SURVEYING AS PRACTISED BY CIVIL ENGINEERS CHAPTER I. SURVEYING WITH THE CHAIN ONLY. Instruments: Chain. For the purpose of making actual linear measurements on the ground, the chain is most used. It consists of strong links of steel or iron wire of from No. 7 to No. 12 W.G., connected by rings, with a brass handle at each end (Fig. 1). In English-speaking countries there are two different lengths of chain in common use, the Gunter's or 66 ft. chain and the 100 ft. chain. The 66 ft. chain is most used, and possesses the advantage in computing areas that 10 square chains is equal to 1 acre. Both chains consist of 100 links, and every tenth link is distinguished by a brass tablet, as shown in Fig. 1. The first 10 links from the end is marked by a brass tablet with one point; the tablet at 20 links has two points, that at 30 links three points, at 40 links four points, and 50 links or the centre of the chain is marked by a circular tablet. Each 10 links is marked from the other end similarly, so that the chain can be read both ways. 1 1 Each link of the 66 ft. chain is therefore part of 66 ft. or 7.92 in., and each link of the 100 ft. chain is 1 ft. Distances are thus measured with the 66 ft. chain in chains and decimal parts of a chain, or links as they are called, as 7.85 chains or A 7 chains 85 links; with the 100 ft. chain the measurements are of course in feet only. In the United Kingdom nearly all railway work and ordinary surveying is executed with the 66 ft. chain, the use of the 100 ft. chain being principally confined to works of water supply, sewerage, and municipal works. The 100 ft. chain is, however, universally used in British India and the United States, and generally in the Colonies. In countries where the metric system is used, the usual length of chain is 20 metres. This is almost exactly 66 ft., 20 metres being equal to 65.6 ft. Reading the Chain. In taking measurements with the chain, we must look for the nearest brass tablet short of the point that is being measured to, and count the number of links from it forward to the point in question. If the nearest tablet indicates 20 links, and the point being measured to is 6 links beyond it, the distance is thus 26 links. Custom soon enables one to read the chain at a glance. Some confusion may arise at first from the fact that the tablet with four points indicates 60 links as well as 40 links, according as we reckon from one end of the chain or the other; in the same way the tablet with two points may either indicate 20 links or 80 links. A little practice, however, soon enables one to tell which is the correct reading. A look at the end of the chain or at the 50-link tablet will always decide the point. Laying out Chain on Ground. -When not in use, the chain is rolled up in a bundle (see Fig. 2) and fastened with a leather strap. To lay out the chain on the ground, take both. handles in one hand, and throw forward the chain, keeping hold of the handles. When fastening up the chain after use, take it up at the centre link and bunch it up double, two links at a time. Arrows. pins (Fig. 2). Accompanying the chain are ten arrows or marking These are inserted in the ground to mark the end of each chain. When the ground is too hard to insert the arrow, a scratch may be made with the point for a mark, or the arrow may be simply laid down flat, care being taken that it is not moved while dragging the chain forward. Ranging Rods and Flags.-These are iron-shod wooden poles, slightly tapering from the bottom, which are used to range out lines and mark points. They are usually 5 or 6 ft. long, and are painted in divisions alternately black and white or red and white, to render them visible at a distance. When they are at a considerable distance, they should have a piece of white or red cotton cloth tied on the top as a flag, to make them conspicuous. A very convenient length is 10 links, painted in ten divisions. These rods come in very useful for measuring short offsets. Three rods are necessary to range out a straight line, but the surveyor should have from six to twelve ranging rods to mark the ends of the chain lines and important points of the survey. For very long lines it is necessary to have poles of greater length, 10 to 15 ft. long or more, according to circumstances. In wet weather for flags use woollen cloth instead of cotton, which clings to the pole when wet. Laths. A bundle of laths sharpened to a point sometimes comes in useful in ranging long lines and marking points to be levelled in taking cross sections. They are cheap and easily procurable, and are easily sharpened to a point or cut to any desired length. Being white, they are easily seen, and they may be discarded when done with, without serious loss. Whites. When laths are not to be had, small twigs sharpened to a point and cleft in the head, with a piece of paper inserted in the cleft, are good marks. Steel Tapes. For accurate measurements a steel tape must be used. This consists of a steel band about in. wide, with feet, inches, and eighth parts of an inch marked on one side, and links or decimals of 66 ft. on the other, in the case of a tape 66 ft. long. Steel tapes may be had of any desired length, and are made to roll up inside a small circular leather case. Linen Tapes.-For ordinary measurements a linen tape is used. The chief use of the linen tape is the measurement of offsets, the chain being allowed to lie on the ground in the chain line while the offset is measured from it with the tape. Linen interlaced with fine metallic wire has been used as an improvement in point of strength and stretching. It is, however, inferior to a good plain linen tape. New tapes may be bought separately, and are easily inserted in the leather case when the old tape is worn out. Linen tapes are not to be depended on for accurate or important measurements on account of their stretching, especially if wet, while on a windy day the tape is useless for long measurements, as it catches the wind, sags and pulls. If the tape gets wet and dirty, it should never be rolled up until it has been cleaned and dried. It should be coiled loosely until it can be washed in clean water. It may then be hung up to dry, and afterwards rolled up in the leather case. Steel Band. As a substitute for the chain a steel band may be used. This consists of a steel band with a brass handle at each end, similar to the ordinary chain handles. The advantage of the steel band is that it is practically unalterable in length (for ordinary small surveys); while the chain is liable to be stretched or to have its links bent, and must therefore be tested occasionally and adjusted. The steel band does not accommodate itself to the inequalities of the ground when laid down, as well as the chain. It is, however, no doubt better and more accurate for measuring long lines, especially when not many offsets are required, and it is consequently not necessary to allow the chain to lie on the ground; in such work, for instance, as chaining out the centre line of a railway. Adjusting the Chain.-This is effected by removing some of the rings connecting the straight links if the chain is too long; or by straightening out the links and inserting some of the additional rings furnished on the chain if it is too short. The chain may be tested by comparing it with a steel tape. Cross Staff. This is used for setting out long offsets at right angles to the chain line. The simplest form of cross staff |