Holland Circle

In the late 16th century, a Dutch scholar and instrument maker named Gemma Frisius suggested that a good surveying instrument could be made by attaching a magnetic compass to the back of an astrolabe. Building on this idea, Jan Dou designed an instrument with two pairs of fixed sights attached to a graduated circle, and another pair of sights attached to either end of a movable alidade. Dou published an account of this instrument in Dutch in 1612, and the form became popular with surveyors in the Netherlands. The name Holland Circle was coined in the 19th century. The Holland Circle is similar to but easily distinguishable from the common theodolite.

The outer circle of this example is graduated every thirty minutes, and has additional scales for tangents, sines, and secants. The small compass in the center has a fleur–de–lis at north, and an offset line probably indicating magnetic variation. Inscriptions read "JACOB SECOES" and "ANNO 1622."

Ref: Jan Pietersz. Dou, Tractaet vant maken ende Gebruycken eens nieu gheordonneerden Mathematischen Instruments (1612).

H. C. Pouls, De landmeter Jan Pietersz. Dou en den Hollandsce Circle (Delft, 2004).

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