A cruising rod is a simple computing device that to quickly estimate the number of pieces of lumber yielded by a given piece of timber. Similarly to a yardstick, it is a rod with markings. The estimation is carried out as follows. Standing at arm's length from the tree, estimate its average diameter by taking a note on the rod's markings. Walk away to see the whole tree; hold the rod upright at the distance from the eye at which the rod and the tree appear of the same diameter; the noted mark on the rod will show an approximate location of an 8-foot log cut along the tree height.
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Cruising rod Wikipedia(Text) CC BY-SA