Publication details: New Zealand Government Railways Department, Wellington

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Part of: The Railways Magazine

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# The Unit of Measurement

#### The Unit of Measurement.

Engineering surveying is a supplementary branch dealing more particularly with the configuration of the earth's surface and the subsequent location of lines, curves, grades, levels, angles, etc., of all works and structures included in the profession of Civil Engineering. The unit of measurement is the link and 100 of these go to the chain. A chain also equals 66 feet, so that a link is approximately eight inches. The one chain measure is divided into 100 links, and in accurate measuring the links are further divided into tenths. The colonial practice is to have a steel band 6 or 11 chains in length and about one-eighth inch wide. This band is wound upon a drum. The end length of the steel band, for a distance of one chain is marked in links by brass studs, and each chain by a numbered disc. This length of measuring band permits of a rough surface to be spanned in one operation, correction being made from standard tables, for the sag in the band, and varying expansion through temperature.

The unit of area is an acre, which equals 10 square chains, or 100,000 square links, practically a metric system. The area of any rectangular section can be found by multiplying the frontage by the depth in links and taking off five places of decimals thus: 100 links frontage by 250 links depth = 250 × 100 = 25,000, and with five places of decimals off = .25 or quarter of an acre. (In surveying, all measurements and areas are the horizontal equivalent.)

If you reside on a property with a natural slope, you have a surface beyond that shewn on your title deeds, but you could not place a larger building upon page 11 it, grow more trees vertically, or catch more rainfall than on a similar area on flat ground. (See Fig. No. 1.)

Let us first consider a survey made with the chain measure only, and without the use of an instrument (theodolite). (Such a survey can be undertaken by any layman and will be approximately correct.)

In figure No. 2 is shewn an irregular shaped field. Each of the four sides is measured on the ground, and diagonals as check lines. Having these measurements and deciding upon a suitable scale, the various measurements are taken, one at a time, on a pair of compasses and arcs drawn, the points of intersections of the arcs being the corners of the field. If the ground is on a slope, the measurements are taken on the horizontal, and a “plumbob” used to mark this length on the slope. (See Fig. No. 3.)

The width of a river, too wide to be spanned by direct measurement, may be ascertained by proceeding as shewn in figure No. 4. At C, in the line AB, a line CD, is laid off close to the river bank and at right angles to AB. The line CD is halved at E, and the line DF laid off parallel to CA. As soon as the point F comes into line with BE then DF equals CB. Deducting the distances of B and C from the actual river bank, the width of water is given. In laying off the right angle a triangle with sides measuring 3, 4 and 5, or any multiple thereof is set out with the chain. (See Fig. No. 5.)

A survey of a stream meandering through a field may be completed with the use of a chain only, as shewn in Fig. No. 6. Check measurements or tie lines across the angles, as shewn by line ABC, ensure the accuracy of the survey. The distance of the stream bank from the adjoining survey line is observed and noted at each chain, when measuring along the lines.