The first six paragraphs of thus section cover the basis of stringlining, the equipment the fieldwork

requires, the marking off of stations, the measuring of ordinates, the evaluation of the figures, and the calculations

involved. The next paragraph discusses the marking of the ends of tangents, spirals, etc. The last two paragraphs

discuss some problems you may encounter in stringlining.

3.17.

BASIS

When a string is stretched tautly between two points on the inside or gage side of the outside rail of a

curve, the distance between the midpoint of the string and a point five-eighths of an inch from the very top of the

rail is proportional to the degree of curvature. This is the basis of stringlining. A longer distance between the

midpoint and the rail signifies a sharper curve than does a shorter one. This distance is known as an ordinate,

though some railroaders call it an offset. The measurements marked m in figure 3.4 are ordinates.

When the string used is 62 feet long, the

ordinate in inches equals the degree of curvature.

For example, if a 62-foot string is stretched between

two points on the inside of a rail on the simple

portion of a curve and the ordinate measures 2

inches, then it is a 2-degree curve. If the ordinate

were 3 1/2 inches, it would be a curve of 3 degrees

30 minutes. For this reason, a 62-foot string is

usually used in stringlining.

On a smooth or properly lined simple curve, the

degree of curvature is the same at all points. Since

measured ordinates are equal to curvature, all those

in such a curve should be of equal length. If they are

not, the curve is out of line. Similarly, on a properly

lined spiral, curvature changes at an even rate.

Therefore, ordinates on a spiral measured at equal

distances should vary an even amount from one to

the next.

Figure 3.4. Ordinates on a Simple Curve.

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