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Corey J. Hughes, P.S.
Riparian Bottomlands Apportionment & Boundary Consulting
VERTICAL CONTROL IN THE GREAT LAKES AREA
Approximately one hundred and sixty years ago, the Congress of the United States appropriated the money for the "Survey of the Northern and Northwestern Lakes." The U. S. Lake Survey was established to carry out the task of making this survey under the direction of the Corps of Topographical Engineers of the U. S. Army, setting up headquarters in Buffalo, New York, where it began work.
Vertical control to most engineers and surveyors consists of stable points called "benchmarks" whose elevations above or below some reference point are known. In hydrographic surveys, where depth measurements are made from a fluctuating water surface, much more is required than a network of benchmarks. First, a fixed plane of reference must be established for each lake to which all depths may be referred.
The first reference planes established on the Great Lakes were at what was considered to be the highest level ever observed. Although surveys were in progress and water-level gages were in operation providing data on the relative fluctuations of each of the Great Lakes by 1860, the elevations of the lakes above sea level were not known until 1875. In that year, instrumental levels were run from Oswego, New York, on Lake Ontario, up the Oswego River, eastward across the divide to Rome, and then following the water level route along the Mohawk River to Albany, where a connection was made to a U. S. Coast and Geodetic Survey bench mark on its line along the Hudson River from New York City. Working on the theory that the mean water surface of any body of water would be at an equal elevation throughout, the new sea-level elevations at Oswego were transferred to the mouth of the Niagara River and other harbors on the lake itself by a comparison of water level gage readings between Oswego and the other sites. This method of transferring elevations between points on any lake is known as a "water level transfer." The extension of these first sea-level elevations from Oswego to the remainder of the lakes was by water-level transfer across each lake, and instrumental levels along the rivers between the lakes.
By 1900 the Lake Survey had rerun most of its level lines between the lakes, and the U. S. Coast and Geodetic Survey (USC&GS) had extended its level network westward past the Mississippi River. As the USC&GS work progressed through the Great Lakes area, the Lake Survey level lines were incorporated into the national system, and in 1903, when an adjustment was made of all levels east of the Mississippi, these lines were included.
The Lake Survey adopted the 1903 elevations as computed by the Coast Survey, and by means of new water-level transfers and some additional leveling determined new elevations for all bench marks. Water-level gages were set to give lake elevations on the new datum that was to become known around the Great Lakes as "U. S. Lake Survey 1903 Datum."
At this point one might think that further levels and adjustments would not be required, however, this is not the case in the Great Lakes area. Because of a natural phenomenon known as “differential crustal movement.” Gages set in 1903 on Lake Michigan began to show different elevations, and as time progressed the differences increased.
By 1930 it was obvious that a new evaluation of benchmark elevations would be necessary, and in 1935 water-level gages were installed and operated in practically every United States Harbor on the Great Lakes. The level lines between the lakes had been releveled, but as a new connection to sea level had not been made, it was decided not to adjust the elevations of the lakes with respect to sea level, except for Lake Superior. So, at Oswego, NY, Cleveland, Ohio, and Harbor Beach, MI., the benchmark elevations as determined in the 1903 adjustment were adopted as 1935 Datum elevations, and these harbors were considered to be control points for their respective lakes. In the case of Lake Superior, the elevations at Point Iroquois were determined from Lake Huron values by the new level line along the St. Marys River, which was considered to be better than the old line used in the 1903 adjustment. The 1935 gage data were then used to determine new elevations of the benchmarks in each harbor, so that all marks on one lake were in harmony with the control point for that lake.
At the time the 1935 Datum was established, it was realized that in about 20-30 years it would be necessary to make another adjustment of elevations because of crustal movement. In 1935, in the Canadian waters of the Great Lakes, Canadian agencies were operating about a dozen gages, a few of which had been in operation since before 1900. As these gages were installed, they were set to read water surface elevations on 1903 Datum by making water-level transfers from the nearest United States gages. At the time the Corps of Engineers adopted the 1935 Datum, the Canadian Government decided to continue using the 1903 adjusted values. As a result, lake level data published by agencies of the two governments were not identical for the same lakes and rivers.
These differences were not great, and were considered insignificant until the advent of international power development on the St. Lawrence River, at which time it became very important that basic hydraulic and hydrologic data pertaining to the Great Lakes system be the same in both countries.
In 1953 the U. S. Lake Survey and its counterpart agencies in Canada started on a program of coordinating these data, and one of the many results of this coordination was the development of International Great Lakes Datum, later to be known as IGLD1955. In establishing this new international datum it was necessary to meet certain basic requirements:
It would have to be acceptable to both governments, so that the old uncoordinated references could be abandoned. This requirement meant that the new datum would have to extend along the entire St. Lawrence River, and thus have its reference zero somewhere in the Gulf of St. Lawrence.
