the thing about dead reckoning is that anything less then perfect tends to have degraded usefulness after a while unless supplemented with some outside data to

the thing about dead reckoning is that anything less then perfect tends to have degraded usefulness after a while unless supplemented with some outside data to reference again. Now thing about a soldier underground who might be subjected to explosions nearby, shooting, being shot at, having to dive for cover and these are all things that are going to screw up any inertial guidance system which is why the suggestions of paint or something make a whole lot of sense.