Gage Datum and Elevation
The term "datum," such as used in the "Location" or "Gage" paragraphs in USGS publications and on webpages, refers to the reference, or 0.0 foot elevation (stage), for a tide gage. The term datum is used when the reference elevation has been surveyed in from known benchmarks or with precision GPS. The datum or elevation will be reported to either NGVD29 or NAVD88, or to a different datum which is noted in the description of the gage.
The USGS in New York State has used the National Geodetic Vertical Datum of 1929 (NGVD29) as the datum for tide gages to determine elevation of estuary or ocean water surface data. Since the early 1990's, the emphasis has been changing to use the new North American Vertical Datum of 1988 (NAVD88). See NGVD29 and NAVD88 for a description of the differences between the two datums. You can compute the difference with the National Ocean Service VDatum webpage. Additional information is available from the National Geodetic Survey and their FAQ webpage.
Currently, all USGS tide gages in New York have been converted to report the gage datum and elevation data in NGVD29 and NAVD88. The dual references are indicated on the individual station-data webpages and in the manuscript for the individual stations in the Water Year Summaries. You can follow the links on the webpages to the information for the station(s). Gages reporting estuary or ocean water surface elevation will display up to four plots during our conversion process. One each for elevation observations referenced to NGVD29 and NAVD88. And one each for elevation predictions (where available) referenced to NGVD29 and NAVD88.
U.S. Department of the Interior, U.S. Geological Survey
Maintainer: New York District