The observed values of gravity survey must undergo normal field and elevation correction. Therefore,the plane positions and elevations of gravity measuring points must be measured; the elevation survey, in particular,must be very precise. In theodolites, surveyor's levels and range finders used previously for geodesic work, the “normal elevation system” of geoid surface was adopted for the elevation system on the has of “specifications”. In recent you, the Gladal Positioning System (GPS) has been introded into gravity survey, and the elevation of GPS survey is “geodetic elevation system” of WGS-84, so that the “geodetic elevation” must be converted into “normal elevation”. This conversion requires considerable amounts of work and adds to possibilities of errors. Based on analysing theoretical formula of normal gravity field of the earth, the author holds that the geodetic elevation of GPS survey might ha adopted for cavity altitude correction.