Greater Bay Area Map, Hanging

For measuring distances of roads, streams, etc. on maps with Four scales; 1:75k, 1:50k, 1:25k, 1:20k. The wheel features kilometre measurements on both sides. Use method: Move the pointer to zero, at the scale of 1:1. The small wheel alignment measuring address as a starting point, along rounting slip(rolling) to the end, according to the map of the ratio of the corresponding scale, is up point to the end of the number of kilometers.