I have a PE client requesting I determine some elevations of water well casings relative to mean sea level. Usually I tie my vertical work to county benchmarks that are based on NAVD88 2001 Geoid99. I dont really know how that relates to mean sea level but im guessing they are not the same. How does one determine MSL? If its a huge pain I may try to talk him into using the county benchmarks or turn down the project. Thanks
How does one determine MSL?
Mean Sea Level is different every where you go. You have to find the nearest Tidal Benchmark and refer to that.
Chances are; your PE client hasn't got a clue, as to what he or she is asking for. Ask her or him why they need the elevations relative to Mean Sea Level.
Keep in mind that prior to 1973 NGVD29 was known as 'Sea Level Datum of 1929" and there are a number of long term legacy projects/agencies floating around that are still using 29.....such as the eighth largest water/wastewater utility in the U.S. (The Washington Suburban Sanitary Commission)
I second Radar's comment.
By looking at adjustment date and geoid, I am guessing Harris County Texas. There has been so many relevelings/adjustments done since original published NGVD29 data (because of subsidence in area) that MSL doesn't mean much unless client knows or lets you know actually what he needs. Being in Harris County, I tell client that I will use latest Harris County reference marks and go from there.
"MSL" can sometimes be a shorthand for NGVD29.
NGVD29 was "MSL29" or simply "MSL" until NGS changed the name to NGVD29 in 1973.
Ask him to clarify what datum he needs. He could be working from old drawings and needs to match something for flow. Apparently some FEMA stuff is still in NGVD29.
I saw a job once where they installed a whole neighborhood worth of sewer using NAVD88 to stake NGVD29 elevations.
Rather than dig it all out they bought a vacant parcel and added a pump station.
Rather than dig it all out they bought a vacant parcel and added a pump station.
I wonder if they compensated the public agency owner (i.e. the taxpayers) for the cost of maintaining a pump station in perpetuity.
What a coincidence, I'm just starting out on a monitoring well project that requires MLLW on monitoring well casings. In this case it makes sense since I'm dealing with an old Coast Guard Station dump right on the Columbia River near the Pacific Ocean. In my case the conversion from NAVD88 to MLLW is simple since in nearest tidal station is on the base. If you are on the coast you should check out the NOS tide station pages. They give conversion factors for each tidal station.
The fun part of my job is going to be the request to stake out MLLW along the adjacent shoreline in December. The Columbia tends to run high this time of year. I don't think the engineers thought about that.
Data for Morgan Point in the Houston area is found here:
https://tidesandcurrents.noaa.gov/datums.html?id=8770613
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
As previously noted NGVD29 was previously called the Sea Level Datum of 1929.?ÿ It was never the Mean Sea Level Datum of 1929.?ÿ Actually the original designation was the Fifth General Adjustment of 1929, but SLD 29 was adopted shortly after the adjustment was completed.?ÿ It was because so many users were making the mistake of referencing the heights as MSL that NGS drafted the Federal Register Notice changing the name in 1973.?ÿ It should also be noted that for anyone using any tidal datum (MSL, MLLW, MHW etc.) that in addition to providing the height value you should also provide the metadata indicating which tide station was referenced and specify which National Tidal Datum Epoch was used, the current is 1983-2001.?ÿ Look for the Center for Operational Oceanographic Products and Services (CO-OPS) to release a new NTDE around the same time that NGS will release the new geodetic datums.
?ÿ
Comet meted?ÿ
I have been working on CEUs for CFM and Comet MetEd has at least 3 online courses that take less than am hour that go into the details.
Last night I took Storm Surge and Datums.?ÿ One answer was to simply ask for the datum.?ÿ In your case, ask which tidal BM, go to the NOAA CO OPS site.?ÿ 50 50 shot it has a reference to NAVD or go OPUS.
Share the lesson with the PE to male sure.
Also remember that old timers forget that in 1973, Congress changed MSL datum to NGVD.?ÿ There is a nearby firm that still lists MSL even though it has not been called that in my lifetime.
Good luck.
A request for Mean Sea Level means asking questions until you are certain what they need and stating it in the contract. Usually, I found it to mean a some vertical datum and not an assumed elevation.
I think you can find Benchmarks with 1929 information attached still extant in the Houston area. If you can locate the Benchmark, obtain a current elevation bu observation.
Paul Kwan has a large amount of information concerning the Lake Houston elevations, which are based on 1936 data. Probably little or no subsidence out there at that time.
Good luck,
Jack Chiles?ÿ
Chances are; your PE client hasn't got a clue, as to what he or she is asking for. Ask her or him why they need the elevations relative to Mean Sea Level.
This turned out to be the case. He had no clue. Thanks for all the info though. Will come in handy.
