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

Question

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

Answer 1

I'm amazed at how many incorrect answers you have received for this question.

Mean sea level is determined by tidal gauges, along with mean high tide and mean low tide. The measurements are taken by permanently established tidal gauges that measure the changes in sea level throughout each day, and for periods of many years.

The 'sea level' that the USGS refers to does in fact change from time to time. In 1988 a new sea level datum was established for North America called the NAVD88, which is based on a tidal gauge at the mouth of the St Lawrence River in Quebec. Whenever these changes are made the old maps are revised, and older versions will be considered incorrect.

Because the new NAVD88 used a different method for determining the geoid than the older NGVD29 the errors are not simple systematic errors, but instead vary according to the region of the country. For this reason for applications where exact elevation is important, it is important to use the newer datum. Here is a map showing the corrections made between the two datums:
http://www.ngs.noaa.gov/PUBS_LIB/NAVD88/NAVD88_2.gif

Answer 2

If you look on a U.S. Geological Survey topographic map you will see the statement, "Datum is mean sea level." This means
that the zero elevation line is at the point that is
the mean (average) level of the sea when all changes of tide level are taken into account.

For important reference points metal plaques
called benchmarks are usually placed to mark
the spot. If all the world's polar ice melted and
the sea level rose of course the mean sea level
would rise. In this case the map references would
probably not change because it would be too much
work and expense to make the necessary changes. There would probably be just a notation
saying subtract so many inches or feet from the
old measurements.

Answer 3

You do not have to measure sea level at any particular time or tide level. If you take lots of measurements and then average them you will get the average sea level. The accuracy will be determined by the number of measurements you take. It is simple statistics. Take a look at the plot labeled "Recent Sea Level Rise" in the link below and you will see what I mean.

Answer 4

No, the measurement is not made from tide high, but from average height of the tide. So if there is high tide there is also low tide and this measurement are also year lowest and year highest