# Orders of Accuracy Definition for Land Surveyors

**orders of accuracy**—Standard orders of accuracy used until FGDC (Federal Geographic Data Committee) Geospatial Positioning Accuracy Standards were issued in 1998; known as first (meaning the most accurate). second, and third (the least accurate) orders. In the past, the orders of accuracy for unadjusted horizontal distances, which express the closure error as a fraction of the overall distance measured between points of a known, higher order of accuracy, were: first (1:25,000), second (1:10,000), and third (1:5,000). For the unadjusted angles in traverses, the orders of accuracy were expressed as the following number of seconds of arc times the square root of the number of horizontal angles measured, i.e., first, 2; second, 10; and third, 30. For triangle closure in triangulation the seconds of arc in the maximum and average error for each order of accuracy are: first, 3 and 1; second, 5 and 3; and third, 10 and 5 the unadjusted closure error in vertical distances were expressed as the following values times the square root of the length of the level circuit in miles or kilometers, respectively: first, 0.017 feet and 4.0 mm; second, 0.035 feet and 8.4 mm; and third, 0.05 feet and 12.0 mm. Any survey in which the closure error is larger than third order would be known as fourth order. Also, in supplemental control for the photogrammetric compilation of topographic and other maps, fourth-order control could have errors of closure as large as 1:2,500 in the horizontal. 60 seconds times the square root of the number of angles measured in a traverse, ma of 30 seconds and average of 15 seconds in triangle closures in triangulation, and in the vertical of 0.50 feet or 120 mm, respective! times the length of the lever circuit in miles or kilometers. See *accuracy **standards (horizontal, ellipsoid height, and orthometric height).*

**orders of accuracy—**See *control survey classification; Appendix A, Standards for Geodetic Control Surveys.*

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

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