This is the reason why, in 1973, the National Geodetic Survey changed the name from the Sea Level Datum of 1929 to the National Geodetic Vertical Datum of 1929. The only place in the North American Continent where zero mean sea level was equal to zero Sea Level Datum in 1929 was the primary benchmark of the United States - Galveston, Texas.
The density of the Earth's crust and gravity varies elsewhere ... that's why zero for either NGVD or NAVD is not equal to mean sea level except at one particular index point. For NAVD88, it's at Father's Point, Rimulski on the St. Lawrence Seaway.
This is pretty old news.
Question For Cliff
Why Father's Point on the St. Lawrence Seaway? Do they have minimal tide ranges? I've always wondered why they chose that particular mark in that particular area.
Thanks for sharing your knowledge with us.
Dale Yawn
Savannah, Ga.
So that is why everything fit MSL so well when I was surveying in the Houston area. I knew about the gauge at Galveston, but I didn't realize it was the basis for the country at that time.
:good:
Question For Cliff
Dale,
As I recall the story told to me by Dave Zilkoski, the former Director of the NGS, Canada took a dim view of SLD29/NGVD29 and did not use it. In an attempt to woo Canada to recognize this new datum for both Canada and the United States, they chose that point on the St. Lawrence Seaway.
Guess what?
Canada still didn't bite.
🙂
Cliff?
I thought the Galveston tide level was only held in the two "Special Adjustments" made by Bowie to gage the accuracy of the system, which were not published for general usage.
And that NGVD was warped to 28(+/-) tide stations around the US and Canada and that was the data released to the public.
http://www.pobonline.com/Articles/Features/bdfd817cac0f6010VgnVCM100000f932a8c0____
Cliff?
Per the quote you referenced,
"In 1927 a special adjustment of the level net was performed for theoretical purposes. Only closed circuits were adjusted and when the net was “consistent within itself," the elevation of the Houston, Texas, junction point, whose elevation was determined from sea level at Galveston, was carried through the net to other tide stations. As published in SPECIAL PUBLICATION NO. 134, GEODETIC OPERATIONS IN THE UNITED STATES, JANUARY 1, 1924, TO DECEMBER 31, 1926, by Bowie, the results showed the Pacific sea level to be higher than Atlantic sea level, and that sea level rose in both towards the north, and west along the Gulf of Mexico."
In 1929, that datum was "indexed" to Zero at Galveston, as the Primary Bench Mark of the Sea Level Datum of 1929.
Cliff?
Many thanks for clearing that up.
Gentlemen such as yourself really should write books for those of us less informed.
Thanks again for the info.
Question For Cliff
Politics is everywhere, isn't it?
Thanks,
Dale
Origin of NAVD 88 and relation to LMSL
Neither NGVD 29 or NAVD 88 should be considered the same as Local Mean Sea Level. Depending on which datum you are using you could easily have differences of a few cm to more than a 1 meter. For addition info take a look at the NGS/CO-OPS webinar "Introduction to Geodetic and Tidal Vertical Datums" -- http://www.ngs.noaa.gov/corbin/class_description/Tid_Geo_Vert_Datums.shtml.
Father Point (Rimuski) was selected as the origin for NAVD 88 for two principal reasons. It is also the origin of the International Great Lakes Datum of 1985 (IGLD85), so the datums can be considered the same with the exception that NAVD 88 heights are orthometric and IGLD are dynamic heights. The other major reason is that by selecting that tide station the differences in heights with respect to NGVD 28 are minimized in the eastern US (basically east of Denver). This was important to the U.S. Geological Survey because did not significantly impact their National Mapping Program for 7.5' topos.
