Blog · September 30, 2026
What Is Mean High Water?
Mean high water is a tidal elevation, the average of the high tides over a long epoch, used in many coastal states as the boundary between private upland and public tidelands. It is not the seaweed line from one afternoon, and it is not ordinary high water on an inland lake.
Platbookmapper is an Amazon Associate. If you buy through an Amazon link on this page, we earn a commission. The link is a product, not the county map.
A tidal datum, not a stain on the sand
On tidal shores, surveyors and courts talk about mean high water, mean low water, and sometimes mean higher high water. Mean high water is computed from tide observations over a tidal epoch, not from a single high tide and not from a debris line you noticed on a walk. The National Ocean Service publishes tidal datums at gauges. Transferring that elevation to a particular beach requires survey work, because the water surface at the gauge and the line where that elevation meets your sloping beach are a surveying problem. A phone that says sea level is not on this datum and is not a survey. The boundary line on the ground is where that elevation intersects the shore, and it can move if the shore accretes or erodes gradually.
How private deeds use it
A coastal deed may run to mean high water, to the Atlantic, or to a bulkhead. The first of those is an ambulatory tidal call. The bulkhead may be fixed. State law decides how much of the foreshore is public even when a deed uses generous words. Some states hold public trust rights to mean high water. Some use a different tidal plane. Copying a rule from the wrong coast is a serious error. The plat of a beach subdivision may also show a bulkhead line, a dune setback, and the lot line as three different lines. The lot you buy may stop at a line landward of mean high water, with the dry sand in a public or association parcel. Read the courses. County GIS will draw some shoreline from imagery. Platbookmapper links to that map and does not compute tidal datums.
What you do not mark
Do not set a fence at the darkest seaweed and call it mean high water. Debris lines are evidence a surveyor may consider and are not the datum. Do not fill seaward of the line to create yard. That can be a public-trust and a permit problem as well as a neighbor problem. If you need the line for a permit, hire a surveyor who can tie a tidal datum to the site. Until then, read the deed and the plat as they are. An inland lake’s ordinary high water is a different concept, built from different evidence. Do not use a tide table on a reservoir.
What to do next
- Use mean high water on tidal deeds, and look for the state’s tidal boundary rule.
- Do not substitute a debris line or a phone elevation.
- Read whether the lot actually runs to that line or stops at a bulkhead or a plat line.
- Keep inland ordinary high water in a separate category.
The mix-up to avoid
Marking yesterday’s high tide as the property corner. One tide is not the mean, and the mean is an elevation from a tidal epoch, surveyed onto the beach.
Questions people ask
Does mean high water move?
The datum is long-term, and the place where that elevation meets the land moves if the beach erodes or builds gradually. A storm can also change the profile suddenly, which raises avulsion questions. The elevation and the intersection are related and not identical.
Is the wet sand always public?
Often the public trust includes the shore seaward of the private line, and the private line is often mean high water, but the state and the deed can change the answer. Read both. A general slogan about wet sand is not a survey.
Find the county map
Start at Platbookmapper, search the county, and open the local GIS link. Write down the parcel number before you compare the screen to a paper plat.
Not a survey. County GIS and tax parcel maps are assessment and planning layers. They are not a substitute for a recorded plat, deed, or licensed survey.