▁ 1 9 8 8 ▁( NA VD ▁ 8 8 ) ▁consists ▁of ▁a ▁level ing ▁network ▁on ▁the ▁North ▁American ▁Cont inent , ▁r anging ▁from ▁Al aska , ▁through ▁Canada , ▁across ▁the ▁United ▁States , ▁aff ixed ▁to ▁a ▁single ▁origin ▁point ▁on ▁the ▁continent . ▁ ▁In ▁ 1 9 9 3 ▁N AV D ▁ 8 8 ▁was ▁aff ir med ▁as ▁the ▁official ▁vertical ▁dat um ▁in ▁the ▁National ▁Sp atial ▁Reference ▁System ▁( NS RS ) ▁for ▁the ▁Con termin ous ▁United ▁States ▁and ▁Al aska ▁( see ▁Federal ▁Register ▁Notice ▁( FR N )). ▁Although ▁many ▁papers ▁on ▁N AV D ▁ 8 8 ▁exist , ▁no ▁single ▁document ▁serves ▁as ▁the ▁official ▁defining ▁document ▁for ▁that ▁dat um . ▁D etailed ▁information ▁about ▁N AV D ▁ 8 8 ▁is ▁available ▁here . ▁ ▁The ▁definition ▁of ▁N AV D ▁ 8 8 ▁uses ▁the ▁Hel m ert ▁orth ometric ▁height , ▁which ▁calcul ates ▁the ▁location ▁of ▁the ▁ge oid ▁( which ▁approxim ates ▁MS L ) ▁from ▁mode led ▁local ▁gravity . ▁The ▁N AV D ▁ 8 8 ▁model ▁is ▁based ▁on ▁then - available ▁measurements , ▁and ▁remains ▁fixed ▁despite ▁later ▁improved ▁ge oid ▁models . ▁ ▁Since ▁N G VD ▁ 2 9 ▁used ▁a ▁simple ▁model ▁of ▁gravity ▁based ▁on ▁latitude ▁to ▁calculate ▁the ▁ge oid ▁and ▁did ▁not ▁take ▁into ▁account ▁other ▁variations , ▁elev ation ▁difference ▁between ▁points ▁in ▁a ▁local ▁area ▁in ▁it ▁and