Gage Datum and Elevation
The term "datum", such as used in the "Location" or "Gage" paragraphs in USGS publication and on Web pages, refers to the base, or 0.0 foot gage-height (stage), for a gage. The term datum is used when the base gage-height has been surveyed in from known benchmarks or with precision GPS. The datum or elevation will be reported to either NGVD of 1929 or NAVD of 1988, 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 (NGVD) of 1929 as the datum for gages to determine elevation of gage-height data. Since the early 1990's the emphasis has been changing to use the new North American Vertical Datum (NAVD) of 1988, see a description of the differences between the two datums. You can compute the difference with the National Geodetic Survey Web page. Addition information is available from the NGS and their FAQ Web page.
Currently most USGS stream and reservoir gages in New York have been converted to reporting the gage datum and elevation data in NAVD of 1988. Many older gages still reference the NGVD of 1929 as well. The change is indicated on the individual station-data Web pages and in the manuscript for the individual stations in the Water Year Summaries. You can follow the links on the Web pages to the information for the station(s). Elevation gages will display two plots, one for NGVD of 1929 and one for NAVD of 1988, during our conversion process.
U.S. Department of the Interior, U.S. Geological Survey
Maintainer: New York District