Once the preserve of runners, GPS watches have become popular with cyclo-cross riders, mountain bikers, triathletes and now road cyclists. The latest generation not only do the job of a head unit but are fitness trackers and more.
Among the many upgrades that have appeared since the Casio PRT-1GP (the first GPS-enabled watch) is the optical heart-rate sensor, which is designed to eliminate the need for a chest strap.
But many athletes and coaches have found some of the data from these sensors to be less than accurate. Manufacturers have been
