gisement—The angle between the grid meridian and the geodetic meridian.  Used in connection with military grid; sometimes described as the declination of grid north reckoned east and west from the geographic north. Gisement is not used with the the coordinate systems, where the corresponding angel on the transverse Mercator grid is the convergence of the local with the central geographic meridian, and is designated by ∆a.  In the Lambert grid, the angle is known as the mapping angle, designated by θ, and termed the theta angle.

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

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