Current epoch
http://tidesandcurrents.noaa.gov/data_menu.shtml?stn=9414290 San Francisco, CA&type=Datums
superceded
http://tidesandcurrents.noaa.gov/data_menu.shtml?stn=9414305 NORTH POINT [PIER 41] S.F.BAY, CA&type=Superseded Bench Mark
Here is the question HT0728 is right across the street from my project and it appears to be superceded for tidal datum purposes. So would I be correct to figure that I have to find a tidal bm from station 9414290 and level from there to get an accurate tidal datum elevation for my location...
thanks for your help
Dane, I think you would be safe to use the tidal datum to NAVD 88 offsets for station 9414290 to apply to HT0728. Just to be safe, send your request to Mike Michalski -- Michael.Michalski@noaa.gov, who is the tidal datum team leader for NOAAs Center for Operational Oceanographic Products and Services (CO-OPS).
Dane,
At the bottom of the Tide Station Page are elevations of the various water levels (MLW, MHW, etc.) and the primary fixed datum related to the tidal datum of that station (MLLW at the Tide Station = 0.00).
The superceded Tidal Epoch (That used for reporting data for 9414305) is in feet and gives the NGVD29 0.00 as being 2.99 above that local Tidal Datum.
The current Tide Station of 9414290 is on the current Tidal Epoch, is reported in meters, and shows 0.000m NAVD88 to be 0.018 below the local 0.000 of the Tide Station (MLLW). If you hit the link just below that data titled "North American Vertical Datum (NAVD 88)", it brings up a chart that also has NGVD29 on it and reports in both feet and meters. That should give you all the data you need to relate the two to whichever fixed datum you're working on.
Additionally, below the tide level info for the tide station (for each tide station), there are reported distances above MLLW and MHW for several BM monuments related to each of these stations, set at various locations in the vicinity (within a few blocks most likely) of each tide station. If any of these still exist, you can compute their elevation in NGVD29 for 9414305, and in either NGVD29 or NAVD88 for 9414290 using the data for each respective tide station, or for either fixed datum for those related to 9141305 by using the data from both tide stations.
HT0728 is 14.66 feet above MLLW for 9414305 in the old Tidal Epoch. NGVD29 is 2.99 feet above MMLW at this tide station for that epoch. Therefore the elevation of HT0728 = 14.66 - 2.99 = 11.67 feet NGVD29.
Remember, the fixed datums are fixed (aside from the various readjustments along the way). The Tidal Datums change with each new Tidal Epoch calculated by NOS. And each tide station is on its own datum, they are determined by the level of MLLW at that tide station per 19 years worth of measurement data at that station. 0.000 at one tide station will not be 0.000 at another.
If you have additional questions about this, feel free to email.
