4 
e The tidal information is limited to a shoreline representing an approxi- 
mate line of mean high water and a statement giving the approximate mean 
range of tide. Depth curves and soundings are Obtained from NOS charts 
and based on a datum of mean low water (Atlantic and Gulf coasts) or 
mean lower low water (Pacific coast). The low water line, from which 
some private property boundaries and the Federal and State seaward 
boundaries are defined, is not shown. 
e The datum for contour and elevation information is stated as mean sea 
level on maps published before 1973. However, as will be explained 
later, there is a distinct difference between a tidal datum and the e e 
geodetic datum used for elevations. 
e Subsidence is appreciable in some areas, particularly along the Gulf 
coast, so that the contour delineation is probably in need of revision. 
TIDAL AND GEODETIC DATUMS 
Tidal datums are vertical datums defined by a phase of the tide and used 
as reference planes. The law has traditionally recognized the validity of using 
tidal datums to establish seaward boundaries, Federal-State jurisdictions, and 
private ownership and to regulate navigable waters. The Submerged Lands Act 
of 1953 and the 1958 Convention on the Territorial Sea and the Contiguous Zone 
provide that the low water line as shown on large-scale maps may be used as the 
base line for cartographically delimiting the territorial sea. This is a prag- 
matic solution to a technical problem since the ordinary low water line and not 
the high water line was previously mapped on large-scale coastal navigation 
charts. Low water is more critical to the sailor than high water. It does 
not invalidate the use of other tidal datums for riparian rights. Accurate 
values for tidal datums such as mean high water or mean low water are obtained e eo 
by averaging height readings at a location over the 19-year tidal epoch. While 
tidal datums are required for property boundaries, they have several limitations 
for describing and mapping the coastal wetlands. / 
e. Tides are usually measured &t sheltered areas of open water with free 
access to the sea. Selected estuaries, bays, and rivers may be gaged, 
but rarely the small channels or marshlands. 
e The measurement of tide at a new location requires 1 year of records 
and correlation to a control tide station for an accuracy of 0.1 foot 
or better. Tide records for a few months will give accuracies of 
several tenths of a foot. The control tide stations require records 
spanning the 19-year tidal epoch. 
e Tides in a river or estuary are combined with the stage and flow of 
fresh water. This gradually becomes the primary influence with 
increasing distance from the ocean. The influence of the ocean may 
be observed at extreme high tides or low river flow, but measurement 
of an accurate tidal datum is nearly impossible.