NEED FOR NEW DATUM
DESCRIPTION OF HISTORICAL DATUMS
DEVELOPMENT OF INTERNATIONAL GREAT LAKES DATUM (1985)
METHOD OF COMPUTATION OF DYNAMIC ELEVATIONS
ESTABLISHMENT OF REFERENCE ZERO AT POINTE-AU-PÈRE/RIMOUSKI
ESTABLISHMENT OF INTERNATIONAL GREAT LAKES DATUM (1985)
RECOMMENDATIONS TO RESPONDING AGENCIES
1. Requirement for Internationally Coordinated Hydraulic and Hydrologic Data. The Great Lakes-St. Lawrence River system extends some 3,200 km from the headwaters of streams tributary to Lake Superior to the Gulf of St. Lawrence. The system drains a great interior basin of about 775,000 square km to the outlet of Lake Ontario, reaches almost halfway across the North American continent, and borders upon eight states of the United States and two provinces of Canada. This vast series of lakes and rivers is shared by the United States and Canada. The joint use of these waters poses numerous international problems for which the solution requires using coordinated basic data.
2. Prior to 1953 data pertaining to the hydraulic and hydrologic factors of the Great Lakes and the St. Lawrence River were collected and compiled independently by the responsible federal agencies in Canada and the United States, with only superficial and informal coordination of some of the data. As a consequence, the data in many instances were developed on different bases and datums and were divergent in other respects.
3. Establishment of International Study. International problems were greatly increased by the advent of extremely high lake levels in 1952 and by the imminent power and navigation development in the St. Lawrence River. Recognizing that continued independent development of the basic data was illogical under the circumstances and that early agreement upon the hydraulic and hydrologic factors was of paramount importance, the Corps of Engineers, United States Army, and the Departments of Transport, Mines and Technical Surveys, and Resources and Development, Canada, opened negotiations early in 1953 to establish a basis for the development and acceptance of identical data by both countries. The negotiations culminated in a meeting of representatives of the interested agencies at Ottawa on 7 May 1953.
4. At the meeting the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data was formed to study the problem and to establish a basis of procedure. Recommendations of this committee were to be advisory to the agencies of the United States and Canada which are charged with the responsibility for collecting and compiling the Great Lakes hydraulic and hydrologic data. The committee was constituted as follows:
Canada United States
T. M. Patterson, Water Resources Gail A. Hathaway, Office,
Division, Department of Chief of Engineers
Resources and Development, Department of the Army,
J. E. R. Ross, Geodetic Survey Edwin W. Nelson, Great Lakes of Canada, Department of Division, Corps of Engineers, Mines and Technical Surveys U.S. Army
D. M. Ripley, Special Projects W. T. Laidly, U.S. Lake Survey Branch, Department of Transport Corps of Engineers, U.S. Army
The present membership of the Coordinating Committee is as follows:
Canada United States
P. P. Yee, J. W. Kangas, Water Issues Division, Corps of Engineers, Environment Canada, Department of Army, Chairman Chairman
P. D. Richards, P. C. Morris, National Oceanic Canadian Hydrographic Service, and Atmospheric Administration, Department of Fisheries and Oceans Department of Commerce
C. Southam, R. E. Wilshaw, Inland Waters Directorate, Corps of Engineers, Environment Canada, Department of Army, Secretary Secretary
Messrs. C. M. Cross, A. T. Prince, R. H. Smith, D. F. Witherspoon, B. J. Tait, and J. Robinson have also served as Canadian members of the Committee while Messrs. L. D. Kirshner, F. F. Snyder, H. F. Lawhead, F. A. Blust, B. G. DeCooke and C. I. Thurlow have served as United States members of the Committee.
