Student Surveyor

Mean Sea Level (MSL)

Hello All,

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 an MSL tidal gauge and/or NAVD88.  I believe some coastal areas still 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 (upland of gauge), but not found any reference to this in a white paper form or scientific type documentation, but there must be something?

Thanks in advance for your help with this, greatly appreciated.

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  • Land Surveyor

    I can't imagine what reason would necessitate the need to use the old National Geodetic Vertical Datum (1929) in preference to the more recent calculation. There will of course be localised and in all probability negligible differences simply as a result of the change in datum and subsequent additional considerations in the calculation of the geoid in the more recent NAVD88.

    Not being familiar with the equivalent systems in mapping in North America I can only assume that the more recent NAVD88 is similar in nature to the Ordnance Survey's OSTN15 and OSGM15 geoid calculation to facilitate the use of GNSS and remove the requirement to rely upon physical benchmarks which by their very nature have the potential to be rendered unreliable through geological and/or structural movement. Deep mining of coal and the subsequent widespread subsidence throughout South Yorkshire kept me quite busy throught the 1990's.

    Your two datums will in most cases deliver very similar results but it is potentially a significant undertaking to verify values on the old network if said network is no longer maintained.

    In short I would suggest that NGVD29 should always be considered inaccrate and thus inapplicable unless extensive checks are undertaken relative to multiple benchmarks in the given area. Any such check can only verify local values so the nature of the specified requirement to use this datum would need to better understood. 

  • May be a bit of a stretch for your circumstance, but I would begin by looking at some of the FEMA El Cerificate specs.  Might get you pointed in the right direction.

  • The engineer may need elevations referenced to mean sea level (MSL) because it is required by local government or he/she needs to compare the existing elevations to local flood maps and so on.

    Below is a link to NGS's data maps with horizontal and vertical control.

    Just locate the control near your project and  use the monumentation(s) elevations to gather the information needed.  

    NGS Data Explorer

    National Spatial Reference System, Geodetic control map
    Geographical interface for accessing the National Spatial Reference System.
  • Land Surveyor

    If the site is on a river and only a few miles inland from the coast, quite likely, there’s a gaging station near the site.

    If can be more specific about details, I might be able to better help you.


    • He should be able to interpolate between the two datums from a single point of measure.

      • convert would be the better operative.

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