Stadimeter

A stadimeter is an optical instrument that estimates distance to a ship or other object of known height by measuring its apparent angular height.

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Angular diameter: The angle an object subtends at an observer, determined by its size and distance. A stadimeter measures this angle to estimate the distance to a known-height object.
Naval warfare: Armed conflict conducted by or between naval forces. Shipboard stadimeters supported visual range estimates during naval operations.
Small-angle approximation: The approximation that an angle in radians is nearly equal to its tangent when the angle is small. At ordinary viewing distances, this makes distance approximately proportional to target height divided by angular height.
Laser rangefinder: An instrument that measures distance from the travel time or phase shift of laser light. A laser rangefinder measures range directly, while a stadimeter needs a target of known height.
Triangulation: A method of finding a position or distance from measured angles and a known baseline. Like triangulation, a stadimeter turns angular observations into a distance estimate.
Rangefinding: The measurement or estimation of distance to an object. Distance estimation is the stadimeter’s central function.
Radian: The SI unit of plane angle, defined by an arc length equal to the circle’s radius. The distance relation is simplest when the measured angle is expressed in radians.
Radar range equation: An equation relating radar-system parameters to the maximum detectable target range. Radar estimates range from transmitted and returned radio signals rather than apparent target size.
Parallax: The apparent shift of an object’s position when viewed from different locations. It is another geometric route to distance, but requires a viewpoint shift rather than known target height.
Ship navigation: The planning and control of a vessel’s movement from one location to another. A measured range can help establish a ship’s position relative to visible landmarks or vessels.