5. Four working groups, designated the River Flow Subcommittee, the Vertical Control Subcommittee, the Lake Levels Subcommittee and the Physical Data Subcommittee, were formed to assist the Coordinating Committee in its work. These subcommittees were directed to conduct the required technical studies through the collaboration of the appropriate agencies of Canada and the United States. In November 1988 the "Hydrometeorology and Modelling Subcommittee" was formed to explore the coordination of hydromet and hydraulic/hydrologic response models. In September 1969, the "Vertical Control and the Lake Levels Subcommittees" were combined into one body known as the Vertical Control-Water Levels Subcommittee. The Subcommittee was normally composed of three members from Canada and three from the United States. The following persons served as members of Vertical Control Water Level Subcommittee at various times during the progress of the work reported herein:
Canada United States
G. C. Dohler B. G. DeCooke
W. D. Forrester C. F. Feldscher
L. P. Robertson G. E. Ropes
B. E. Russell C. F. Ellingwood
E. A. MacDonald R. M. Berry
J. M. Murakami* D. R. Rondy
M. H. Quast E. P. Kulp
B. J. Tait H. A. Lippincott
F. W. Young R. E. Wilshaw*
D. A. St. Jacques C. T. Whalen
W. M. Archer* D. B. Zilkoski*
P. A. Bolduc P. C. Morris*
R. G. Sandilands* J. A. Oyler*
6. Authority. The Vertical Control-Water Levels Subcommittee of the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data was instructed to provide an International Great Lakes Datum which would be acceptable to all agencies concerned. The following subjects were determined to be within the purview of the subcommittee:
The establishment of the International Great Lakes Datum (1955), or IGLD (1955), was one of the first major accomplishments of the Coordinating Committee. Accordingly the reference zero point was established at Pointe-au-Père, Quebec and first-order leveling begun in 1953 was completed in 1961. The established bench mark elevations were published in September 1961 (A second edition was also published with some revisions in December 1979). The result of this effort was International Great Lakes Datum (1955). This datum was implemented January 1, 1962, and used for the following 30 years, until the effects of crustal movement, the development of a common datum between Canada, the United States, and Mexico, new surveying methods, and the deterioration of the zero reference point gauge location made it desirable to revise the datum. The Vertical Control-Water Levels Subcommittee undertook the revision of IGLD (1955) beginning in 1976 and this effort has resulted in International Great Lakes Datum (1985).
7. Purpose and Scope. The purpose of this report is to document the continued work of the Vertical Control-Water Levels Subcommittee in the development and establishment of International Great Lakes Datum (1985) (IGLD (1985)) and to record the new datum elevations of bench marks at water level gauging stations located throughout the Great Lakes basin.
8. Acknowledgements. The Coordinating Committee acknowledges and expresses its appreciation of the high caliber of service which its Vertical Control-Water Levels Subcommittee rendered in the development of the results presented herein. It recognizes and appreciates also, that other personnel and facilities of the Canadian Hydrographic Service, Department of Fisheries and Oceans; the Geodetic Survey of Canada, Department of Natural Resources Canada; and the National Ocean Service (prior to 1970, the operation and records of Great Lakes water level gauging stations were the responsibility of the U. S. Lake Survey, Corps of Engineers), National Oceanic and Atmospheric Administration, Department of Commerce were employed throughout the study. The individual efforts of Rick Sandilands, Fred Young, David Zilkoski, Emery Balazs, and Harry Lippincott and other staff in their respective agencies are gratefully acknowledged by the committee in collecting and analyzing of historical data, developing the concept for the new datum, applying the mathematical adjustment, and compiling the information for preparing this report.
NEED FOR NEW DATUM
9. As in the establishment of IGLD (1955), there continues to be two principal reasons why it is highly desirable to establish an entirely new datum. One reason is that it will correct for changes in elevation caused by crustal movement prior to the date of the new datum. Because the crust of the Earth in the Great Lakes region is moving with respect to sea level, and because the rate of movement is not uniform throughout the area and because of local marker instability, the elevations of bench marks are changing; with respect to each other and with respect to sea level. The other reason is to provide a common datum which could be used by all agencies interested in vertical control on the Great Lakes-St. Lawrence River system.
