sector— ¹ An instrument composed of a graduated arc equipped with a sighting device (alidade). The sector and the quadrant were the immediate predecessors of the theodolite, both being mentioned it reports of early geodetic surveys in connection with the measurement of horizontal angles. These instruments were constructed with arcs having radii several feet in length. ²Designation of special types of instruments such as zenith sector and track sector. ³ Designation of certain map features, e.g., red sector.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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