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Horizontal Curves
Basic Properties Of
Horizontal Curves
Location Of Tangent
Points
Setting Out
Offsets From The Tangent
Offsets From The Chord
Deflection Angles
Deflection Distances
Quartering
Coordinates
Transitions Curves
Location Of The Tangent Points
For a given pair of straights, there is only one point at which a curve of given radius or degree may leave the first straight tangentially in order to sweep tangentially into the second. The points of commencement and termination of the curve must therefore be determined in the field with greater precision than would be possible by merely scaling their positions from the plan.
Having located the two tangents and defined them by ranging poles, peg out the first tangent EA up to about the estimated position of A, the theodolite being placed on EA and align two pegs a and b a few feet apart, one being placed on each side of C, the position of which is estimated by from the line of the poles on BF.
Transfer the instrument to some convenient point on the second straight, and produce the latter to meet a string stretched between a and b. The point of intersection C of the two tangents thus obtained is marked by a peg.
Set up the theodolite over C, and measure the angle ECF. By subtracting the result from 180°, the value of intersection angle I is obtained. Calculate the tangent lengths.
From C, measure back the lengths CA and CB = T, the tangent points A and B being aligned from the instrument at C. Mark A and B in a distinctive manner, either by painted pegs or by three ordinary pegs, the centre one of which defines the point.
Transfer the instrument to A, and set it over the tangent point peg. Measure the angle CAB, which should equal
I. This provides a convenient check on the equality of the tangent lengths, which may, however, both be in error by the same amount through a mistake in the measurement of I or in the calculation of T.The chaining of the first straight may now be completed, the chainage of the point A being noted.