If land is surveyed from ocean shoreline (high-tide). How is it figured years later when tides have changed?

Meaning, does the constant change in the oceans shoreline (over time) effect land suveys and measurements for mountain altitudes?

If land is surveyed from ocean shoreline (high-tide). How is it figured years later when tides have changed?
It is periodically readjusted. In 1988 a new North America Vertical Datum was established and was completed by 1991. This replaced the old NGVD 29 datum that was established in 1929. This is one reason that old maps often have erroneous data on them (as well as changing delcination).





Here is a map of the elevation changes that resulted from this new datum being applied:


http://www.ngs.noaa.gov/PUBS_LIB/NAVD88/...





Here is the original NGS report:


http://www.ngs.noaa.gov/PUBS_LIB/NAVD88/...





The primary tidal benchmark for North America is at the mouth of the St. Lawrence River in Quebec. This datum is used for Canada, the US, and Mexico. The prior vertical datum in use was NGVD29 that used a combination of 26 different tidal benchmarks. The difference, as I understand it, is that the 26 different locations caused localized errors because of the differences in sea level from one location to another. The newer datum, being based on only one point, does not introduce this type of error into the elevation reference.


Here is an article by a surveyor that discusses this:


http://www.profsurv.com/archive.php?issu...





One thing to also consider is that many elevation changes are not the result of sea level change. In many parts of the US there is significant ongoing subsidence, and in other areas there is significant uplift taking place. These are the results of groundwater pumping, natural subsidence related to isostacy, and uplift from tectonic forces. Out of about 17,000 benchmarks, about 25 percent have been found to be disturbed by seismic or subsidence activity, Correcting these errors is an ongoing effort.
Reply:I think Geologists measure changes too and punch in the numbers, on the other hand maybe they overlooked because the changes were too small.
Reply:Elevation data is based on points of known elevation on the ground called benchmarks. The height above sea level is approximate depending on where you are. Valid elevation data is established by NVD1929 in my area and is accepted by most surveyors in the area. A special tidal survey has to made to measure what the mean high water elevation is for coastal areas.

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