A stadimeter is an optical instrument used to estimate the range to an object of known height by measuring the angle subtended by its vertical extent. The accuracy and convenience of the tool is undercut by its small size and the need to know the height of the object (whose particulars and even identity may be in doubt), but its charm was that it was small in size and suitable for hand-held use or in a submarine periscope where a large rangefinder could not be fitted.
Hand-held stadimeters were ideal for use on the bridge of a ship to determine the range to nearby friendly vessels in order to ensure a precise mooring position or for accurate station keeping. The ranges would be accurate given how short they'd be, and the height of friendly vessels can be committed to memory or recorded in a handy reference suitable to the same near-to-hand application.
Use in Submarine Periscopes
Large rangefinders require a lengthy base arm to separate their objective lenses, and this makes their use in a slender, stealthy periscope difficult, although some vertical-base models were developed (e.g.: the Barr and Stroud FY1 ordered in 1917). Stadimeters were much easier to fit in the head of a submarine periscope.
- Range and Vision, pp 83-4.
- Moss, Michael; Russell, Iain (1988). Range and Vision: The First Hundred years of Barr & Stroud. Edinburgh: Mainstream Publishing. ISBN 1851581286.