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Of Surveying Harbours.
194. There are two objects to be attained in the survey of a harbour.
1st. To survey the shore along high or low water mark, to trace its windings, to note the points and inlets, and to ascertain and fix the places at which rivers and creeks discharge themselves. And,
2dly. To discover the channels, their direction, depth, and width; the position of shoals, the depth of water upon them, the nature of the bottom, and in short, whatever may contribute to easy and safe navigation.
To determine the principal points and trace the shore.
195. Having provided a boat and crew, row once or twice around the harbour, mark the more important and prominent points; at which, let station-staves with flags upon them be erected.
Then, measure a base line, and form a series of triangles, having their angles at the stations already chosen. Let the angles of these triangles be measured with the theodolite, and their sides calculated; after which, the high or low water mark may be traced along the shore with the compass, as hereafter explained.
Let us suppose that Plate 6 is a map of a harbour to be surveyed.
We see, by inspecting it, that the upper end of the lake at A, the termination of the harbour at B, the rocks at C, the point at D, the fisheries at E, and the two bays at F and G are all prominent points. At these points, therefore, let station-flags be placed. Then, measure the distance from A to B, for a base line, and let the work be begun at A.
Remove the staff at A, and place, by means of a plumb-line, the axis of the theodolite over the station. Then, having levelled the instrument, bring the o of the eyeglass vernier to coincide with the o of the limb, and tighten the clamp-screw
the body of the instrument until the telescope comes nearly on the base line AB: then tighten the clamp-screw K, and by means of the lower tangent-screw L, and the thumb-screw Z, bring the intersection of the spider's lines to coincide with the bottom of the staff at B. Then, direct the lower telescope to the same point, without moving the limb.
Having thus placed the instrument, examine the opposite vernier, and if it stands exactly at 180°, enter the direction from A to B, 00, as in the field notes below.
But if the reading of the opposite vernier exceeds 180o, enter half the excess for the direction. If the reading is less than 180o, take half of what it falls short, from 360°, and enter the remainder for the direction from A to B.
The two verniers are used to avoid any error which might arise from a defective graduation of the limb, or from an imperfect centring. A false centring, is when the centre of the limb or vernier plate is out of the axis of the instrument, and when this is the case, it is a fruitful source of error.
Both verniers should be read at every observation, and a mean between the readings taken for the true direction.
Having thus placed the instrument, loosen the clamp-screw of the vernier plate, and direct the telescope to station E. Note the degrees, and take a mean between the readings of the two verniers for the minutes, and enter the result opposite direction AE, as in the field notes. Do the same for the station G, and then enter in a column to the right, the angle formed by the lines which join the stations. The angle will either be the difference of the readings, or the difference between 360° and the larger reading, plus the smaller reading.
Having sighted to all the stations which can be seen from A, remove the instrument and replace the station staff.
Take the theodolite to B, the other extremity of the base line. It is now required to place the instrument in such a manner that the horizontal limb shall have the same relative
For this purpose, after having levelled the instrument, add 180° to the direction from A to B, and place the 0 of the eyeglass vernier at the point so found. Then clamp the vernier plate, after which direct both the telescopes to station A. It is now plain that the line of the limb drawn through 0 and 180° will coincide with the base line AB, the o being towards ., as before; hence the theodolite is like placed.
Having clamped the limb, loosen the clamp-screw of the vernier plate, and sight to stations E and C, and enter the directions as below.
Having sighted to all the stations which can be seen from B, replace the station-staff and remove the instrument to station C. To the direction BC 57° 12' add 180°, and the sum is 237° 12'. Having levelled the instrument, place the o of the eyeglass vernier at 237° 12', and then sight to station B. The limb of the theodolite will then have the same relative position as at the stations A and B. Then sight to E and D, and enter the directions as below.
Remove the instrument to E. To the direction CE=18027′, add 180°, and the sum will be 360° 27'. Then place the 0 of the vernier at 27', and direct the telescope to C. Or, the theodolite may be placed at E by adding 180° to the direction AE, as taken from A, or to the direction BE, as taken from B, and then directing the telescope to A or B.
By placing the instrument in a similar manner at every station, the line of the limb passing through 0 and 180o, continues parallel to the base AB, the o being constantly in the direction towards A. The instrument is thus placed at all the stations, and the following are the results of the measurements of the angles.
The measurements which have been made, enable us to calculate the lengths of the lines joining the several stations. For, commencing with the triangle AEB, we know all the angles and the base line AB; we can, therefore, find the sides EB, EA. We shall then know one side and all the angles of the triangle CEB, and by pursuing the calculation, the sides of all the triangles can be readily found.
Since the third angle of a triangle can always be found when two of the angles are known, it may seem unnecessary to measure all the angles. But when the three angles are measured and their sum found equal to 180°, the work is proved to be right, and this verification should never be omitted.
It is not probable that the sum of the three measured angles will be exactly equal to 180°. But they ought not to
times, and a mean of the measurements be taken, the errors of observation and of the instrument will be much diminished.
196 The method of determining points by a series of consecutive triangles, is called the method by triangulation. It may be extended to any number of triangles, and if the three angles of every triangle be measured, and the work carefully verified at each step, there is little danger of error. We have applied the method only in the survey of a harbour, but it may be used with equal advantage in all surveys in which long lines are to be determined, and is, indeed, the only one that can be relied on, where great accuracy is required.
Of the Manner of using the Compass.
197. The compass is often used in connection with the theodolite, and although a rude instrument, may yet be relied on for the shorter lines and smaller parts of a survey. The following is the manner of keeping the field notes.
Divide a paper into two equal parts, by two parallel lines near to each other, and consider each part as a separate leaf or page. Each leaf is divided into three spaces, and the middle one is generally smaller than either of the others, which are equal.
The notes begin at the bottom of the first page, and run up the page to the top. They then commence again at the bottom of the next page, and run up to the top; thence to the bottom of the third page, and thus, for as many pages as the work may require.
When the compass is used in the way we are about to explain, the distances to objects which lie on the right or left of the courses, are determined by means of offsets.
The beginning of every course is designated in the middle column by 0, and the bearing is entered directly above. The other figures of the middle column, express the distances from the beginning of each course to the offsets, and those in the side columns indicate the lengths of the offsets, or the distances to objects on the right or left of the compass lines.
The stations, at which the compass is placed, are designated by 0 in the middle column, and the bearing of each course is