FICURE I

COMMENCEMENT OF

LONCITUDINAL SECTION

PLOTTING OF LEVEL BOOK-PACE 2

NOTE THE CHAIN USED IN THE FIELD FOR MEASURING THE HORIZONTAL
DISTANCES CONTAINS 100 LINKS. EACH FOOT ON THE LEVEL STAFF
USED FOR TAKING THE VERTICAL HEIGHTS IS DIVIDED INTO TENTHS AND
SUBDIVIDED INTO HUNDREDTHS-PLOTTING SCALES THEREFORE ARE
DECIMALLY DIVIDED AND EXPRESS CHAINS AND LINKS WHEN USED FOR
THE HORIZONTAL MEASUREMENTS, THE SAME DIVISIONS DENOTING FEET
AND DECIMAL PARTS OF A FOOT WHEN USED FOR SCALING THE VERTICAL
HEICHTS. IN A CONCITUDINAL SECTION ALL DISTANCES ARE EXPRESSED
IN THE LEVEL BOOK, AS MEASURED FROM THE STARTING POINT

NOTE THE DATUM LINE SHOULD BE FIRST INKED IN. FROM BEGINNING TO END OF THE SECTION.
WITH A FINE BLACK LINE. DRAWN PERFECTLY STRAICHT, AND ITS HEIGHT FIXED IN TERMS OF
THE COLUMN OF REDUCED LEVELS IN THE LEVEL BOOK SO AS TO SUIT THE SECTION TO BE
PLOTTED. THE LEVEL OF THE DATUM LINE, HAVING BEEN DETERMINED AS SO MUCH ABOVE
OR BELOW SOME KNOWN BENCH MARK OR DATUM POINT, THE HORIZONTAL MEASUREMENTS ARE
MARKED ALONG THE DATUM LINE IN PENCIL, AND THE SECTION IS THEN PLOTTED. PACE BY PACE
AS FOLLOWS:-THE VERTICAL LINES SHOWN THUS
-ARE THE LINES NECESSARY TO
BE DRAWN IN PENCIL FOR PLOTTING THE HEICHTS ABOVE THE DATUM LINE-
THESE HEIGHTS ARE OBTAINED FROM
LUMN OF REDUCED LEVELS IN THE LEVEL BOOK.
AND ARE SCALED FROM THE DATUM LINE, THE HEICHTS BEING FOR THE SAKE OF ACCURACY
CAREFULLY MARKED THEREON WITH A FINE NEEDLE POINTER IN PLOTTING WITH AN ORDINARY
SCALE, THE VALUE OF THE SECOND DECIMAL EXPRESSING HUNDRED PARTS, IS ESTIMATED BY THE
EYE, WHEN MARKING OFF THE LENGTHS AND HEIGHTS REQUIRED, AND AS THESE HEICHTS CANNOT
AFTERWARDS BE SCALED WITH SUFFICIENT ACCURACY TO BE STATED CORRECTLY TO TWO PLACES OF
DECIMALS, IT IS USUAL TO FICURE ON THE SECTION, THE HEIGHTS ABOVE THE DATUM LINE.OF ALL
IMPORTANT POINTS, AS SHOWN IN FICURE No 2. MILES, FURLONCS, AND TOTAL LENGTHS ARE
INDICATED ALONG THE DATUM LINE IN MOST LONCITUDINAL SECTIONS.
ORDNANCE DATUM

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

PA

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, inter-
mediate 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

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LONGITUDINAL AND TRANSVERSE SECTIONS.

181

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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