EM 1110-2-1100 (Part II)
30 Apr 02
Thus, a general adjustment was made in 1929 in which it was assumed that the geodetic and local sea levels
were equal to zero at 26 selected tide gauges in the United States and Canada (Rappleye 1932). The
differences previously computed were treated as errors and were distributed over the network of observation
points (Harris 1981). The tide gauge locations used in this computation are shown in Figure II-5-18. The
datum, originally called the "Sea Level Datum of 1929," was officially renamed the "National Geodetic
Vertical Datum (NGVD) of 1929" in 1963.
(3) Index maps of tidal benchmarks and lists of established references between the 1929 NGVD and
the local MSL are available for each coastal state through the NOS. An NOS tidal benchmark sheet is shown
in Figure II-5-19. These sheets often describe several benchmarks established in the vicinity of each tidal
observation point and describe the relationship between the various datums common to the area. NGVD and
tidal benchmarks are discussed more thoroughly in Harris (1981).
(4) The primary distinction between the NGVD and all other tidal datums is that the NGVD is defined
as a fixed surface whose elevation does not change with time. Therefore, the procedure adopted to account
for recognized changes in relative mean sea level is to compute updated MSL (or other) datum relationships.
Relative changes in sea level will be discussed in the following section.
Great Lakes datums.
(1) A separate water surface elevation datum was established for the Great Lakes basin and St.
Lawrence River. The datum was originally established by an international coordination committee composed
of representatives of the U.S. Army Corps of Engineers, the National Oceanic and Atmospheric
Administration, and the Department of the Environment, Canada. This first datum was called the
International Great Lakes Datum (IGLD) of 1955. The "zero" of the datum was established as the average
of all hourly water surface water level readings at Pointe-au-Pere, Quebec, located on the Gulf of
St. Lawrence for the period between 1941 and 1956.
(2) First-order leveling lines from Pointe-au-Pere were used to systematically define datums for Lake
Ontario, Lake Erie, Lakes Michigan and Huron, and Lake Superior. Lakes Michigan and Huron are assumed
to have the same elevation because of the deep and wide connection of both lakes at the Straits of Mackinac.
The IGLD of 1955 was replaced by the IGLD of 1985 to reflect certain corrections to the elevations assigned
to the various lakes. Elevations of the IGLD of 1955 and IGLD of 1985 are given in Table II-5-8. This
revision was implemented in January 1992. The zero reference of the IGLD 1985 has been specified to be
Rimouski, Quebec. Revised lake elevations are shown in Figure II-5-20.
e. Long-term variations in datums. Water level observation based datums such as MSL or IGLD vary
over time periods much longer than a tidal cycle. These variations can be seasonal or of much longer
duration. Long-term changes are often described as a relative change in sea level and necessitate the 25-year
interval updating of the 19-year tidal datum period. In the following section, some of the contributing factors
to sea level change are discussed. In the final section, factors contributing to long-term elevation changes
in the Great Lake are presented.
(1) The apparent rise in worldwide sea level has been of great concern to the United States, as well as
other countries, for several years. Much of this concern stems from the claims of some climatologists and
oceanographers that the rise will accelerate in the future due to warming of the atmosphere associated with
the "greenhouse effect," a global warming produced by increased levels of carbon dioxide and other gasses
in the atmosphere. Because of the potential consequences associated with sea level rise, a Committee on
Water Levels and Long Waves