Spectra Precision FAST Survey Reference Manual - Manual (Page 228)

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 Add (Spiral-Curve-Spiral):
The Spiral-Curve-Spiral element is really just two implementations of Spiral Only
(line-spiral-curve and curve-spiral-line). The advantage of Spiral-Curve-Spiral is that it completes three elements at
once and is a fairly common application on high-speed highways. This routine also allows the user to define the
spiral-curve-spiral using known points as long as the spiral in and spiral out lengths are entered first.