5 
® Many tidal surveys have been conducted for safety of navigation and 
coastal charting and not for boundary purposes. 
e The critical upper wetland boundary may be thousands of feet hori- 
zontally and only a few inches vertically from the point where the 
tide can be reliably measured. The ground supporting the vegetation 
resembles a sponge, and its average slope may be # percent or less, 
i.e., an elevation change of 0.5 foot or less per 100 feet hori- 
zontally. Therefore, a 0.1- or 0.2-foot error in elevation can 
easily cause a 10- or 20-foot horizontal error even if the ground 
were firm. The soft footing and prolific vegetation prohibit a 
reliable transfer of elevations by conventional leveling except at 
well-defined points and with considerable expense. Recently a tech- 
nique using a sequence of simultaneous water level observations has 
been used to transfer tidal datums from a tide gage in open water 
to a new location in the wetlands. 
e The tidal datum is not a mathematically defined surface but an 
undulating geoid that can only be defined by direct measurement 
at a location. A tidal datum defined at one location cannot be 
extended more than a short distance along the shore, for when the 
area comes under the influence of new tidal conditions, another 
tide gage is required. 
e The tidal datums are ever-changing. Along the southern Atlantic 
coast the mean tide level has increased 0.4 foot since 1924. NOS 
has a policy of revising the 19-year base epoch every 25 years. 
The present epoch for tidal surveys is 1941-59. 
e While mean high tide and mean low tide are the common legal datums 
for establishing boundaries, the morphology, biota, and environmental 
value of the coastal wetlands may be more dependent on the high spring 
tides. 
e In recent years many coastal charts and orthophotomaps have been 
compiled using serial photographs taken at the desired stage of 
tide. Infrared photography provides good contrast between land 
and water and is particularly useful for showing the waterline on 
an exposed beach, mudflat, or rocky coast. On the other hand, in 
the marshes behind the sandy beaches the tide flows beneath thousands 
of acres of vegetation without a unique land/water infrared signature. 
The geodetic datum adopted as a standard for elevations in North America 
is not synonymous with local mean sea level. It is a computed surface based 
on a 1929 adjustment of first-order level nets of both the United States and 
Canada using 26 primary tide stations as a measure of mean sea level. The 
results of the 1929 General Adjustment have not been, and probably never will 
be, published in a single package; geodetic bench mark elevations are available 
from Fedéral and State mapping agencies. Since the 1929 adjustment, it has 
been determined that the ocean surface is actually tilted with respect to the 
computed surface. The ocean has also been rising and there are local variations