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The New Zealand Railways Magazine, Volume 5, Issue 9 (April 1, 1931)

Surveying New Zealand

Surveying New Zealand.

I will now deal with surveying in its general application, and take the period when New Zealand was a virgin country, as far as surveys were concerned.

The first work undertaken was the major triangulation, or the accurate location of prominent landmarks such as mountain peaks, hills, or in flat country, page break
Surveying in Relation to Railway Engineering. Particulars explanatory of the above drawings are given in the accompanying letterpress.

Surveying in Relation to Railway Engineering.
Particulars explanatory of the above drawings are given in the accompanying letterpress.

page 14 any small rise with an extensive outlook. This major triangulation was then further subdivided into minor triangulation and permanent stations placed on prominent points. These points are known as “trigs,” and are seen throughout the country with a beacon erected over them for observation from adjoining trigs. In figure No. 11 I give a diagram of major and minor triangulation, the latter shewn dotted, also the base line used in the preliminary work.

The first operation is to locate a base line some 10 to 12 miles long on level country. This line is measured with great accuracy many times, and the mean measurement adopted. Extreme care is taken in this measurement, and allowance is made for the expansion of the measuring band through changes of temperature. Permanent monuments are placed at the terminals, and the bearing of the line in relation to true North is obtained by repeated theodolite observations of the sun by day and certain stars by night, the true bearing being found from astronomical tables. The base measurement and its bearing having been fixed, the horizontal angles to some distant prominent major mountain peak, or hilltop, are observed from each end of the base line, and the angles made with the base line are recorded. (See ABC in Fig. No. 11.)

A triangle is thus obtained with the base as the known side, two angles known and the third computed, being the difference of the sum of the two known angles and 180 degrees. The unknown sides are then computed. These sides are then used for bases of other triangles, without actual measurement, and observations are made from these sides to other major points, and the process is carried on ad infinitum. Minor triangulation is a further breaking up of the major triangulation to provide points of ready accessibility for surveyors.

It will be seen that the whole of New Zealand can thus be surveyed with the taking of only one actual measurement, i.e., the original base line, all other measurements being computed. Cook Strait could be spanned by observations to prominent landmarks on each side. There is no possibility of error provided the original base line and angles are correctly observed, as each subsequent triangle in the triangulation automatically checks itself with the adjoining triangles. Minor triangulation is further subdivided in town areas into a standard survey for the town, and standard blocks or stations are placed permanently at street intersections. These are placed with great accuracy as the value of the adjoining property is high and any discrepancy would be costly.

An instance of this may serve to lighten this somewhat serious subject. An ingenious person once “raised the wind” by obtaining the loan of a decrepit theodolite, and planting it in the main street of a small town, spent most of the day making great pretence of taking observations and measurements to the hotel on the corner. Later in the day he waited upon the publican and informed him he regretted that he found the hotel encroached about two feet on the public street. The publican was very disturbed, and gave the man £10 to say nothing about it.