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<br />6.3A <br />Page 26 <br /> <br />Historically, the most common vertical datum used for the Federal Emergency <br />Management Agency's (FEMA) flood hazard studies/restudies and map revisions <br />has been NGV029. NGV029 assumed that 26 tide gages in the United States <br />and Canada all represented the same zero elevation, which was mean sea level. <br />As survey technologies became more accurate, it became increasingly apparent <br />that NGV029 constraints were incorrectly forcing surveys to fit different tide <br />stations (all zero elevation or mean sea level) that actually had different <br />elevations relative to each other. <br /> <br />In the 1970's, the National Geodetic Survey (NGS), and counterpart agencies in <br />Mexico and Canada, decided to adopt a vertical datum based on a surface that <br />would closely approximate the Earth's geoid. The new adjustment, NAV088 was <br />created by adding 625,000 kilometers of leveling and performing a major least <br />squares adjustment that constrained only a single tide station at zero elevation. <br />Subsequent to the establishment of NAV088, new flood hazard studies are <br />preferably referenced to that datum. As part of FEMA's Map Modernization <br />program, all elevations shall be referenced to the NA V088 datum. <br /> <br />In response to the datum conversion, the City has completed establishing all City <br />benchmarks in the NA V088 datum. The next step is to survey all the levee <br />segments in the NAV088 datum. Meanwhile, pending the new survey, all <br />elevations in this manual is referenced to the NGV029 datum. <br /> <br />For the County of San Mateo, FEMA has adopted a uniform conversion factor to <br />convert from NGV029 to NA V088 with the understanding that a margin of error <br />does exist with using a conversion factor. The FEMA adopted conversion factor <br />for the County of San Mateo is NGVD29 + 2.75 = NAVD88. <br /> <br />19 <br /> <br />April 2008 <br />