When survey leveled from these markers to the tide station, the relationship calculations for these marks must match within 9 millimeters. If the individual benchmark has a NGS record, this is recorded by the IDP or OPUS PID numbers of that mark. The survey benchmarks associated with each station are provided in the Benchmark Sheets for a station. Our standard requires a minimum of 2 survey benchmarks, installed within a short distance of the station, which have a NAVD88 elevation recorded by the National Geodetic Survey (NGS). If that field is blank, then no relationship has been established. This relationship is noted on the Tidal Datums for that station, by providing an elevation of NAVD88. Where possible we provide a relationship between tidal datums and the geodetic datum of NAVD1988. NOAA, and it predecessor organizations, have been measuring tide data long before the development of a standard geodetic datum, such as NAVD1988.
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