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This document shows the way that spiral curve data should be shown on Highway Design plans and briefly describes the meaning of each item listed. For more information, see Drafting Spiral Curves From Old Plan Data.
Sample Curve Data

spiral curve variables

spiral curve variables
Detail in leading spiral curve area.

Spiral Curve Data
PI= 23+96.20 PI Station of intersection point of start and end tangents
N = 405083.92 N North coordinate
E = 964724.93 E East coordinate
Δ = 77°-47'-44.8" RT Δ Angle between start and end tangents
Δc = 39°-35'-54.9" RT Δc Angle turned by the circular portion of the curve
Dc = 19°-05'-54.9" Dc Degree of curve on Imperial alignments
Ts = 346.15' Ts Distance from tangent point to intersection of tangents
Es = 92.58' Es Distance from PI point to curve
Ls = 200.00' Ls Length of spiral
Lc = 207.34' Lc Length of circular portion of curve
Rc = 30 0.00' Rc Radius of circular curve
LT = 134.12' LT 'Long Tangent' of spiral curve
ST = 67.38' ST 'Short Tangent' of spiral curve
θs = 19°-05'-54.9" θs Angle turned by spiral portion of curve
P = 5.53' P Offset of circular curve from tangent line at the point where the circular curve would be parallel to the tangent if it were extended.
K = 99.63' K Distance along the tangent to the point described immediately above (marginally less than half the length of the spiral)
Xc = 197.79' Xc Distance along tangent from point of tangency to end of circular curve
Yc = 22.05' Yc Distance perpendicular to tangent from tangent line to end of circular curve
LC = 199.01' LC 'Long Chord' distance between ends of the spiral

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