Page images

outwards, to the right, like an inverted pendulum, or else he may hold the elbow of his left arm with his right hand and move the left hand palm outwards to the left. By such means he can readily indicate to the assistant that the bottom of the pole is correctly placed, but that the top requires to be moved more, either from or towards the assistant, in order to appear upright. If the line to be set out be a very long one, the surveyor may render his signals intelligible at a distance to his assistant by waving a pocket handkerchief in the direction required by either hand, or by fastening a pocket handkerchief to the end of a spare ranging rod and employing this as a flag to indicate his instructions.

When any intermediate pole is set accurately, both as regards position and uprightness, the surveyor indicates that it is correct by raising both hands, palm down, one upon each side of the pole at A, and, while slightly bending his body, moves them from about the level of his head towards the ground. The assistant then presses the pole into the ground with both hands, or, if the ground be hard, props it up with loose stones or other material, and looks towards the surveyor for final instructions as to leaving it perfectly vertical before he proceeds to set the next pole. If both the stations A and B are fixed and the intermediate points upon the line have to be determined, the best order in which to place the poles in position, is to commence near the distant station and allow the assistant to work towards the surveyor along the line, as by this means each pole that is being fixed is distinctly seen by the surveyor, and is not in any way hidden behind the other poles. Where possible the surveyor should observe in each case the bottom or lower end of the ranging rod. If the pole marked 5 in fig. 1 is fixed first, the remaining poles might appear in line when a complete diameter out as indicated upon the diagram, because daylight could not be seen between them in the direction A B, and the base line A B would, if continued beyond B or if the pole B does not stand perpendicular, take the direction of the dotted line. Although this would not affect the actual length of the line measured between the points A and B, it would affect the length of lines

measured off A B upon either side of it, as it is clear that if the chain be laid out of line, an offset will be measured either too short or too long, the consequences being an incorrect plan leading to inaccurate computation of area.

Figs. 6 and 7 (pages 6, 7) illustrate the ranging out of a base line between two fixed stations C and F when a hill intervenes. The poles C and F are fixed perpendicularly in the ground, and the surveyor with one of the intermediate poles D or E and his assistant with the other intermediate pole, each proceed to a distance as far apart from one another as will enable the top half of the pole at C to be viewed from E and the top half of the pole at F to be viewed from D. The diagram, for the sake of clearness, shows the poles D and E shifted about upon one side of the line C F only. It is probable that one or both of them in the actual setting of the lines, C D E and D E F would be brought too much over the line C F, and would have to be shifted back and wriggled into proper position by the process illustrated in fig. 7. It is evident that the farther apart the points D and E are from one another the more accurate will the line between C and F be ranged out, as a longer length of line will be common to the two sides C E and D F (figures 6 and 7).

When the ground is very undulating the surveyor is compelled to set up the poles at very short distances apart. When the line crosses a thickset hedge, a gap must be cut through with a bill-hook, sufficient to make the continuation of the line upon the opposite side clearly visible.

Poles are generally understood to be longer than ranging rods, but are the same in principle. Both are usually shod with a pointed iron shoe for convenience of planting in the ground. The most suitable ranging rods taper from the bottom to the top, being over an inch in diameter at the shoe, and about inch at the top. When the position of a station point upon or at the termination of a base line is marked by a wooden peg driven into the ground, it is advisable to observe the precaution illustrated in fig. 8 (page 6) in order that the pole may appear in the direction of the line viewed. In rough open moorland iron pegs fixed in concrete, as shown in figs. 9 and 10, may be advantageously b

employed for the main stations. The hollow portion serves to hold a ranging rod, which can be easily wedged up vertically in setting it up with the aid of a plumb-bob (see fig. 2). The top of this iron peg (fig. 10, page 7) serves as a reliable bench mark when taking the levels.

A moderately gentle slope is by no means objectionable, as it enables the surveyor to obtain a better view of the country in ranging out his lines (see also pages 57 and 72, but in railway work the precaution named upon page 309 must be observed). That the setting out of a straight line on the ground from one point to another demands some care and skill is easily tested by attempting to cross a field covered with snow, or other ground leaving footmarks, directly from one object to another. You must view two objects in line, one visually covered by the other to guide you for the continuation of the line in a straight direction. With one object only, as a guide, an irregular course will be pursued, while by means of continuously covering coincident sights, a nearer approach to a straight line is obtainable. Hence it is necessary to have at least one intermediate pole upon a line in addition to the poles at each end.



ALL lines upon which a survey is based are measured in a perfectly straight line from end to end, for which purpose a surveyor has to select one of two kinds of chain and tape which are commonly employed in the measurement of land, each divided into 100 links. The total lengths of the best known short chain and tape are each 66 ft., and of the long chain and tape 100 ft. The former is called Gunter's chain, from its inventor, the Rev. Edmund Gunter (1620 A.D.), and its use is quite peculiar to this country. It consists of 4 poles, or 22 yards, decimally divided into chain links, with eyes at each end. Each link, being one hundredth part of 66 ft, will be equal in length to 7'92 in., but this fact is more interesting than useful, as any portion of a chain is invariably expressed in links. Chains of metre lengths, divided to fifths of metres and tallied at every two metres, are also sold for work requiring a metrical


Link Chains.-A long chain saves time over level ground in fixing arrows in the ground; but where the ground is uneven and rough, the links being so long as compared with those of a Gunter's chain, are apt to be twisted in being drawn through hedges and rough places, which of course produces errors in the measurements. In open ground a long chain aids accuracy, because the more often the whole length of a chain has to be shifted in measuring a base line the greater are the chances of error in recording the total measurement. A chain 100 ft. long is a suitable length, as a longer chain would be found very inconvenient to drag, especially in wet weather.

In folding up a chain the most expeditious plan is to roughly lay it out as in fig. 1 (pages 14, 15); then take the links

nearest the 50 mark in one hand and fold the chain double, until the handles are reached, taking care so to cross the links which come next to one another in folding, that the body of the chain, when folded, may be smaller in the middle than at the ends, as shown in the figure representing the chain folded up. A strap is first passed round the middle to bind the links together and is then passed through the handles together, with the heads of the arrows, and fastened by a buckle. When you wish to unfold the chain, having unstrapped the arrows, take both handles in one hand and, having freed the first two or three links, take the remainder of the chain in the other hand and throw it out from you taking care to keep hold of the handles. When thus thrown out upon the ground you must then straighten and adjust the links where necessary and close any of the connecting rings that may be open at the joints before proceeding to measure the base line.

Every chain is accompanied by ten arrows, each about one foot in length, made of stout iron or steel wire, and pointed at the bottom. The reason why ten is the number adopted is probably that ten chains of 66 feet in length equal one furlong. They are usually bent in a circular form at the top for convenience of handling, and a piece of red cloth is often attached to the ring of each, in order that they may be more easily observed when fixed in the ground. When in use they may be best carried by attaching the buckle end of the strap, which is taken off the chain, to the ring or top of one arrow, and passing the strap through the rings of the remaining nine arrows from which they can be easily removed, one at a time, as required. The use of the arrow with the strap upon it will then indicate that the whole ten arrows have been employed in the measurement of any given base line. Some surveyors adopt the plan of using ten supplemental arrows, distinguished from the others by brass or other marks, to be put down in succession at each tenth chain. In chaining long base lines it is advisable to put down a peg at every ten chains, entering it in the field book (which will be described further on in these pages), although, as here stated and subsequently referred to, the usual practice is for the ten chains

« PreviousContinue »