I highly suggest you take a look at Wndsn XPD. They currently offer a number of products including their very slick Distance Meters which are trigonometry based with no moving parts and no electronics. These cards allow the user to make use of a specific length of cord, a scale printed on the card, and an object of known size to determine distance.
http://jerkingthetrigger.com/2017/03/30/wndsn-xpd-distance-calculator/
