782.—False Picket.—For the placing of a picket it is usual to clear the sod with a small spade where possible, so as to suspend the plummet from the theodolite into the hole made by the picket to triangulate from its position. In marshy lands and under many conditions this is not easily done. It will generally be found more expeditious to carry about one of the author's false pickets, to place directly in the hole from which the picket is removed, which saves the trouble of removing the grass. This is shown in Fig. 384. It consists of a wooden peg, upon the top of which a cross is sawn to represent the axis. This cross is filled in with a veneer of ebony, and the whole is polished over to keep it clean. It will be readily seen that any picket accidentally broken will make a false picket. In setting up the theodolite over it the plummet is brought to verticality with the centre of the cross. In moving the false picket the original one is easily replaced, if required, in the same position for continuing the work.
783.—Spur-shod Picket.—Much stouter poles than may readily be pressed in by hand, as for instance, of 2 inches diameter, may be driven into the ground by having a spur or cross-bar of steel, about 7 inches long and about 3/8 inch diameter, placed through the pole, say at 1 foot distance from the point, a form which is much used on the Continent. This picket may be jerked down for a certain distance by pressure of the foot on each side, and then jerked home to the ground by standing upon it, to make a 10-feet or 12-feet pole stand sufficiently rigid for temporary work, Fig. 385.
Fig. 386.—Socket for station pole.
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784.—Permanent Stations are commonly constructed upon hilltops or other commanding positions. A very general way is to set up a long pole of fir or other wood at command, from 10 to 20 feet in height, according to the circumstances. Occasionally it is desirable to remove the pole and place the theodolite centrally over its vertical position. A very good way to do this is to have a slightly tapered wooden socket, Fig. 386 S, constructed of stout boards, say 1½ inches thick, made into a hollow square with a cross of boards, WWWW fixed to it. The socket is placed in a hole dug out entirely below the ground, and is rammed in and fixed as an ordinary gate post. The pole P is squared at the end to fit the tapered socket up to shoulders which are formed by leaving the other part of the pole round. The socket for a 15-feet pole should be 18 inches deep; for a 20-feet one, 2 feet deep. Where these poles are properly prepared they may be jointed together in two or more parts for portability. Bunting flags, red and white, about 18 inches by 9 inches, may be fixed at the tops of the poles. In fixing the socket the pole should be erected in it to be able to keep it constantly vertical during the ramming. A plummet suspended at arm's length, at a distance from the pole in two positions at about right angles to each other from the centre of the pole, will provide a means of keeping it erect during the fixing of its socket. The socket hole, upon lifting the pole out, forms the centre for erecting the theodolite over its position.
785.—Referring Object.—It is desirable that all arcs taken by the theodolite from an important station should contain one point in common, for which the best defined object to be found at a distance may be selected. Colonel Clark, of the Ordnance Survey, recommends as a referring object two rectangular plates of metal placed with their edges parallel to each other in the vertical plane, at such a distance apart that the light of the sky seen through the opening appears as a vertical line of about 10″ in width. The best distance for this object is from 1 mile to 2 miles. Two pieces of board, fixed a small distance apart by ledges screwed thereon, answer the same purpose. The description fully conveys the method without illustration.
Stations Visible at Great Distances are formed by means of reflection of the sun's rays or by artificial light.
