Ne Reproduced from the holdings of the National Archives Pacific Southwest Region o™, VERTICAL CONTROL There has been no requirement for an overall vertical control network and such a network would involve extensive observations over a conSiderable period of time. Bench marks for vertical control have been established independently at each of the project ereas from tidal observations and the accuracy is considered consistent with project requirements. A check was obtained of the datum established by this method at Eniwetok island when a tide gage was operated at . this location for several months during Operation Greenhouse by the U. S. Coast and Geodetic Survey. A differential of 0.1) foot was determined which would be of no consequence in the to project structures. tidal relation