obtain the distance of the object when any number of degrees between the two
bearings has been observed, is supplied with all chart sets.
The use of a gander angle in passing outlying rocks with land behind should also not be forgotten. In employing this method, however, caution
is necessary, as should the chart not be accurate, i.e. should the objects selected not be quite correctly placed, the angle taken off from it
may not serve the purpose. It should not, therefore, be employed when the survey is old or manifestly imperfect.