Mean Sea Level

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01-14-2019 07:04 AM
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Occasional Contributor II

All,

I am using ArcMap 10.4.1.

I see many engineers using MSL, but I believe, should be using NAVD88 as a vertical datum.  I imagine there must be a rule of how many miles upland (distance from gauge) the sea level gauge is good for.  Does anyone know how far upland one can make use of Mean Sea Level accurately, before having to switch to NAVD88 or similar? 

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MVP Esteemed Contributor

I don't know if I would rely on mean sea level at all if you are looking at it as a reference point for upstream reference points... given the 40-50 foot tides in the Bay of Fundy NB/NS in Canada.

What exactly do you want to use msl as a reference point for?

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Occasional Contributor II

Dan,

Thanks for your interest. I have a project in Alabama that is located on the south end, not very close to the coast, but close to a river connected to the coast. I have never used MSL, but I am working with an engineer that has a need to use MSL. As a designer, I would like to know what the cut-off distance is between use of a tidal gauge and/or NAVD88. I know some coastal areas use MSL, but there must be a rule for knowing when it is no longer accurate or applicable. In my limited research, I have heard of 1-2 miles as rule of thumb, but not found any reference to this in a white paper form or scientific type documentation, but there must be something?

Regards,

Chuck Turlington

CAD Designer

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MVP Esteemed Contributor

Chuck, I haven't come across anything...

So much for me suggesting... 'ask an engineer, they often work with those vague things'

Melita Kennedy‌ might have stumbled across this type of question over coordinate system discussions

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Esri Notable Contributor

I'm not sure I've seen anything either. If it's anything to do with flood mapping / FEMA, it should be NAVD88.

Melita

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Occasional Contributor II

Dan,

 

Thanks for your interest.  I have a project in Alabama, USA that is located on the south end, not very close to the coast, but close to a river connected to the coast.  I have never used MSL, but I am working with an engineer that has a need to use MSL.  As a designer, I would like to know what the cut-off distance is between use of a tidal gauge and/or NAVD88.  I know some coastal areas use MSL, but there must be a rule for knowing when it is no longer accurate or applicable.  In my limited research, I have heard of 1-2 miles as rule of thumb, but not found any reference to this in a white paper form or scientific type documentation, but there must be something?

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Regular Contributor

Hi Chuck,

In your first question you mention "I see many engineers using MSL, but I believe, should be using NAVD88 as a vertical datum." I think this blog from this place might be of interest as there's a bunch of ambiguity around MSL etc...

https://community.esri.com/groups/coordinate-reference-systems/blog/2014/08/14/mean-sealevel 

Regarding the next question: "what [is] the cut-off distance is between use of a tidal gauge and/or NAVD88."

With regards to the validity of a tidal gauge reading against distance from that gauge. I would treat the gauge as a single point of reference much like a survey triangulation point but arbitrary and local so maybe not like a survey triangulation point. This is because local variations in sea level are really subject to large hydrodynamic influences especially close to the shore. So, there isn't a rule of thumb as every tidal gauge would have to be seen in the context of it's local situation (e.g. topography, bathymetry, currents, etc...) perhaps a number could be achieved after hydrographic modelling. A rule of thumb would work however, it would be impossible to have a regular assumed error of vertical error based on distance from the gauge.

NAVD88 is based upon levelling of height observations across the USA and I understand that a model was built after hurricane Katrina to interpolate height, from local sample locations (i.e. tide gauges) so that an accurate model of MSL along coastal regions could be achieved. There is documentation on this on NOAA's National Geodetic Survey (NGS) but the website is unavailable until the Federal Closure ceases.

I hope this helps.

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Regular Contributor

Now you're back in business have a look at NOAA's vertical datum transformation tool (VDatum). There's information about the tool and more detail on vertical datums in general.

NOAA/NOS's VDatum 3.9: Vertical Datums Transformation  

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New Contributor III

Here is an article that explains the difference in very simple terms:  Elevation for Beginners: What to Know About GPS Receiver Elevation.

Hope this helps

Jean-Yves

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