NGVD 47

What is the area you are working in? Do you have a specific NOAA tide station you are looking at? Also note that there is no such thing as NGVD 47. There was a supplemental adjustment of NGVD 29 in 1947 for the NW region. Is that what you are referring to?

If you pull up an NGS datasheet for an older point in your area, does it show the 47 readjustment values in the superseded section?

Unfortunately NGS does not identify which of the supplemental adjustments (1933, 1947 or 1956) the superseded NGVD 29 heights are related to in this region. They are simply the final value published prior to the release of the NAVD 88 and in most areas of OR/WA that will be the 1956 values.

The project is in Des Moines Washington, near puget sound so the nearest tidal stations are in Seattle or Tacoma, the link and numbers I posted are for the Seattle station.

Awesome, I never knew that they ever updated MSL 1929 in areas.

According to the Federal Register, MSL was renamed NGVD in 1973.

As far as the question at hand, can you find a nearby benchmark with published elevations on both datums?

Sorry I missed the link you provided. I find those confusing as well and I've worked with this stuff for decades.

The difference from NAVD 88 to NGVD 29 heights at this station is - 3.58' meaning the NGVD 29 datum level is above the level of NAVD 88. This translate to the highest tide level of 8.56' in NGVD 29.

I would suggest that you send a request to the NOAA Center for Operational Oceanographic Products and Services (CO-OPS) tidal datums team and ask them if they have any supplemental unpublished stations closer to Des Moines -- nos.coops.datums@noaa.gov

I had a pretty good file of older tide stations in my files at NGS before I retired but after I left I understand someone trashed most of them.

The NOAA Center for Operational Oceanographic Products and Services (CO-OPS) updates the national tidal datum epoch (which includes MSL among other tidal datums) about every 20-22 years in most parts of the country. The 1973 Federal Register you referenced did not rename MSL as NGVD 29. It renamed the Sea Level Datum of 1929 to the National Geodetic Vertical Datum of 1929 (note that the older name did not include the word "MEAN"). This was done specifically to try and remove the confusion between the name of the geodetic datum and Mean Sea Level which is a very specific and localized tidal datum.

READING TOO FAST

I read spledeus's post too fast.

The U.S. Coast & Geodetic Survey/National Geodetic Survey often performed regional readjustments of both the older datums - NAD 27 & SLD 29/NGVD 29 as newer data was added to densify the networks. The NW regional supplemental adjustments is one of the best examples of a regional readjustment of SLD 29/NGVD 29, but others cover areas such as Houston-Galveston, Southern Florida and the Virginia Beach/Norfolk, Virginia areas.

READING TOO FAST

> I read spledeus's post too fast.
>
> The U.S. Coast & Geodetic Survey/National Geodetic Survey often performed regional readjustments of both the older datums - NAD 27 & SLD 29/NGVD 29 as newer data was added to densify the networks. The NW regional supplemental adjustments is one of the best examples of a regional readjustment of SLD 29/NGVD 29, but others cover areas such as Houston-Galveston, Southern Florida and the Virginia Beach/Norfolk, Virginia areas.

I have heard that Florida was not even part of the original NGVD29 adjustment and was just tacked on piecemeal. I have wondered if that was the reason that there was 0.8' of slop between the east and west coasts of the Florida peninsula.

FL PART OF NGVD 29

The level line connecting the tide stations at Cedar Key and St. Augustine was the most southern extent of USC&GS leveling at the time of the publication of NGVD 29. Subsequently extensive leveling was conducted across the state during 1932-33 including rerunning the line from Cedar Key to St. Augustine and these data were then added as a supplemental adjustment. Based on tide gauge and leveling records the relationship of local Mean Sea Level from east to west is approximately 0.8 ft across the peninsula and the uncertainty of First-Order Class II leveling across the area could possibly account for about half of that (.005m * Sqrt 510 km = 0.11m /0.4 ft)

> Sorry I missed the link you provided. I find those confusing as well and I've worked with this stuff for decades.
>
> The difference from NAVD 88 to NGVD 29 heights at this station is - 3.58' meaning the NGVD 29 datum level is above the level of NAVD 88. This translate to the highest tide level of 8.56' in NGVD 29.
>
> I would suggest that you send a request to the NOAA Center for Operational Oceanographic Products and Services (CO-OPS) tidal datums team and ask them if they have any supplemental unpublished stations closer to Des Moines -- nos.coops.datums@noaa.gov
>
> I had a pretty good file of older tide stations in my files at NGS before I retired but after I left I understand someone trashed most of them.:-X :-@ :-O o.O

Thanks for the info Dave, and too bad some people just don't understand the value in holding onto older data.
Cheers
See you in January @ PLSO

> But this plan has the vertical datum as NGVD 47 which I gather is some sort of tidal datum. There is one other clue on the plan which is a "Tidal Maximum Elevation 4.67 feet"
>

NGVD[Anything] is not a tidal datum. Tidal datums are unique to the tide station it is reported for and uniquely related to the benchmarks associated with that tide station.

Tidal datums are:

MHHW: Mean Higher High Water
MHW: Mean High Water
MTL: Mean Tide Level
MSL: Mean Sea Level
MLW: Mean Low Water
MLLW: Mean Lower Low Water

The definitions of which points of the daily tide cycles these each represent are explained thoroughly on the Tides & Currents website. The means are calculated from 19 years' worth of data (covering a full 18.6 year tide cycle) gathered during the most recent Tidal Epoch, currently the epoch running from 1983 to 2001. The datums for each station will change with each new epoch's data.

Often, but not always, the tide station is also related to one or more fixed datums.

Note that for each tide station, the level of MLLW is used as the 0.00 elevation for reporting all other elevations. If an elevation is also listed for one of the fixed datums (NAVD88 or NGVD29), you have a means to then compare the tidal datums to the datums of other tide stations which are also related to the fixed datum.

Most likely, the elevation given to you as "Tidal Maximum Elevation 4.67 feet" is the elevation of MHHW at whichever tide station is mentioned (or the nearest one of non is listed), based upon whichever tidal epoch was in effect when the 1947 adjustment of the NGVD29 fixed datum was the adjustment in use.

I suspect that that data should have more properly been reported as MHHW = 4.67 feet, NGVD29(47). The tidal epoch in effect for that data was probably the one ending sometime around 1953 (I don't recall the exact year, but a new tidal epoch is put into effect roughly every 25 years). There is a possibility that it might be the epoch ending in 1978, or the one ending around or just prior to 1930.