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Mean Sea Level
Posted by sacker2 on March 29, 2012 at 7:29 pmAnother surveyor was asked to use the term “Above Mean Sea Level” for a ‘1A’ letter on an existing cellular tower. His measured elevations are based on NAVD 88 and from what I have read, MSL is irrelevant to NAVD 88. I told him I wouldn’t use the “Above Mean Sea Level”, only the datum. Am I wrong ?
loyal replied 12 years, 1 month ago 16 Members · 19 Replies- 19 Replies
We have found the relationship between NGVD and NAVD to be almost equal to calculated sea level rise, at least here about. But to answer your question, if you’re looking for an exact relationship check the local tidal benchmarks and derive the conversion, or you can call it “close enough”. I forget the vertical accuracy requirement but it’s most likely within your tolerances.
You can make the comparison, however.
> Another surveyor was asked to use the term “Above Mean Sea Level” for a ‘1A’ letter on an existing cellular tower. His measured elevations are based on NAVD 88 and from what I have read, MSL is irrelevant to NAVD 88. I told him I wouldn’t use the “Above Mean Sea Level”, only the datum. Am I wrong ?
Nope, you’re right on. AMSL is based upon the average observations of a Tidal Station, making the value a local one.
NAVD 88 cannot be translated to AMSL unless one takes an observation on the Tidal Station and establishes a NAVD 88 value for it.
If I had to guess, this request for mean sea level comes from someone who doesn’t understand the subject matter, and is just trying to check off all the boxes.
Do you have VDatum in that area?
I just did a least squares interpolation to establish 1934 mean sea level for a given LAT and Long. Basing any project on tidal datum is silly. MHW is usually used in most Civil Marine Projects.
Ralph
I used to do alot of celltower 1-A and 2-C letters and the AMSL language is pretty much universal. I figured it must have come from above or all letters wouldn’t be using the same language
I traced it to FCC form 601 Schedule D block 13 FCC 601 Schedule D. Doesn’t make it right, but at least we know the source.
I believe NAVD is tied to sea level at a single point in New York. Given that you’re in the NE, you probably don’t see the same difference we do down here. I’m thinking it’s about 1 foot difference in Florida.
But yes, the OP is correct, NAVD is not considered a “sea level” datum.
Newtonsapple is absolutely correct. NAVD 88 and even NGVD 29 are not the same as Mean Sea Level. MSL, which should actually be referenced as Local Mean Sea Level (LMSL) can only be defined in the very local vicinity of the shore line. Both NGVD 29 and NAVD 88 were referenced to LMSL, but they do not define it. Real tidal data is a function of NOAA’s Center for Operational Oceanographic Products and Services CO-OPS. Any interested can download a webinar on the subject of Geodetic and Tidal Vertical Datums — Datums
That’s it Stephen. The client sent about 5 example copies from various states and everyone of them had the AMSL on them prepared by a licensed surveyor. I suggested to my fellow surveyor that he should educate the client and not be one of the “rollover” surveyors. Perpetuating any misnomer instead of attempting to stop it is lazy.
Pilots are used to the AMSL idea, that’s what they think their altimeters are set to. This number is so they won’t bump into the cell towers. You have a tolearance of 3 feet so your elevations should fall within those parameters.
“NGS develops and maintains the current national geodetic vertical datum, NAVD 88. In addition, NGS provides the relationships between past and current geodetic vertical datums, e.g., NGVD 29 and NAVD 88. However, another part of our parent organization, NOS (National Ocean Service), is the Center for Operational Oceanographic Products and Services (CO-OPS). CO-OPS publishes tidal bench mark information and the relationship between NAVD 88 and various water level/tidal datums (e.g., Mean Lower Low Water, Mean High Water, Mean Tide Level, etc.). The relationships to NGVD 29 are not published, but may be calculated independently from specified tidal bench mark sheet links to the NGS data base. Tidal bench mark information, water level/tidal datums, and their relationship to geodetic vertical datums are available at the CO-OPS web site.”