10. There have been several vertical datums which can be identified in the Great Lakes that have been used by the two governments, the most notable of which are the Canadian Geodetic Datum (CGD) of 1928, the Georgian Bay Ship Canal Datum, the National Geodetic Vertical Datum of 1929 (NGVD (1929)), the U.S. Lake Survey 1903 Datum, the U.S. Lake Survey 1935 Datum, and the IGLD (1955). While IGLD (1955) was a much improved datum over the earlier datums, it was recognized that a new datum would be necessary in approximately 25-30 years due to movement of the earth's crust in the Great Lakes region.
DESCRIPTION OF HISTORICAL DATUMS
11. As stated, prior to 1900 there were numerous datums used on the Great Lakes as references for water levels, charting, and river and harbor improvements. In 1903 the U.S. Coast and Geodetic Survey (now National Ocean Service) made an adjustment without the use of the orthometric correction based on level lines and tide gauge records in the United States east of the Mississippi. This adjustment was available at a number of places on the Great Lakes and provided the basis for the U.S. Lake Survey 1903 Datum. This datum was extended to all major harbors around the lakes, along the connecting rivers, and down the St. Lawrence River to Cornwall, Ontario, by the U.S. Lake Survey and the Canadian Hydrographic Service through water level transfers and leveling height differences. The latter agency made use of leveling height differences supplied by the Geodetic Survey of Canada.
12. The Georgian Bay Ship Canal Datum was in effect an extension of the U.S. Lake Survey 1903 Datum, and was established by leveling differences and water level transfers. The leveling for this datum was done by the Canadian Department of Public Works in the years 1904 to 1908, and consisted of level lines from Rouses Point, New York, through Montréal, Quebec, and North Bay, Ontario, to French River, Ontario, on Georgian Bay, and from Toronto, Ontario, to North Bay, Ontario, with a connection to Collingwood, Ontario. Water level transfers were made from U.S. Lake Survey gauges to French River, Collingwood and Toronto. Elevations on Georgian Bay Ship Canal Datum were determined after adjustment around the several loops in the system. Instrumental elevations were released before the adjustment was made, so that a distinction was necessary between the adjusted and unadjusted elevations in the system. The Georgian Bay Ship Canal Datum was not in general use since the Geodetic Survey of Canada took over the Public Works leveling and adjusted it to the Canadian Geodetic Datum of 1928. However, it did survive locally in some areas, notably along the St. Lawrence River between Summerstown, Ontario, and Montréal, Quebec, where it was employed as a reference for water level gauges of the Canadian Hydrographic Service, and along the Trent Canal System, where it was employed as a reference datum by the Canal Services Branch of the Department of Transport. It has been replaced by IGLD and CGD.
13. By 1935, differential movement in the earth's crust was causing gauges at harbors on the same lake to show appreciable differences in water surface elevations, and the U.S. Lake Survey reopened the study of datums. A control point was chosen on each lake; Oswego, New York, on Lake Ontario; Cleveland, Ohio, on Lake Erie; Harbor Beach, Michigan, on Lake Michigan/Huron and Point Iroquois, Michigan, on Lake Superior. The bench mark elevations at the control points were adopted as given on U.S. Lake Survey 1903 Datum except for Point Iroquois, where elevations were derived from Harbor Beach by water level transfer and leveling of 1934, between Lake Huron and Lake Superior. Bench mark elevations at other sites on the United States side of the Great Lakes were computed from these control points by water level transfers supplemented by local leveling. These resulting elevations were referred to as U.S. Lake Survey 1935 Datum. In other words, the elevations derived were the elevations of the bench marks as of 1935 with respect to their particular control point. The Canadian Hydrographic Service continued to use U.S. Lake Survey 1903 Datum.
14. The IGLD (1955) was an entirely new datum developed and established as the culmination of the international study under the charge and authority of the Coordinating Committee formed in 1953. In establishing this datum, the committee agreed that the Great Lakes-St. Lawrence River system would be considered as a unit with datum and reference surfaces based on mean water level at the outlet of the system in the Gulf of St. Lawrence. Pointe-au-Père, Quebec was chosen as the site for the new reference zero because: a) it was the outlet of the system, b) the tide gauge at that location had a long period of reliable record, c) the mean water level at that point approximates mean sea level, and d) it had been connected to the remainder of the system by first-order levels. When a new datum is established, it brings the elevations of all bench marks in the system into harmony -- that is, the assigned elevations are measurements of their respective places in the vertical. An analysis of the many miles of first-order levels and many months of gauge records used to determine the new datum indicated the year 1955 to be the best year for adoption. IGLD (1955) is a dynamic height system. For more information on IGLD (1955), readers are referred to the report by the Coordinating Committee dated December 1979, and entitled "Establishment of IGLD (1955), Second Edition". Figure 1 - IGLD (1955) LEVEL LINES
DEVELOPMENT OF INTERNATIONAL GREAT LAKES DATUM (1985)
15. Reference Zero. The committee again agreed that the Great Lakes-St. Lawrence River system should be considered as a unit with datum and reference surfaces based on mean water level at the outlet of the system in the St. Lawrence River. The gauge at Pointe-au-Père, Quebec was used as the reference for IGLD (1955). Due to the deterioration of the wharf at Pointe-au-Père, the gauging station at the site was discontinued in 1984 and moved approximately five kilometers upstream to Rimouski, Quebec. The reference zero for IGLD (1985) was transferred to the new site at Rimouski, Quebec.
16. Reference Year. Because crustal movement causes bench marks to shift in position, it becomes very important to show the year in which the assigned elevations are true. Extensive crustal movement studies have shown that rates of movement are small enough to be negligible over a span of three to five years, and in most instances it is not necessary to make an over-all adjustment of elevations more often than once every 25-30 years. An analysis of the gauging data used to determine the new international datum has shown the year 1985 to be the best date to adopt for the period 1982-1988.
17. Network Concept. The development of IGLD (1985) coincided with the development of a new North American Vertical Datum of 1988 (NAVD (1988)). This datum was to include vertical control networks of the U.S., Canada, and Mexico. Early discussions of the Coordinating Committee considered the inclusion of Great Lakes datum data in the network with the exceptions that for the Great Lakes the reference zero would remain at Pointe-au-Père, Quebec, or near the mouth of the St. Lawrence River, and the elevations would be published in the Dynamic height system, very similar to IGLD (1955). In determining the appropriate network for IGLD (1985) three separate adjustments were performed, the U.S. Network, the Canadian Network, and the U.S.- Canadian Network (combined). The U.S.-Canadian Network concept containing 78 loops of leveling and connecting to over 80 water level gauging stations in the basin was adopted for IGLD (1985).
18. Dynamic values. The surveying and mapping community uses several different heights systems. Two systems, orthometric and dynamic heights, are relevant to the establishment of IGLD (1985) and NAVD (1988). The geopotential numbers for individual bench marks are the same in both height systems. The requirement in the Great Lakes basin to provide an accurate measurement of potential hydraulic head is the primary reason for adopting dynamic heights. It should be noted that dynamic heights are basically geopotential numbers scaled by a constant of 980.6199 gals, normal gravity at sea level at 45 degrees latitude. Therefore, dynamic heights are also an estimate of the hydraulic head. Consequently points that have the same geopotential number have the same dynamic height.
19. Following are some of the advantages of dynamic heights:
20. The foregoing reasons define the decision to use dynamic heights in establishing IGLD (1985).
METHOD OF COMPUTATION OF DYNAMIC HEIGHTS
21. General. For determining a vertical control network in the Great Lakes region, the latest leveling data and water level transfer observations in the U.S. and Canada were made available. The primary network consisted of 78 loops based on the latest leveling data. Also, the network connected to over 80 U.S. and Canadian water level gauging stations. Additionally, 25 connections were made between the U.S. and Canadian national vertical control networks.
22. Most of the leveling data involved were observed between the years 1965 and 1986. For the water level transfers, data for the four months June to September for the seven year period 1982-1988 were used to calculate a mean water level (MWL) at each permanent gauging station.
23. Corrections applied to the U.S. leveling observations were rod scale, rod temperature, level collimation, astronomic refraction and magnetic effects. These corrections to observed leveling data are to minimize the effects of known systematic errors. Approximately one-half of the vertical control network was generated from Canadian leveling data and this data in turn was corrected for rod scale, astronomic, refraction, and magnetic effects.
24. As stated earlier in paragraph 18, geopotential numbers relate dynamic heights and orthometric heights. A geopotential number (C) of a bench mark is the difference in potential measured from the reference surface to the equipotential surface passing through the mark. It is the amount of work required to raise a unit mass of 1 kg against gravity through the orthometric height to the mark. A geopotential difference is a difference in potential which can indicate hydraulic head.
25. An orthometric height of a mark is the distance from the reference surface to the mark, measured along the line perpendicular to every equipotential surface in between. Equipotential surfaces can be used to represent the gravity field. One of these equipotential surfaces is specified as the reference system from which orthometric heights are measured. The surfaces defined by the gravity field are not parallel because of the rotation and shape of the Earth as well as gravity anomalies in the gravity field. Two points, therefore, could have the same potential, but may have two different orthometric heights. The value of the orthometric height at a point depends on all the equipotential surfaces beneath that point.
26. The orthometric height (H) and the geopotential number (C) are related through the following equation:
where G is the mean gravity value between the point and the reference surface, estimated for a particular system. Height systems are called different names depending on the G selected. When G is computed using the Helmert height reduction formula (Helmert 1890), the heights are called Helmert orthometric heights: when G is computed using the international formula for normal gravity, the heights are called normal orthometric heights: when G is equal to normal gravity at 45 degrees latitude, the heights are called dynamic heights. The elevations published in appendix A and published with respect to the IGLD (1985) are dynamic heights. Dynamic heights should be visualized as geopotential numbers. A dynamic height is a geopotential number simply scaled by a factor close to unity to transform the units of kgal*m to m for user convenience. Therefore, the data used to define the IGLD (1985) are:
ESTABLISHMENT OF REFERENCE ZERO AT POINTE-AU-PÈRE/RIMOUSKI
27. The original reference zero for IGLD (1955) was chosen as the mean water level at the Canadian Hydrographic Service gauging station at Pointe-au-Père, Quebec. Mean water level was calculated as the arithmetic mean of eleven yearly means between 1941 and 1956. The primary bench mark, 1248-G, had an elevation of 3.794 metres (12.447 feet) on IGLD (1955). Because of the deterioration of the Pointe-au-Père wharf and gauge, a new gauge was built in 1984 in the harbour at Rimouski, about five kilometers upstream. Both gauges were to be kept in operation for a period of time to compare the water level data and to facilitate the transfer of the reference zero to the new gauge at Rimouski. However, the data recorded at Pointe-au-Père were affected by the deterioration of the structure and a comparison was not possible. Therefore, the reference zero had to be transferred by first- order leveling to bench mark 1250-G at Rimouski.
28. In 1983, the Geodetic Survey of Canada (GSC) releveled the line between Rimouski and Pointe-au-Père and determined an elevation of 6.263 metres IGLD (1955) for bench mark 1250-G. This elevation is identical to the original IGLD (1955) elevation (20.549 feet, 6.263 metres) assigned to bench mark 1250-G. Since there is no appreciable difference in the character of the tide between Pointe-au-Père and Rimouski and there is good agreement in the first-order level line results, bench mark 1250-G can be used to define the reference zero for IGLD (1955). To determine the reference zero for IGLD (1985), mean water level was computed as the arithmetic mean of the daily mean water levels as recorded at Pointe-au-Père from 1970 to 1983 and at Rimouski from 1984 to 1988. The data set for this period was 87.3 percent complete. The arithmetic mean of the daily means for the period was -0.010 metres IGLD (1955) or 0.010 metres below the IGLD (1955) reference zero. Since the reference zero at Rimouski for IGLD 1955 is 6.263 metres below bench mark 1250-G, the reference zero for IGLD (1985) is defined as 6.273 metres below GSC bench mark 1250-G at Rimouski, Quebec.
