A n-degree curve turns the forward direction by ndegrees over some agreed-upon distance. The usual distance in US road work is 100 ft (30.48 m) of arc. US railroad work traditionally used 100 ft of chord and this may be used in other places for road work. Other lengths may be used—such as 100 metres where SI is favoured, or a shorter length for sharper curves.
Since rail routes have very large radii, they are laid out in chords, as the difference to the arc is inconsequential and this made work easier before electronic calculators.
The 100 ft (30.48 m) is called a station, used to define length along a road or other alignment, annotated as stations plus feet 1+00, 2+00 etc. Metric work may use similar notation, such as kilometers plus meters 1+000.