“Sea Level Datum of 1929” — Did You Know?

This entry is part 7 of 21 in the series Field Notes

I recently was consulted about the use of the term “Sea Level Datum of 1929” on plans and in engineering reports dating from the 1940-60s. The inquirer was concerned that this possibly meant that the work was based on something other than the National Geodetic Vertical Datum of 1929 (NGVD 29). As is often the case in situations like this, it provided an opportunity for my continued education through researching the correct response.

As always, the first place I visited was the NGS website, where I quickly found the answer. Up until May 10, 1973, the national vertical datum was officially known as “The Sea Level Datum of 1929.” On that date, the name was changed to “The National Geodetic Vertical Datum of 1929” because it was known that the datum did not represent sea level, the geoid, or any other equipotential surface. Therefore, to call it “Sea Level” anything was incorrect.

So, if you see the term, “Sea Level Datum of 1929,” cited as the vertical reference datum on plans or reports prepared between 1930 and 1973, don’t be alarmed. The term was being used correctly and is synonymous with NGVD 29.

To learn more about how this datum was originally defined based on 26 tide gauges (21 in the United States and 5 in Canada), please visit the NGS website, which is almost an endless library of information.

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