Have you ever noticed the abbreviation NAVD 88 or NGVD 29 written after an elevation marker on a survey or construction drawing and wondered what it means? What’s the difference between the two? Or maybe you’ve wondered why we don’t always use Above Ground Level (AGL). Well, there are a few simple explanations to answer why we need and use these complex systems.
NAVD 88 is the abbreviation for the North American Vertical Datum of 1988. NAVD 88 is one of five current National Geodetic Datums, which are coordinate systems that act as standard reference lines to measure points on the earth’s surface in the region that they apply. A datum cannot be seen or physically measured because is a calculation of the Geoid/Mean Seal Level (MSL), which is the average global height of the ocean without any variables acting upon it, such as wind, climate, etc.
NGVD 29 is the abbreviation for the National Geodetic Vertical Datum of 1929, the predecessor to NAVD 88. Due to the advancement of technology and surveying methods, the increased amount of available data and the level of accuracy of that data led to the new datum. The original datum used 26 tide stations throughout the United States and Canada and recorded changes in tide levels to establish a starting line of 0 ft. elevation to measure and compare heights of buildings, floodplains, mountains, etc. Though NGVD 88 only used one of those tide stations, Pointe-au-Pere, Rimouski, Quebec, Canada, it recorded data continuously for the entire tidal epoch of 1970 – 1988. This created a record of more consistent observations, rather than a combination of varying periods of time at different locations.
A few of the reasons why NAVD 88 is more accurate than its predecessor is that it includes adjustments for observed gravity, 1,001,500 (km) of land was leveled, and 450,000 benchmarks were created. In contrast, the process for calculating NAVD 29 included only 106,724 (km) of leveling and 100,000 benchmarks.
It’s important to note there is roughly a one-foot difference between the two datums, depending on the location in the country. Therefore, you don’t ever want to confuse or combine the two in your calculations or interpretations. This could happen by accident if someone were to reference older surveys that used NGVD 29 and then designed a structure using NAVD 88. This mistake could cause a shift in elevation, putting the design at risk for serious and/or expensive consequences such as an increased risk of flooding, not meeting city/state requirements, etc.
AGL is not always the best choice since it is a measurement that is relative to the ground at that specific time; the ground level around that structure could change and then the AGL would change. Knowing the true height of grading, structure, etc. relative to a datum line is useful in design because the level of a terrain can change with time or even suddenly, such as repaving a road or in the case of a natural disaster.
While these datums are all meant to measure the same point, it is vital to confirm which datum your reference documents use, which datum is right for your documentation/design, and, if you have two datums, use a conversion factor to adjust to one single datum for the entire project. This simple step could save your design!
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