Hypsometer

Modern hypsometers use a combination of laser rangefinder and clinometer to measure distances to the top and bottom of objects, and the angle between the lines from the observer to each to calculate height.

The principle of operation of such a scale hypsometer is based on the idea of similar triangles in geometry.

Then as in step 1 in the illustration, a sighting is taken on the top of the object whose height is to be determined, and the reading on the horizontal scale, h', recorded.

Similarly as in step 2 of the illustration, a sighting is taken on the base of the object whose height is to be determined, and the reading on the horizontal scale, d', recorded.

[1] The instrument consists of a cylindrical vessel in which the liquid, usually water, is boiled, surmounted by a jacketed column, in the outer partitions of which the vapour circulates, while in the central one a thermometer is placed.