I would imagine these are mm of which there are 10 in a cm and 1000 per meter. There are roughly 30cm (300mm) in a foot. So easiest way is probably to convert mm to cm and divide by 30.
14500mm = 1450cm/30 = 48.33 feet approx
or convert to meters = 14.5meters * 3.33 (number of feet in meter) = 48.33