Although not sufficiently accurate for certification purposes,such devices can alert you to the upcoming location of intermediate split points and measurement end points on your first measurement. Such devices are available in many bicycle stores. If you get a flat, all measurements made since the last calibration are invalid. You must fix the flat and start over with a new calibration. Solid tires are one way of avoiding flats.
Solid tires have two major advantages.
First, you eliminate flat tires and the wasted measurements that accompany a flat tire. This allows you to ride the shortest possible route with more confidence since glass fragments are no longer a problem. Solid tires do not eliminate the need to calibrate before and after measuring but they do reduce differences between the working and finish constants. Many races have signs indicating kilometer or mile points and may have times read to the runners at several points. Very little time is available to place signs and drop off timers on race day and such a guide helps assure the runners will get splits at the proper points.