« PreviousContinue »
PLOTTING OF LEVEL BOOK-PACE 2
NOTE THE CHAIN USED IN THE FIELD FOR MEASURING THE HORIZONTAL
NOTE THE DATUM LINE SHOULD BE FIRST INKED IN. FROM BEGINNING TO END OF THE SECTION.
NOTE THE HEICHTS ABOVE THE DATUM LINE IN THIS
SECTION CIVE THE SURFACE LEVELS.
BY SETTING THE EDCE OF A LONG PARALLEL RULER TO THE DATUM LINE, AND THEN ROLLING IT A LITTLE BELOW
LONGITUDINAL AND TRANSVERSE
At least three assistants are required when taking a section,
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
LONGITUDINAL AND TRANSVERSE SECTIONS.
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,