How do you....

Much as I respect NGS and their work, just say it's the Federal Government. Most people will understand that.

I know this sounds political - it's NOT. It's a fact of life.

Tell them sea levels rose 1' over time and they adjusted elevations to that.;-)

Just tell them the truth,

All the space dust collecting on the moon between 29 and 88 caused the moon to get much heavier, Increasing the gravitational pull and causing the water in the ocean to swell. That coupled with all the pollution and the fact that the gauges could not tell the difference in sea water and the foam floating on top caused the sea level to be higher so they adjusted it.
😀
James

I agree with Mighty Moe.....I just tell them sea level rose so relative to sea level the new elevations are lower.

[sarcasm] Al gore says that global warming melted the polar ice caps, thus the new datumn. If they drive a Volvo with an Obama sticker, they will buy that explanation.[/sarcasm]

It's not a problem here. We are so far from any sea and sea level that very few people have any concept of elevation numbers. Of those, very few would be aware of the elevation of anything---benchmark, flowage line, that concrete thingy with a brownish-metallic thingy in the top that has the word "ELEVATION" on it but no numbers.

Our differential is up about 0.45 feet such that 1050 is now 1050.45.

I had a similar issue with a site on the Sound in Rye. The long time owner was very confused until I gave him a short chart comparing his first floor and the FEMA flood elevation with both the old and new datums. Once he saw all that together, he was fine with the new datum.

Ken

Face it. If you were to explain to the average person the difference between an old sea level elelvation based on a 19 year tidal guage average and a sea level based on a complex gravitational model, all your going to get is a blank glazed look of confusion. Just tell them the localized isostatic rebound isn't keeping pace with rising sea levels due to melting of the Greenland ice sheets, because of a bunch of darn butterflies in Thailand keep beating their wings causing a lateral shift in the Gulf Stream and that the USGS is working on a fix.

http://www.ngs.noaa.gov/faq.shtml#WhyNAD

What are NGVD 29 and NAVD 88?
"The National Geodetic Vertical Datum of 1929: The name, after May 10, 1973, of (the) Sea Level Datum of 1929." (Geodetic Glossary, pp. 57)

"Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929."

"Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is 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. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level."

"The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929." (Geodetic Glossary, pp. 56)

The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-U.S. leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.

Why did NGS change from NGVD 29 to NAVD 88?
NAVD 88 was computed for many of the same reasons as NAD 83. About 625,000 km of leveling had been added to the NGVD since 1929. Thousands of bench marks had been subsequently destroyed and many others had been affected by crustal motion, postglacial rebound, and subsidence due to the withdrawal of underground fluids. Distortions amounting to as much as 9 meters had been seen due to forcing the new leveling to fit the NGVD 29 height values.

Well it is the northeast, so most people tend to think that way around here.....maybe you should throw in some global warming BS as well

Yeah...that'll clear things up for them.;-)

-V

"Better technology"

> explain to people the difference in datums from '29 to '88?
Anyone who has visited the beach knows that "sea level" changes rather dramatically twice every day. The tide goes in, the tide goes out. Not exactly the same amount every day, depending on the weather and the phase of the moon. And the ocean never stops moving to let us measure it. People try to take an average over time and call that "sea level", but what an average is is subject to interpretation.

What they may not realize is that the "sea level" at one beach is not the same as "sea level" at every beach. Ocean currents pile up water onto the beach by degrees that differ by location and with the seasons. There are other effects, too.

There have been numerous attempts to define what is meant by "sea level". One such attempt was made in 1929. It was thought to be pretty good for a time, but by 1988 it was realized that there were flaws in that data serious enough that a new attempt had to be made.

But most of all we just have to know that there are different interpretations of what "sea level" is, and deal with it.

> explain to people the difference in datums from '29 to '88?

Well, it's a well known fact that there's a secret society of the five wealthiest people in the world, known as The Pentavirate (The Queen, The Vatican, The Getty's, The Rothschild's, and Colonel Sanders before he went tits up) who run everything in the world, including the newspapers, and meet tri-annually at a secret country mansion in Colorado, known as The Meadows.

At their 1988 meeting, in conjunction with Henry Kissinger and the Trilateral Commission, they forced the Government to adjust the official vertical datum of the United States to mitigate the effects of subsidence caused by underground nuclear explosions as part of the ongoing war against the mole people.

Here is what I send the local engineers and architects who wonder why my topo differs from the previous plans by 2.2 feet (I get the question all the time):

For decades the standard elevation datum was the 1929 National Geodetic Vertical Datum (NGVD29), which was somewhat based upon Mean Sea Level (MSL). That datum was replaced by the 1988 National Adjusted Vertical Datum (NAVD88). The difference between the two in the Tucson valley is generally 2.2 feet, but that varies by a tenth of a foot at various locations across the valley.

I always explain datum's to people by using temperature as an analogy; most people understand the difference between Fahrenheit and Celsius. I just say that vertical scales are the same thing, it's just a different system for measuring height.

> and Colonel Sanders before he went tits up)...

how can you hate The Colonel?

[flash width=560 height=315]//www.youtube.com/v/YKRFlNryaWw?hl=en_US&version=3[/flash]

> ...... That datum was replaced by the 1988 National Adjusted Vertical Datum (NAVD88). The difference between the two in the Tucson valley is generally 2.2 feet, but that varies by a tenth of a foot at various locations across the valley.

I thought it was the North American Vertical Datum of 1988

The difference here in Western Washington is about 3.4'....

How do you... 29 flawed

> Some folks think we should stick to 29 with all of its flaws just for convenience?....

Would these be the same folks that don't like the metric system? :-S

B-)
:snarky: