HISTORY AND THEORY
OF DATUM PLANES OF THE GREAT LAKES
by F ra n k A. B l u s t
Chief Engineer, La k e Survey Center,
National Ocean Survey, Detroit, M ichigan
INTRODUCTION
L ik e other natural watercourses t1», the Great Lakes (figure 1) have
levels which fluctuate in response to natural phenomena. T h ere are
seasonal variations: the lakes are generally higher in summer than in
w inter. Superimposed on the seasonal variations are m ore random longperiod variations. In addition, the level at any given location on a given
lake w ill vary over irregular periods fro m several minutes to several hours
because o f forces disturbing the lake surface, principally w ind. T h erefore,
in describing depths in the lakes it is convenient to have fixed planes o f
reference or datum planes. This plane on each lake is com m on ly referred
to as the L o w W a ter Datum.
Th e datum planes o f the Great Lakes have tw o m ain purposes. First,
they provide a reference fo r depiction o f depths on navigation charts.
Second, they provide a reference fo r determ ining depths to w hich im proved
navigation channels are dredged. The latter represents an econom ic m atter
since the positions o f the planes are related to (but do not necessarily
determ ine) the number o f cubic yards o f m aterial that must be dredged
to g ive the desired depths. Other purposes can be ascribed to datum
planes as references descriptive o f the behavior o f lake levels. Term s such
as “ Mean High W a te r ” and “ Mean L o w W a te r ” can be defined and can
be significant, fo r example, to shore property owners. T h e term “ L o w
W a te r Datum ” is generally used w ith reference to navigation, and the
discussions herein are confined to navigation considerations.
Th is paper describes the past and present datum planes o f the five
Great Lakes as used in the United States and describes the bases fo r
selection o f planes. Since Lakes M ichigan and Huron are considered
hydraulically as a single lake, they have a common datum. L a k e St. Clair
is generally considered a part o f the L a k e Erie basin and usually not
(1)
The natural fluctuations o f Lakes Su perior and O ntario are m o d ifie d b y a r tific ia l
control o f th eir ou tflow s.
identified separately in a discussion o f the Great Lakes. H owever, it is
an im portant segment o f the Great Lakes system and a datum plane has
been established for it. No attempt is made to describe the use o f datum
planes in Canada other than to note the coordination o f the present planes
by the two countries.
HISTORY OF DATUM PLANES <2>
In the developm ent of the Great Lakes as w aterw ays recognition o f the
need for datum planes was not immediate, as the earliest lake hydrographers often recorded water depths only in terms o f water levels at the
times o f their surveys. An evolutionary process spanning a period o f over
50 years was required to develop the m odern concept o f datum planes.
Early planes
Unusually high water occurred on all of the lakes in 1838. Planes
of reference used by the Lake Survey fo r lake levels prior to 1876 were
intended to be the highest levels observed in 1838. In the Annual Report
of the Chief o f Engineers, U.S. Arm y, for 1876, these planes w ere officially
defined as distances below particular benchmarks. H ow closely the 1876
planes corresponded to the high water o f 1838 is problem atical since level
records o f 1838 are incomplete.
The 1876 planes o f references were considered too high for charting
or harbor im provem ent. The first datum plane used on published Lake
Survey charts was for Lake Erie during the period 1870-1875 and was
the mean level o f that lake over the period 1860-1870. Beginning in 1876
and until 1901, the datum planes used on nearly all Lake Survey charts
were the mean lake levels 1860-1875. These planes w ere from tw o and
one-third to slightly over three feet low er than the 1876 planes.
Connection to Sea Level
Elevations fo r the Great Lakes based on sea level were first obtained
by (a) spirit levels perform ed by the U.S. Coast and Geodetic Survey (now
National Ocean Survey) from N ew Y ork City to Greenbush, N ew Y ork , on
the Hudson R iver (now Rensselaer) and (b) spirit levels and water
transfers (3) perform ed by the Lake Survey from Greenbush to Lake Ontario
(2) Most o f the discussion of the form er planes to the yea r 1932 is based on an
unpublished Lake Survey report “ Datum Planes on the Great Lak es” by Sherman M o o r e ,
1939, F ile 3-2869. A history o f the datum planes is also given b y R o p e s , 1965.
(3) The setting o f water level gages at opposite ends of a lake to give the same
average readings, based on the assumption that averaged over the selected period the
lake has a le v el surface.
and through the chain o f lakes (U.S. Deep W a terw a ys Commission, 1896).
Th ese elevations are said to be based on the levels of 1877 although the
fie ld w ork was accomplished during a number o f years.
