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Revising and Improving Parliamentary Location.— The working survey is usually conducted on the 25 in. Ordnance scale, on the sheets of which the Parliamentary line and the limits of deviation should now be pencilled. A careful examination of the Parliamentary section and plan and a comparison of the ground with the larger 25 in. scale map will usually result in various modifications of the line presenting themselves, by means of which cuttings and embankments may be reduced.

As a rule, it will be possible to decide definitely upon these merely by inspection of the ground, but in difficult country it may be necessary to take levels and cross sections and plot one or more longitudinal sections of the proposed alterations before fixing upon the final line.

Final Location of Line in Difficult Country. The ruling gradient and minimum radius of curve have of course been already fixed on the Parliamentary survey and plans, and the method of locating the line in difficult country is as follows:Take levels over the Parliamentary line laid down on the map and leave marks where cross sections are to be taken. The points where cross sections are to be taken will be determined by the general configuration of the ground. They should be such that the surface between adjacent cross sections is approximately plane, or roughly speaking, cross sections should be taken wherever the slope of the ground changes. For methods of cross sectioning see Chapter III. As a rule, the clinometer is the most rapid to take cross sections with. Having taken and plotted the cross sections as shown in Fig. 169, the contours are to be taken off them and laid down on the plan as follows:-Draw a vertical AB to represent the centre line of the railway as laid down on the map. Suppose the reduced level of the surface of the ground at the centre line of the railway is 102.80, and that contours are required 5 ft. apart vertically. Then the first contour above the centre line of the railway will be the 105 ft. contour, ¿.e., the contour whose reduced level is 105.00 and 105.00 102.80: = 2.20. Plot up, therefore, 2.20 ft., and ab, bc, cd, each equal to 5 ft. Similarly the first contour below the centre line of the railway will be the 100 ft. contour and 102.80 Plot down, therefore, 2.80 ft., and ef, fg, gh, each = 5 ft. The points h, g, f, e, a, b, c, d, will then be at the reduced levels 85.00, 90.00, 95.00,

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100.00, 105.00, 110.00, 115.00, 120.00 ft. respectively, and the horizontal lines hs, gr, fq, ep, ak, bl, cm, dn, will give the horizontal distances from the centre line of the railway to the 85, 90, 95, &c., contours. These horizontal distances are to be measured off the centre line of the railway on the plan, and will give the position of the contours at that point.

Each cross section is similarly treated, and the positions of the contours plotted on the plan. By joining these corresponding points on the plan we then get the contours of the surface of the ground. A quicker method is given in Chapter III., graphic interpolation of contours, page 154.

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Fig. 169.-Position of Contours from Cross Section.

If the scale of the general plan is too small to plot contours 5 ft. apart vertically, the centre line of that portion of the railway to be cross sectioned and contoured should be laid down on a larger scale on a separate sheet and the contours plotted on it.

Having then decided on a trial gradient, calculate the distance in which the rise or fall of the gradient is 5 ft. if the contours are 5 ft. apart vertically; if 10 ft. apart, the distance for a rise or fall of 10 ft. Take now this distance in the compasses, and step from contour to contour, marking the points where the compass touches each contour. A line joining these points is then the line on which there is neither cutting nor bank, and is called a "surface line.” The proper location of the railway is then the line which can be

drawn most closely to agree with this line, and as a rule the smaller the minimum radius of curve the closer will the location agree with the "surface line." A longitudinal section of this proposed location may now be plotted from the contours and examined with a view to adoption or otherwise. If not satisfactory, a new line with. cross sections on it may have to be surveyed and the operation repeated until a satisfactory location is obtained.

It is perhaps unnecessary to say that all alterations must be within the limits of deviation as marked on the Parliamentary plans.

As regards sharpening curves and steepening gradients, there are certain specified limits which must not be exceeded, for which see a copy of the regulations. These matters must of course be strictly attended to, as before the inspection or opening of the railway plans and sections of the final and Parliamentary lines must be handed to the Board of Trade for approval. In certain cases, if sufficient cause can be shown, exception may be made to the regulations under special circumstances.

Pegging out Line. The line having been definitely fixed in this way and laid down on the 25 in. Ordnance sheets, these should be pasted together and mounted, and cut up into continuous rolls convenient for use in the field, or a tracing on linen made for field use. The next operation is now to set out the line by driving in pegs 1 chain or 66 ft. apart along the centre line. The field details of this operation are entered into later (see page 233 et seq.).

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Pegs. The pegs should be set out with the theodolite, and should be about 1 to 2 in. square, and 15 to 18 in. long. every 10 chains an index peg should be driven in slantwise, slightly to the right hand of the centre line peg, with the chainage marked on it in miles and chains with a blue pencil.

Tangent Points.-Tangent points should be distinguished by pegs driven in slantwise, one on each side of the centre line peg, and to right and left of it.

Chainages and Survey of Existing Detail.-The chainage of all roads, ditches, fences, &c., crossed by the railway must be carefully taken and entered in the field book. Any alterations to fences, boundaries, new buildings, &c., must be carefully sur

veyed, and the 25 in. map in fact corrected and brought up to date. The survey of existing features should extend for about a chain beyond the probable position of the railway fence as nearly as that can be judged. This may be estimated approximately from the probable height of bank or depth of cutting.

When important works, such as large bridges, &c., occur, it may be necessary to survey the road or river, &c., crossed for some distance on each side of the railway.

Longitudinal Section. The line having been pegged out, a careful section is next taken along it, levels being taken at every chain peg and also at any intermediate points where the slope of the ground changes.

Cross Sections.-Cross sections are also to be taken wherever the ground slopes at right angles to the centre line, or where a section at right angles to the centre line is rough and broken.

Working Plan.-The working plan is prepared by tracing from the 25 in. Ordnance on which the line was laid down or by replotting from it and the field notes. The chainage should be marked at every chain and numbered at every 10 chains. The radii of the curves, the tangent points and their chainage, the intersection points, the lengths of the tangents, and the intersection angles of the curves, are also to be marked on the plan, together with span, width, height and description of bridges, number of the drawing which shows the details of the bridge, the level crossings, road and stream diversions, accommodation works, culverts, pipes, &c. &c.

The fence lines should also be shown as well as the lines representing the tops and bottoms of the slopes, and the space between the latter, i.e., the excavation or embankment, should be coloured red.

The centre line, fence lines, tops and bottoms of slopes, and in fact all new works, are to be in red ink, all other details existing on the ground being in black ink.

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Working Section. The section is usually plotted to the same horizontal scale as the plan, viz., 500; the vertical scale is then usually 30 ft. to 1 in. The levels are invariably referred to Ordnance datum, and the datum line may be plotted at any convenient height above or below Ordnance datum, it being so stated on the section.

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