First, what is a “datum?”

About 2015, many Federal, State and County organizations began moving from the long held measurement standard of NGVD29 to more accurate methodology known as NAVD88.  Because we have thousands of Lake Holden elevation records dating back to the 1950s, all collected before the NAVD88 was even developed,  it makes sense – at least at this point, to keep the records in the NGVD29 format. For Lake Holden elevation data, it is more important to compare the data collected to itself, using the same standard – rather than worry about which methodology is used.

For most purposes, (for Lake Holden’s location): to convert from NAVD88 to NGVD29: simply subtract an inch (actually 1.029 inches). And, to convert from NGVD29 to NAVD88 simply add an inch ( 1.029 inches).

Now, on to the good stuff:

If we say that a flood will rise to 100 feet, one must ask “100 feet above what?” The starting point for measuring elevations is our datum. We need a consistent starting point so we can compare flood and ground elevations. In most cases, we mean “above sea level.”

The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation (altitude) above, and depression (depth) below, mean sea level (MSL).

Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. The adjustment required a total of 66,315 miles (106,724 km) of leveling with 246 closed circuits and 25 circuits at sea level.

Since t29 he Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) in 1973. NGVD29 was superseded by the North American Vertical Datum of 1988 (NAVD 88), based upon an equipotential definition and a readjustment, although many cities and U.S. Army Corps of Engineers projects with established data continued to use the older datum.

(source wikipedia)

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