I am a Civil Engineer with limited survey knowledge and have been reviewing geodetic concepts and the subject of vertical datum.  In the civil engineering profession, many times a day I see reference to MSL but know the surveyor has indicated NAVD88 as the vertical datum.  It seems that anything related to tide stations such as NAVD88  or NGVD1929 would in some way be considered MSL.  However, it remains a source of confusion, as a novice surveyor, I am unfamiliar with the surveyor definition of MSL.  Can someone elaborate on the difference(s) between "MSL" and NAVD88, if any ?

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MSL is a local measurement of actual tides over the span of 1 epoch. Ngvd29 is a national leveling network giving a common reference datum over long distances. NAVD88 is a hybrid datum that combines leveling data with gravity measurements. NGVD29 and NAVD88 have no direct correlation to MSL and the value will vary depending on location. Check a local tidal gauge to determine the MSL in your desired datum.


The next datum we are discussing is the tidal datum. The movements of the earth, sun, and moon create the movements of the tides along with other influences. These are generally periodic and take 18.6 years to cycle through. “A tidal datum is a standard elevation defined by a certain phase of the tide. Tidal datums are used as references to measure local water levels and should not be extended into areas having differing oceanographic characteristics without substantiating measurements.” “The NTDE (National Tidal Datum Epoch) is a specific 19-year period over which tide observations are taken to determine Mean Sea Level and other tidal datums such as Mean Lower Low Water and Mean High Water.” (NOAA) Mean sea level may be determined at a tide gage as the average of mean high water and mean low water observations.


Once upon a time in a galaxy far, far away… no that’s another script. Back in the day, before the invasion of electronics into surveying, there wasn’t much concern about some universal datum, especially when performing a survey of limited scope. Such datums are irrelevant to the purpose at hand. Local surveys used local datums. The boundary had an origin at the POB and was aligned along the first side line called, &c. There may be a local benchmark that everyone nearby used or a TBM was set for any project.


There was an official vertical datum called MSL29 that was the result of trying to mean the mean sea levels from 26 tide gages, 21 in the United States and 5 in Canada. Since without government sized resources one had little hope of accurately carrying levels for any great distances, the accuracy of MSL29 was not as important as was providing “official” bench marks for putting on “official” documents. As people became more aware of those facts the name of the datum was changed from Mean Sea Level 1929 to National Geodetic Vertical Datum of 1929.


With the intrusion of electronics and satellite positioning surveys that were no longer local in scope became more common even for the smallest survey practices. The newer techniques presented a problem to NGS in that every day surveyors were making observations with greater precision than the underlying datum. NGS responded first with NAD93 (horizontal) and later with the North American Vertical Datum 1988. NAVD88 became the official datum of the U.S. in 1991. NGS can no longer say what the NGVD29 values would be at any monument.


The recognized method of determining an estimated MAVD29 value is to use the best current method to determine NAVD88 and stick those values into an program you can get from NGS called VERTCON.


  • Real MSL only available at tide gage
  •  NAVD88 official datum for U.S.
  •  An “official” estimate of NGVD29 is possible using VERTCON





Very good Anthony. There isn't much left to say. The problem with MSL is it isn't consistent, ie: it varies from location to location. If you run levels from one tidal bench mark to another down the beach a number of miles away, the levels probably won't close. NGVD' 29 & NAVD' 88 were created to correct this. NGS is currently readjusting NAD, 83 & NAVD' 88. This new adjustment is scheduled to be effective in 2022.

I guess nobody told FEMA, many of their flood maps are still on '29 datum. I use the published NGVD' 29 datum when I need that datum if it is available. So far, I haven't needed VERTCON.

Now if I can just figure out how to convert my LORAN C fishing coordinates to Lat. & Long.! I found two on line programs that claim to do so but agreement isn't too good. My older Furuno GPS will work with either lat. & long. or LORAN but I don't see a direct conversion yet.

Simple answer: NAVD88 is a reference datum that is now the current standard. Mean Sea Level is measured over time. MSL can be related to any datum. NGVD 29 elevation # will be different than a NAVD88 # but the MSL is the same. Just related to a more modern adjustment of the datum. Hope that made sense.

Here is three pages about MSL


    this one is from NOAA


good question, Great answers

Nice  understandable reply with picture thank u Mr.Billy

Antonhy and Charlie have already given you great answers. I recommend you read this: Why Am I “Mean” About Sea Level? 

It’s a little bit technical but you will understand it!!!    

Mr. Alex Roses,

I think you misunderstood  my reply, I never compete with other people reply.s on who's the best.

I meant that everyone's answers was great, and what a great site this is. I have the deepest respect for Mr. Cavell and Mr. Aycock. And I asssure you both of them know it.

This is not a Yahoo chat room this is Land Surveyor United ,not divided. And I do not apply insults in my reply's ,so everyone can see where you are coming from. So if you are looking for arise you will not find

one. Respect can sometimes be technical for some people.

Best Regards

Mr Billy Brooks

My apologies to you and all those who have offended. My intention was not to criticize, but to contribute to the subject.

I think your comment it’s because my final line: “It’s a little bit technical but you will understand it!!!” I wrote because I suppose Mr Turlington is not familiar with most of the term. I quote him: “as a novice surveyor, I am unfamiliar with the surveyor definition of MSL”. I meant “It´s a little bit difficult but you will understand!” I messed up.

My bad if looks like I’m trying to arise or something like that. Land Surveyor United, not divided. Just like you write.

Best Regards

 Mr. Roses, Ditto back to you, look forward to here from you in the future.


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