29. The reference zero was calculated from the water level data recorded at the Pointe-au-Père and Rimouski gauging stations between 1970 and 1988. This period was chosen: (1) to use a nineteen year period of water level data (one tidal epoch), (2) to use as much data as possible form the new gauging station at Rimouski, (3) to include the time period of new land leveling up the St. Lawrence River (1978-82), and (4) to include the time period (1982-88) used for the water level transfers between gauging stations on the Great Lakes. In addition, selecting a period near the date of the datum adjustment, minimizes the effect of crustal movement or long-term fluctuations in mean water level on the establishment of the reference zero.
ESTABLISHMENT OF INTERNATIONAL GREAT LAKES DATUM (1985)
30. A special study was undertaken to compile a U.S. and Canadian primary vertical control network using the latest leveling data available in the Great Lakes region. Analyses of these networks proved helpful in determining the effects of datum constraints, the magnitude of height changes from IGLD (1955), and confirmed the validity of using the "Network Concept" in establishing IGLD (1985).
31. The network is comprised of leveling loops starting at the mouth of the St. Lawrence River and included leveling lines which surrounded the Great Lakes (Figure 2). For ease in data computation, all U.S. and Canadian leveling data was merged and compiled in a data base at the Vertical Network Branch, National Geodetic Survey, a Division of the Coast and Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration (Zilkoski et al. 1989).
32. The network formed leveling loops for which misclosures were computed and checked against allowable closure tolerances. Heights of bench marks were computed using a least squares adjustment. The adjustments performed were minimum-constraint least squares adjustments holding fixed the geopotential number of a bench mark, referenced to a zero value of the local mean water level at Pointe-au-Père/Rimouski. Data outliers were detected and removed during this analysis. The resulting primary network consisted of 78 loops containing 1,119 bench marks. Each loop is composed of links based on the latest leveling connecting the junctions of loops. The network connected to over 80 water level gauging stations along the Great Lakes. In addition, 25 connections were made between the U.S. and Canadian vertical control networks.
33. Additionally, water-level transfer data, see paragraph 22, (Coordinating Committee 1979) from U.S. and Canadian gauging stations were included in each adjustment. These differences were used to generate observed height differences from the primary bench mark at the gauge site to the mean water level (MWL) surface. The MWL surface at each station was treated as if it were a bench mark. In this way, the data were used to estimate geopotential numbers at all water level stations.
34. Three separate adjustments were performed. Each adjustment held the elevation of a bench mark referenced to local mean water level fixed at Pointe-au-Père/Rimouski. First, all U.S. data (known as U.S. Network) were combined into a network and adjusted. Second, the Canadian data (known as Canadian Network) was formed and adjusted. Lastly, the U.S. and Canadian data (U.S. - Canadian Network) were combined and adjusted. A comparison of the U.S. Network adjusted heights and Canadian Network adjusted heights showed good overall agreement. The difference between the adjusted heights using independent leveling data from Pointe-au-Père/Rimouski to the west end of Lake Superior is approximately 7 cm. This supports the importance of using a leveling network concept instead of single-route leveling to estimate the heights of bench marks for IGLD (1985).
35. International Great Lakes Datum (1985) and North American Vertical Datum (1988). The development of the NAVD (1988), as mentioned in paragraph 17, was to include vertical control networks of the U.S., Canada and Mexico, as well as International Great Lakes Datum data. For NAVD (1988), a minimum-constraint adjustment was performed also holding fixed the primary bench mark at Pointe-au-Père/Rimouski. Therefore, IGLD (1985) and NAVD (1988) are one and the same. The only difference between IGLD (1985) and NAVD (1988) is that the IGLD (1985) bench mark elevations are published as dynamic heights and the NAVD (1988) elevations are published as Helmert orthometric heights (Zilkoski et al. 1989). Geopotential numbers for individual bench marks are the same in both height systems.
