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 (instructions).
This has obvious applications for outdoor pursuits but if you are a precision shooter, it probably sounds very interesting to you too. Wndsn XPD is currently developing a new version of their Distance Meter and two shooting specific tools (one is pictured below). The upcoming shooting calculator will allow the user to simply stretch a string or straight edge across scales for known size and MIL or MOA reticle reading to determine distance.
Check out Wndsn XPD