AUTHOR OF A SKETCH OF THE CIVIL ENGINEERING OF NORTH AMERICA,

STOR LIBRARY

NEW-YORK

& c.

ADAM & CHARLES BLACK, EDINBURGH;

LONGMAN & CO., AND J. WEALE, LONDON.

MDCCCXLII.

415

TREATISE.

CHAPTER I.

TRIANGULATION.

Selection of Stations-Conditions required to constitute a good Triangulation --Difficulties in selecting Stations-Poles-Flags-Arrangement of Colours of Flags, &c. for distinction-Reference to the Magnetic North —Local variation-Examples of local variation in Surveys-Its effect -Construction of a Compass for the plan-Selection of the Stations from which to determine the Magnetic North-Points to be kept in view in adjusting the Theodolite for observation-Mode of observing and registering the Bearings-Example of Field Book-Rule for adjusting Instrument at the succeeding Stations-Parallelism of the same Bearings at different Stations—Reverse Readings-Rule for reducing reverse Bearings.

THE triangulation is the first operation to be performed in making the survey of a river; of which, indeed, it may be said to form the groundwork. It consists, as is no doubt already known to most of our readers, in the selection of certain well defined points on the banks of the river, called Stations, and the determination of the relative positions of those points by angular observations. Considerable judgment is necessary in selecting these stations; and the bearings by which their positions are determined must be taken

A

with great accuracy, more particularly as they are afterwards to be employed as objects for observation in laying down the positions of sunken rocks, sand banks, and soundings of the depth of water, and in making the survey of the margin or banks of the river; departments of the field work which will be more particularly explained hereafter, when the various uses to which the triangulation stations are to be applied will be fully illustrated.

Before attempting to fix the positions of any of the stations, the observer should walk along the whole extent of the river to be surveyed, examining first the one bank, and then the other. The object of this perambulatory survey is to make himself master of the general configuration of the shores or banks of the river, that he may be the better able, from actual knowledge of the ground, to fix the stations so as best to fulfil the several conditions required to constitute a good triangulation; on which, as may be inferred from what has already been said, the accuracy of the other departments of the survey, as well as the ease with which they are made, chiefly depend. The conditions referred to are,

First, That the triangles formed by the imaginary straight lines joining the adjacent and opposite stations be as nearly as possible equilateral.

Second, That from each station there shall be visible as much of the ground to be surveyed, and as many of the other stations, as possible.

Third, That the stations be not so far distant from each other as to render it inconvenient to employ them as points of observation in determining the positions of the sand

banks, rocks, soundings, or other objects which require to be laid down.

Fourth, That the stations be as few in number as possible, consistently with the foregoing conditions. And,

Fifth, That each station be so chosen as to allow the theodolite, or other angular instrument, to be placed in correct adjustment over the spot occupied by the stationpole; which, as will be afterwards seen, must admit of removal for that purpose.

The fulfilment of these conditions may appear, at first sight, to be no very difficult task, for, in viewing the outline of a river as laid down in a chart or plan, the difficulties which have, in many situations, to be encountered, are not in their full extent discernible; but in practice, many obstacles present themselves, which experience alone enables the surveyor to overcome, and these impediments to the operations render the proper selection of the stations for the triangulation a work in which both care and judgment are required. Inequalities in the level of the banks of rivers, for example, often give rise to great inconvenience in selecting the stations. In some situations, the banks suddenly rise from a low flat to an abrupt head, having a considerable elevation, and projecting from the general line of the shore. When a corresponding elevation and projection take place in the opposite bank, producing a sudden contraction in the bed of the river (a formation of country not uncommonly met with), the triangulation may be said to be divided into two compartments, one on either side of the projecting heads; and it is often very difficult, in such a case, even by the best possible arrangement of the stations,