Great Lakes datums

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.

Tidal datums.

(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

II-5-32

Water Levels and Long Waves