2. It would have to adjust all elevations to compensate for changes caused by crustal movement to the date of establishment, and to correct any errors that might have existed in the old values due to inconsistencies in the work used to determine the original sea-level elevations many years ago. To do this it was necessary to make a completely new determination throughout the area, rerunning first-order level lines and operating many special water-level gages to obtain data for water level transfers.
3. It would have to provide elevations suitable for use in resolving the many complex hydraulic and hydrologic problems existing on the Great Lakes system. This stipulation was the prime reason for adopting dynamic values in the new Datum. Father Point, Quebec, was chosen as the site of the new reference zero because it' is at the outlet of the Great Lakes-St. Lawrence system, the mean water level there is approximately at mean sea level, the tide gage there has along period of reliable records, and it has been connected to the rest of the system by first-order levels. The average of the last eleven yearly mean water levels prior to 1957 from the Father Point tide gage were used as the mean water level and the zero for the new datum. This turned out to be 12.7 feet below benchmark L248-G at Father Point, thus establishing the new IGLD elevation of that mark at 12.447 feet.
IGLD was established along the St. Lawrence River to Kingston, Ontario, at the easterly end of Lake Ontario, by first-order levels from the reference benchmark at Father Point. Water level transfers were made from Kingston to all sites on Lake Ontario where sufficient water level data for the period 1952-1958 were available and at other places by using first-order level lines between transfer points.
The new datum was extended to the easterly end of Lake Erie by first-order levels run along the Welland Canal in Canada and along the Niagara River in the United States, and to other parts of Lake Erie by water level transfer method supplemented with some first-order levels.
First-order level lines along both sides of the Detroit-St. Clair River system as well as the St. Marys River, coupled with water level transfers and additional level lines at selected locations on the upper lakes, made it possible to establish the new datum elevations on bench marks throughout the remainder of the Great Lakes.
When a new datum is established, it brings the elevations of all benchmarks in the system into harmony, that is, the assigned elevations are measurements of their respective places in the vertical. Because crustal movement in the Great Lakes region causes these positions to shift, it becomes very important to show the year in which the assigned elevations were true. Extensive crustal movement studies have shown that rates of movement are small enough to be neglected over a span of three to five years, and in most instances it is not necessary to make overall adjustment of elevations more often than once every 20 years. Over 1,200 miles of first-order levels and many months of gage records had to be used to determine elevations on the new international datum, and an analysis of this information showed the year 1955 to be the best date to adopt.
In the past all elevations published by the Lake Survey have been so-called "instrumental," meaning that the observed differences in elevation as determined by field measurements were used to compute elevations. These differences in elevation between benchmarks, however, are functions not only of the end points, but also the routes along which the levels are run. As such instrumental differences can only be compared when the same route is followed in each case, they were not suitable for use in the new international datum where elevations could be determined by various routes.
Orthometric elevations, commonly in use in the United States and Canada, show that a plane surface, say 1000 feet above mean sea level, is exactly 1000 feet above at all points. All points on an undisturbed surface of a large body of water like Lake Michigan are not the same vertical distance above mean sea level because of variations in the force of gravity. As gravity is greater at the poles than at the equator, the north end of the lake is lower with respect to the sea level reference point than the south end. Therefore, Orthometric elevations which are suitable on land and small lakes are not satisfactory when used to express Great Lakes levels.
The only other alternative is a system of dynamic elevations which was adopted because it has the following advantages:
1. The differences between the dynamic elevations of two points represent the potential head which would exist in a water system joining them.
2. Every point on a mean level lake surface has the same dynamic elevation.
The dynamic elevation of any point as determined from any other point is the same regardless of the route followed in determining the difference in elevation.
The Low Water Datum Planes of reference, the planes to which construction and depths on navigation charts are referred, were fixed in 1935 as being at a given elevation below a specific benchmark. The benchmarks were not physically changed, however as the referenced benchmark was assigned a new elevation on IGLD, it followed that the elevations of the low water datum planes above the sea change also.
The National Geodetic Survey, our Nation's first civilian scientific agency, was established by President Thomas Jefferson in 1807 as the Survey of the Coast. Its mission soon included surveys of the interior as the nation grew westward. In 1878 the agency was reorganized and given a new name, the Coast and Geodetic Survey (C&GS), which it maintained until 1970.
In 1970 a reorganization created the National Oceanic and Atmospheric Administration (NOAA) and the National Ocean Service (NOS) was created as a line office of NOAA. To acknowledge the geodetic portion of NOAA mission, the part of NOS responsible for geodetic functions was named the National Geodetic Survey.
* Information compiled from published materials of the CORPS, NOAA, NGS, USGS, and other governmental agencies.