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lines may be traced at equidistant vertical intervals of from 2 to 10 feet, where the scale of the plan varies from 50 to 500 feet to 1 inch. This plan of the ground should be in the hands of the surveyor on commencing his contouring, as it will be of considerable assistance during the operation; and it is also desirable that sections should be run from the level of some fixed plane of comparison along the principal and best-defined lines marked out by the rods alluded to, leaving pickets at the vertical intervals assigned to the contours. These pickets serve as tests of the accuracy of the work as it progresses and as starting points for fresh contours. The staff is now to be held at one of the pickets; the spirit-level (or theodolite used as a spirit-level) being so placed as to command the best general view of the line of level, and adjusted so that its axis may, when horizontal, cut the staff; and the vane (for a levelling staff of this description is required) raised or lowered till it is intersected by the cross wires of the instrument. The staff with the vane kept to this height is then shifted to a point about the same level between the next row of ranging rods not more than 12 or 15 chains distant from the spirit level, on account of the correction that would otherwise be required for the curvature of the earth (about of an inch in 10 chains), and moved up and down the slope till the vane again coincides with the wires, when another picket is driven. This process is continued until it is found necessary to move the level to carry on the contour line to the extent required.
The same operation of course takes place with the contours above and below that first laid out; and where any bench-marks or points, the level of which can be of importance, come within the scope of the spirit-level, they should be invariably determined. Where the vertical interval is small, the pickets upon more than one line of contours can often be traced without shifting the position of the instrument, if the levelling staff is of sufficient length. Too much should not however, be attempted at one time.
With regard to the second division of this subject, the tracing instrumental contours in connection with a national survey, the best instructions that can be given is a brief outline of the mode at present followed on the Ordnance Survey.
The ground between each of the trigonometrical stations is care
fully levelled with a spirit-level, pickets being left at convenient intervals for the contours to start from. The surveyor to be employed in tracing these contours is furnished with the altitudes of the pickets, or those of bench-marks out of the direct line between the trigonometrical points if they have been so left in preference, from which he has to level up or down to the contour height from whence he is to commence. With a theodolite or spirit-level he then traces the contour lines round the hill features in the manner already described, levelling to certain other bench-marks, whose positions have been given to him, but of whose altitude's he is not informed, in order that a check may be established upon his work; the position of the contour lines being recorded in a field-book, with reference to the measured detail of the houses, fences, &c., in a close country; or by transverse lines in open uncultivated ground.
The whole of the altitudes for the foundation of the contour lines are determined by levelling with the spirit-level; the calculated heights obtained by angles of elevation and depression during the progress of the survey, not being considered sufficiently accurate for the work as it is now performed :-the vertical distances between the contour lines thus traced out on the Ordnance Survey (now published on the scale of 6 inches to 1 mile, or 880 feet to 1 inch), varies-according as the character of the ground is steep or flat-from about 50 to 250 feet. These contours are, however, all interpolated with intermediate horizontal lines, run with the water level at the constant fixed vertical intervals of 25 feet.
By assuming the level of the sea as the datum plane from which these progressive series of contours are to reckon, the altitudes of the several horizontal sections above that point are at once represented, which is a more useful and practical arrangement than the system adopted by the French (who first introduced this method of delineating ground), of fixing upon some imaginary plane of comparison above the highest parts of the plan, similar to the mode still practised with ordinary sections.
On surveys, where pretensions are not made to such extreme mathematical precision, horizontal sections at distant vertical intervals, may be traced with the theodolite or spirit-level, and the