36. HYDRAULIC CORRECTOR. The water surfaces of the Great Lakes are considered to be geopotentially equal. Therefore, on any particular lake, at the time a new vertical datum is established, all Mean Water Level (MWL) values for gauging stations around the lake should coincide. The MWL is the average water surface for the summer months (June - September) for the years 1982-1988 referenced to the gauging station Primary Bench Mark dynamic height (paragraph 18). As indicated in paragraph 33, the MWL at each gauging station was treated as a bench mark in the network adjustment. Following the adjustment (paragraph 32), the MWL values at each gauging station on a lake were slightly different. The differences are due to cumulative differences in the leveling adjustments. The Committee decided to apply a Hydraulic Corrector so each gauge on a lake has the same MWL as the Master Station for the lake. This is accomplished by holding the Master Station as the controlling value and comparing all other gauging stations to it. The Master Stations for each lake are:
37. The Hydraulic Corrector (HC) was obtained by subtracting the MWL at the Master Station (MWLMaster) from the MWL at the subordinate gauging station in question (MWLSub). The answer retains its arithmetic sign. The Hydraulic Corrector may be positive or negative and is subtracted algebraically.
38. The water surface elevation (WSIGLD 1985) is obtained by subtracting the Hydraulic Corrector (HC) from the Dynamic Water Surface Elevation (WSDynamic).
39. The water surfaces of all connecting channels and other rivers on the Great Lakes are considered to be sloping surfaces. Therefore, their Hydraulic Corrector is zero.
40. Example of computations for Kingston, Ontario daily mean for July 1, 1994.
41. Example of computations for Point Iroquois, Michigan daily mean for July 4, 1994.
42. The Hydraulic Corrector at each gauge site on the lake has been incorporated into the data retrieval and storage process. As such, water level information stored at the site mechanically or electronically, at the National Oceanic and Atmospheric Administration (NOAA) or the Department of Fisheries and Oceans (DFO) computers, or in printed form, are in IGLD (1985) and do not require any further adjustment.
43. Hydraulic Correctors for primary gauges are shown in Appendix A.
44. Dynamic Elevations on International Great Lakes Datum (1985). The dynamic elevation of the primary bench mark at each water level gauging station is shown in Appendix A.
45. Differences IGLD (1985) to IGLD (1955). To obtain the Water Level Elevation on IGLD (1955), subtract the DIFF. 85-55 value in Appendix A from the Water Level Elevation on IGLD (1985).
46. Descriptions of Bench Marks. Appendix B lists the addresses where bench mark descriptions can be obtained.
47. Public Information. Appendix B lists address where general information can be obtained.
RECOMMENDATIONS TO RESPONDING AGENCIES
48. The Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data considers and recommends that the establishment of IGLD (1985) along the Rivers and Lakes of the Great Lakes system satisfies the requirements for a new datum in this area. The responsible data collecting and publishing agencies in the United States and Canada implemented its use in January 1992.
49. The advantages of IGLD (1985), leading to the Coordinating Committee recommendations, may be summarized as follows:
50. To ensure that the advantages of having this new datum in general use may be fully exploited, the committee provided its new datum values to all agencies interested in water levels on the Great Lakes-St. Lawrence River system adopt IGLD (1985) as set forth in this report.
Zilkoski, D.B., Balazs, E.I., and Bengston, J.M., 1989, Datum Definition Study for the North American Vertical Datum of 1988 (NGS unpublished technical report), National Ocean Service, Rockville, MD., 20852.
Zilkoski, D.B., Balazs, E.I., Bengston, J.M., 1989, Analyses Performed by the National Geodetic Survey in Support of the Readjustment of the International Great Lakes Datum (NGS unpublished technical report), National Ocean Service, Rockville, MD., 20852.
Coordinating Committee, 1979, Establishment of International Great Lakes Datum (1955), Second Edition, Coordinating Committee of Great Lakes Basic Hydraulic and Hydrologic Data, Chicago, IL., and Ottawa, ON.
Apparent Vertical Movement over the Great Lakes Report - The Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data, July 1977, printed by Detroit District Corps of Engineers
U.S. National Geodetic Glossary, National Geodetic Survey, September 1986, Rockville, MD
Information herein is provided by the Canadian Hydrographic Service, Department of Fisheries and Oceans. Its use and reference is unlimited, upon condition that the source is correctly attributed. Thank you.
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