Wellington Provincial Gazette – 1869 – No. 10

Page 54

✨ Survey Instructions

Hence arises the necessity of applying two distinct series of trigonometrical operations, viz., the Major Series, embracing a large tract of country in as few triangles as the power of the instrument at command and the natural features of the country will permit, thereby attaining the shortest lines of connection; and the Minor Series, the triangles of which derive their bases from the Major sides, and are made to be entirely dependent upon them. In turn these Minor sides afford checks to the chain measurements of the Sectional Surveys, and thus a system is initiated by means of which any undue errors become certainly detected, and limits to the intrusion of small unavoidable errors can be assigned.

The Major Triangulation, although assimilating in principle, not to be compared with the Great Trigonometrical operations of other countries.

It must be clearly understood that the Major Triangulation here spoken of is not of such a character as to be in any wise compared with the "Grand Trigonometrical Surveys of Great Britain or of India." The refinement and care bestowed upon these scientific operations were for the express object of measuring "Arcs of the Meridian" in order to determine the figure and magnitude of the "Earth," a purpose which can only be undertaken by wealthy nations and which would practically be of no service in meeting the wants of a new country, where the primary object is to execute expeditious and economical surveys so as to facilitate the sales of land and the settlement of the country.

In all National Surveys it is the custom to assign certain limits to the errors committed in the prosecution of the works, and this limit is apportioned according to the amount of accuracy known to be attainable in practice. By assimilating the principles of the Great Trigonometrical works to what is here called the Major Series, but without attempting to attain the same refinement and results, a considerable amount of accuracy is arrived at, and attended only with a moderate expenditure. It has been proved in this Province that the Major Series of triangles, the sides of which averaged from 8 to 10 miles, and executed with an 8-inch Transit Theodolite, exhibited errors less than six inches per mile at a cost of 14 shillings the square mile; and that the errors of the Minor Series of triangles, averaging from 2½ to 5 mile sides, and performed with ordinary 5 or 6 inch theodolites were less than 2 feet per mile when costing £2 14s. the square mile. As it would be impossible even in a champaign and well favored country to attain such constant accuracy solely by chain measurements, these results may be satisfactorily considered to be far within a limit of error that could safely be assigned.

To ensure an harmonious conformation of the various surveys, after a previous reconnaissance, the order of procedure should be—First, Major Triangulation; Secondly, Minor Triangulation; and Thirdly, the traverses of the boundaries of blocks, their rivers, streams, road lines, and sectional subdivisions.

But owing to circumstances over which there is no control, the above order cannot always be adhered to, and must therefore be modified in such a manner as to render the surveys easily susceptible of incorporation with the process immediately preceding. Thus a block of unsurveyed land of moderate extent may be commenced by erecting one or more principal stations at distances averaging 8 to 10 miles apart so as to embrace the whole block and to become eventually points for the Major Series. The Minor Triangulation may then be proceeded with. Or small isolated blocks and sections may be at once submitted to the ordinary process of a chain survey after erecting minor trigonometrical stations to be hereafter fixed at intervals of from 2½ to 5 miles apart, to which the traverses can be referred.

But in new surveys it will now be generally found that the localities lie adjacent to and within moderate distances from some fixed trigonometrical points affording the initial elements for commencing the new survey, viz.—a point of departure, the direction of the meridian, and a base of ascertained length for the extension of the triangulation. In such cases advantage is to be taken of all or any of the above data to prevent further surveys partaking of the character of detached operations.

For Major Triangulation the instrument used should be an eight inch Transit Theodolite somewhat similar in construction to the altitude and azimuth instrument.