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When the length of a chord is less than the peg interval, it is known as
Long chord
Short chord
Sub chord
Normal chord
Sub chord
In route surveying, when the length of a chord measured along a circular curve is less than the standard peg interval, it is known as a sub chord ยท A standard peg interval is typically 20 m or 30 m, and full-length chords equal to this interval are called normal chords ยท Sub chords usually occur at the beginning or the end of a curve to ensure full station pegs land on rounded chainages.
In route surveying, when the length of a chord measured along a circular curve is less than the standard peg interval, it is known as a sub chord ยท A standard peg interval is typically 20 m or 30 m, and full-length chords equal to this interval are called normal chords ยท Sub chords usually occur at the beginning or the end of a curve to ensure full station pegs land on rounded chainages.
ฮด1โ=1718.9รRc1โโย minutes โ tangential angle for the first sub chord (c1โ)
ฮด=1718.9รRCโย minutes โ tangential angle for a normal chord (C)
During the setting out of a circular curve using Rankine's method of deflection angles or Rankine's tangential angles, chainage is kept continuous at whole peg intervals ยท Since the point of curve (PC) or point of tangency (PT) rarely coincides with an exact full station chainage, the first and last chords must be shorter than the full peg interval (C<D), requiring deflection angle calculations based on these sub chord lengths.
A normal chord has a length equal to the standard peg interval (20 m or 30 m).
A sub chord has a length less than the standard peg interval.
First sub chord length (c1โ) = First rounded peg chainage - Chainage of Point of Curve (PC).
Last sub chord length (cnโ) = Chainage of Point of Tangency (PT) - Last rounded peg chainage.
Long chord is the chord joining the point of curve (PC) to the point of tangency (PT).
Ensures all intermediate pegs are set out at clean, rounded chainage marks along the route.
Allows accurate curve setting out using standard instruments like theodolites and total stations.
Requires additional individual deflection angle calculations for non-standard chord lengths.
Higher chance of cumulative arithmetic calculation errors if sub chord angles are not rounded properly.
Setting out simple circular curves for highways and railway tracks.
Rankine's deflection angle method for curve alignment in civil engineering surveying.
| Feature | Normal Chord | Sub Chord |
|---|---|---|
Length | Equal to standard peg interval (20ย m or 30ย m) | Less than standard peg interval (c<D) |
Location | Along main intermediate body of the curve | At the beginning (PC) or end (PT) of the curve |
Chainage Marker | Spans between two full station pegs | Connects tangent point to first full station peg |
Option A (Long chord): The total straight distance joining the initial tangent point (PC) to the final tangent point (PT).
Option B (Short chord): A generic non-technical term; not an official surveying classification for curves.
Option D (Normal chord): A chord whose length is exactly equal to the unit peg interval (usually 20 m or 30 m).
C is correct โ A chord length smaller than the unit peg interval is defined as a sub chord.
In Rankine's deflection angle method, remember that the tangential angle for a sub chord uses its actual length c, so ฮด=1718.9ร(c/R) minutes, whereas for normal chords C is used.