error of closure of horizon— The amount by wliich the sum of a series of horizontal angles 

 measured between adjacent Unes in a complete circuit of tlie horizon fails to equal 

 exactly 360° 



error of closure of triangle— The amount by which the sum of the tliree observed angles of a 

 triangle fails to equal exactly 180° plus the spherical excess of tlie triangle. 



error of closure, traverse— The amount by which a value of tlie position of a traverse station 

 as obtained by computation through a traverse faUs to agree with another value of the 

 same station as determined by a different set of observations or route of survey. The 

 traverse may run between two stations whose positions are held fixed, or it may start 

 from and end on the same station. In either case tliere are two values for the position of 

 the final station: one known before the traverse was computed, the other obtained by 

 computations carried through the traverse. The difference between these is the error 

 of closure. It may be resolved into closure in latitude, and closure in longitude 

 (or departure). The total closure is also referred to as closure in position. 



extreme high water— The highest elevation reached by the sea as recorded by a tide gage 

 during a given period. NOS routinely documents montlily and yearly extreme liigh 

 waters for its control stations. 



extreme low water— The lowest elevation reached by the sea as recorded by a tide gage 

 during a given period. NOS routinely documents monthly and yearly extreme low water 

 for its control stations. 



eustatic adjustment— The worldwide increase in sea level due to the increase in the volume 

 of tlie oceans resulting from the melting of glaciers. 



first-order leveling— Precise leveling in which tlie elevation differences obtained in two 

 runnings of a section do not differ by more than 4 millimeters per >/k where k is 

 distance measured in kilometers or by more than 0.017 foot per Vk where k is 

 measured in miles. 



geoid, compensated— A theoretical geoid derived from the actual geoid by the application of 

 computed values of the deflection of the vertical which depend on the topography and 

 isostatic compensation. The data upon which to base tlie effect of topography and 

 isostatic compensation are obtained from readings of topographic maps; tlie effect is 

 then computed in accordance with the assumptions made with respect to isostasy, i.e., 

 the depth and distribution of isostatic compensation. A method for accomplishing this 

 was developed by Hayford, who accepted the Pratt theory of isostasy. If the theory and 

 assumptions with respect to isostasy were exact and there were no anomalies, the 

 compensated geoid would agree with the spheroid of reference. 



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