There is no easy way to measure arcs in investigational work.
Firms also use dividers or stamped metal gauges to do the same chord measurements.
Once the dividers come up on the even markings, the investigator assumes the dividers are properly set. In addition to the previous problem, a one minute time difference is only about 0.014 inch on a typical Taylor OP524 chart. Introducing a 0.014 inch error by making 15 spans or misstamping a gauge is easy to envision. Another problem is the extreme difficulty in extrapolating times from known chord lengths. The geometry for determining the arc or chord length of a circle is shown in Appendix 1.