> Pilots are used to the AMSL idea, that’s what they think their altimeters are set to. This number is so they won’t bump into the cell towers. You have a tolearance of 3 feet so your elevations should fall within those parameters.
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Thanks CPTDent, That explains it.Ralph
I highly doubt an altimeter is even close to within 3 feet. Around here, the difference in NAVD’88 AND NGVD’29 is over 3 feet. I also don’t know if you can have a precise number for “mean sea level”. I would probably publish the NAVD elevation, and say something like “which is so many feet above mean sea level”. Possibly only put that number to the nearest 1 or 5 feet.
I need to watch the referenced video, however. I’m certainly not an expert.
> I highly doubt an altimeter is even close to within 3 feet. Around here, the difference in NAVD’88 AND NGVD’29 is over 3 feet. I also don’t know if you can have a precise number for “mean sea level”. I would probably publish the NAVD elevation, and say something like “which is so many feet above mean sea level”. Possibly only put that number to the nearest 1 or 5 feet.
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> I need to watch the referenced video, however. I’m certainly not an expert.The difference between NAVD 88 and NGVD 29 is not a constant. NGVD29 may approximate mean sea level but it is not MSL. I agree that mean sea itself is an approximation.
In terms of the Altimeter, I’ll have to take his word for it.Ralph
First of all, Sea Level is not mean, but it is unruley….;-)
The level of the worlds oceans is anything but constant and definitely not consistent.
I hope everyone has a great day; I know I will!Right on radar.
Side note: I have been wanting lately to listen to more FZ music, thanks for that reminder.
I believe converting to NGVD29 is more appropriate for these. First off NGVD was supposed to be an “average” of MSL at many tidal stations in the US and secondly, most aeronautic maps use NGVD elevations and topography from Quad maps which are based on 29 datum. That being said, you will likely be within the 3′ tolerance either way. I would not worry about calling NGVD29 elevations AMSL, though its technically incorrect, it is commonly used in these letters and by the FAA.
Au contraire …
NGVD29 “zero” was NOT an average. For NGVD29/SLD29, zero was observed at the “Primary Benchmark” in Galveston, Texas.
For NAVD88 it is at “Father’s Point” in Remulski (sp?), Canada on the St. Lawrence Seaway.
From the NGS FAQ page.
What are NGVD 29 and NAVD 88?
“The National Geodetic Vertical Datum of 1929: The name, after May 10, 1973, of (the) Sea Level Datum of 1929.” (Geodetic Glossary, pp. 57)“Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929.”
“Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level.”
“The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929.” (Geodetic Glossary, pp. 56)
The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-U.S. leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.
Au contraire …
Sicilian below gives the history of the datum as I am aware of it. This is the same as what is being taught in the Geodesy course which I just completed. No mention of Texas at all. Can you elaborate? I would love to hear more about it.
Au contraire …
Maybe this will help:
http://www.ngs.noaa.gov/PUBS_LIB/NAVD88/navd88report.htm
“The differences between adjusted heights from the NAVD 88 general adjustment and the heights of selected tidal benchmarks above LMSL (epoch 1960-78) are shown in Figure 9. This type of difference can be used to determine the difference between NAVD 88 and LMSL for local projects. It should also be noted that like NAVD 88, NGVD 29 height values are not the same as the latest published LMSL values. In 1929, the published 1929 LMSL values at 26 selected tidal stations were used as constraints in the NGVD 29 general adjustment. At these 26 stations, the 1929 LMSL values and published NGVD 29 values were exactly the same. At other tidal stations, these values were not exactly the same. Today, many NGVD 29 height values are significantly different from the published LMSL (epoch 1960-78) values. For example, the difference between the published NGVD 29 height value and LMSL (epoch 1960-78) at Cedar Key, Florida, is 10 cm (0.32 ft) and the difference between the published NGVD 29 height value and LMSL (epoch 1960-78) at Shell point, Florida, is 16 cm (0.52 ft). LMSL values at specific tidal sites are published by NOS’s Ocean and Lake Levels Division.”
Loyal
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