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CHAPTER XVI.

LONGITUDINAL AND TRANSVERSE
SECTIONS.

At least three assistants are required when taking a section, -two men at the chain and a man to hold the staff, and the more experienced and skilful a surveyor is in selecting the spot for setting up the instrument, the more rapidly will he get over the ground assigned to him. The staffman calls out the number of links upon the chain at which the staff is held, and the surveyor books the total number of links in the column headed "Lengths," and describes the various positions of the staff (when necessary) under the heading "Remarks." (See pages 173-176.) When the ground is of an undulating nature at close intervals, intermediate sights are required to be taken. These occur as shown in the form of Level Book above alluded to, between the back-sight and fore-sight at any single setting up of the level. The chain usually employed in the field for measuring the horizontal distances contains 100 links. Each foot upon the level staff used for taking the vertical heights is divided into tenths, and each tenth subdivided into hundredths.

In a longitudinal section (fig. 2, pages 182, 183) all distances are expressed in the Level Book as measured from the starting-point. The actual datum point need not be a point upon the line of section. In transverse sections (fig. 3, pages 186, 187), the distances are expressed as being so many links either upon the left hand or right hand of the centre line.

Plotting scales, as explained upon pages 92-94, are decimally divided, and express chains and links when used for horizontal measurements, the same divisions denoting feet and decimal parts of a foot when used for scaling the vertical heights (pages 178, 179). The vertical scale for a section is usually exaggerated

in comparison with the horizontal scale, in order to render more prominent to the eye the particular irregularities in the surface of the ground, which it is the object of the reduced levels to exhibit.

The datum line should be first ruled in indian ink when plotting a longitudinal section (fig. 1 pages 178, 179), care being taken that the line is perfectly straight and drawn long enough from the beginning to the end of the section to need no subsequent prolongation as in a base line. (See page 90.) Its nominal height is then to be indicated, and should be fixed in terms of the reduced levels in the Level Book so as to suit the section to be plotted. The datum line should be a definite level, as so many feet above or below some fixed mark located upon the plan. In dealing with works where levels are needed to be recorded below water, by assuming a datum as a stated number of feet below a fixed level, all the levels to be plotted become positive thereto. The level of the datum line having been thus determined as so much above or below some known bench mark or datum point outside the section, the horizontal measurements are then marked in pencil for the purposes of plotting the section, along the datum line. In drawing the section, it is the horizontal distances between the stations that must be marked off. In the case before us these distances are to be scaled o, 200, 375, 400, &c., links to 600, upon the datum line, as given upon one page of the Level Book (pages 173, 174). The section is plotted page by page, as follows :-Vertical lines are first ruled in pencil through the marks at o, 200, 375, &c., upon the datum line (see fig. 1, pages 178, 179) with the aid of a set square sliding against a straight edge, set parallel to the datum line. The set square employed should be long enough to draw these vertical lines of sufficient length so as to need no further prolongation when scaling the heights, and its vertical bevelled edge should appear upon the lefthand side as shown unless reversible as in a vulcanite or pearwood set square. The lines should be drawn upwards from the datum line as far as necessary. These heights are obtained from the column of reduced levels in the Level Book and are scaled from the datum line, the heights being,

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Land Surveying and Levelling, pp. 182, 183.

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for the sake of accuracy, carefully marked thereon with a fine needle pointer held perpendicular to the drawing so as to puncture the paper as little as possible. A fine-pointed pencil is not accurate enough when the comparatively small scale of the section is considered. The 100th part upon the scale in plotting is estimated by the eye. Hence, so far as book work is concerned, there is no practical advantage in reading hundredths for an intermediate sight. The points so pricked off are joined by a fine black line ruled in ink, and this is best done if the surface line is inked in before plotting another page of the Level Book (page 174).

The rolling parallel ruler employed should be heavy, with handles to lift it, yet not too long for rapid use, and with its wheels deep enough to pass freely over the paper. length of 18 inches proves satisfactory. If a long straight A edge parallel to the datum line be used, it may be maintained in position by flat weights. The position of the datum point or bench mark is indicated upon the plan in fig. 2 near the end of the longitudinal section. Where the section is taken upon a curve, the radius of the curve (in this case 20 chains) is figured upon the longitudinal section. The tangent lines to this curve intersect at the peg A upon the plan (see fig. 2) off the line of the section. A horizontal line, as shown in fig. 2, is usually drawn above the section, parallel to the datum line. Dotted lines are drawn vertically from the line indicating the surface of the ground to this horizontal line at all points requiring description, and the description is generally written in at an angle of about 45° to this line, as shown. a peg is left in the ground upon the section at this point Where the vertical dotted line is carried down to the datum line, so as accurately to mark its position in a horizontal direction. (See pages 182, 183.)

Transverse sections (as shown in fig. 3, pages 186, 187) have their position marked upon the longitudinal section as well as upon the plan. The height above the datum line is figured upon each transverse section, and also upon the longitudinal section, where the cross section occurs. They are taken for a distance of either one or two chains as required upon

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