Alidada W. H. HARLING, London, 1919.

W. H. HARLING, London, 1919.
Topographic square or plotting rule.
Boxwood and brass.
Signed, dated, and numbered: “8248 / W.H. HARLING / LONDON / 1919”.
Comes with a period leather case.
Condition: in good overall condition, with signs of use, patina, and wear consistent with its age. Case shows scuffs and signs of use.
Measurements: 1.5 x 40 x 4.5 cm (surveying instrument); 2 x 42 x 5.5 cm (case).

An interesting English surveying instrument made by W.H. Harling, a prestigious London firm that has specialized in mathematical, scientific, and precision drafting instruments since the mid-19th century.
Dated 1919 and numbered 8248, the piece consists of a boxwood ruler with scales marked along its edge and two folding metal sights positioned at the ends. When raised perpendicularly, these allowed for establishing a precise line of sight toward a point on the ground, with its direction subsequently transferred to the plan using the edge of the ruler itself.
This type of sighting device, also known in the English-speaking world as a “plotting rule,” was designed for use on a surveying table, allowing for the simultaneous observation of the terrain and its graphical representation. Instruments of this model were used by surveyors, engineers, and military units for cartographic surveys and map-based planning work.