Prob ab ly because o f generally low w ater on the lakes in 1895-1896,
the mean lake levels o f 1860-1875 w ere found to be too high for charting.
A s a result, new planes o f reference, called Standard L o w W a ter, w ere
adopted for La k e Survey charts in 1901. But revision o f planes for channel
and harbor im provem ents was deferred.
In 1903, the U.S. Coast and Geodetic Survey made an adjustment
(w ith ou t an orthom etric correction) o f the levels o f 1877 w hich changed
the values o f sea level elevations established on the Great Lakes and became
know n as 1903 Datum <41. Corresponding changes w ere made in the eleva­
tions o f the 1901 datum planes except that the new elevations (as directed
b y the Chief of Engineers, U.S. A rm y, in 1909) w ere rounded to the nearest
h a lf foot. This resulted in a m axim um change in the physical position
o f any plane o f 0.09 foot. It is w orth w h ile to note here that the 1901 planes,
like the earlier planes, w ere defined as specified vertical distances from
particular benchmarks (Annual R eport o f the Chief o f Engineers, U.S.
A rm y, 1909, p. 2484). The physical position o f a plane was, and is,
considered changed only when the distance from its controlling benchmark
was changed.
T
C om parison
able
1
o f datum
pla n e s
Elevations in feet above mean tide at N ew Y ork , 1903 Datum
Lake
Planes o f 1955
Planes of 1909
Mean
Planes o f
Planes o f
Lake Levels
Nominal Adopted
Nominal Adopted
1933
1901
1860-1875
values
values
values
values
Superior
Michigan-
603.22
600.56
600.56
600.50
601.6
601.63
601.60
Huron
Erie
Ontario
581.64
572.77
246.55
578.51
569.91
242.96
578.51
569.91
242.96
578.50
570.00
243.00
578.5
570.5
244.0
578.54
570.55
244.03
578.50
570.50
244.00
Table 1 shows a comparison o f elevations above mean tide at N ew Y ork
C ity o f the mean lake levels 1860-1875 and Standard L o w W a te r for 1901
and 1909. Since all values shown on the table are referred to the same
datum (1903), the differences between values represent differences between
the physical positions o f the planes, as described above.
(4)
1903 Datura and the la ter 11)35 Datum and International Great Lakes Datum
(1955) should not be confused w ith L ow W a te r D atum ; these datums, which are id en tified
b y the year o f th eir establishm ent, represent coordinated revisions o f benchmark eleva­
tions throughout the region.
1933 Planes
Th e 1909 planes w ere used for ch arting rather than channel and
harbor im provem ents, and it was found desirable to adopt a single datum
plane fo r each lake to be used both for charting and for im provem ent work.
In 1933 the Chief o f Engineers approved such single planes fo r each lake,
also as shown in table 1. Each o f these planes became known as L o w
W a ter Datum on its lake and represented w hat m ight be described as the
lake’s average low w ater level.
Th e physical positions o f the 1933 planes w ere fixed, have rem ained
essentially fixed and are expected to rem ain fixed by defin in g them at a
single point on each lake as a vertical distance from a particular benchmark.
W ater level gages located at the defining points became know n as the
“ m aster” gages since w ater levels transferred from each to any other
location on the same lake served to establish the position o f L o w W a te r
Datum at such other location.
Differential Gage Site Movement
A t the tim e o f the establishment o f the 1933 planes, it w as recognized
that progressive differences between w ater levels at different locations on
the same lake w ere occurring, i.e., over a period o f years w ater levels at
these locations were becom ing higher or low er than w ater levels at the
master gage site. Since w ater levels at any site depend on elevations
assigned to benchmarks at that site, these progressive d ifferen ce probably
were due to rise or subsidence o f benchm arks at the various sites w ith
respect to benchmarks at the master site. The differences amounted
generally to a fraction o f a foot per hundred years. T h e result was
that w ater in most United States harbors was becom ing deeper ( M o o u e ,
1949). Th e phenomenon became known as “ crustal m ovem ent” based on
the conclusion that it was caused by differential m ovem ent o f the earth’s
crust, although the interpretation o f the data rem ains open to some doubt.
A better term at present is “ differential gage site m ovem en t” . W h a tever
the cause, the changes were sufficient to require benchmark elevations in
the harbors on a given lake to be adjusted to provide, at a selected time,
water surface elevations that w ere the same in these harbors as at the
master gage site. Such an adjustment was made for each lake based on
the year 1935. This adjustment is kn ow n as the 1935 Datum.
Present Planes
In the ea rly 1950’ s, a join t decision w as made by the L ake Survey, the
Canadian H ydrographic Service and the Geodetic Survey o f Canada to
adjust again benchmark elevations fo r differential gage site movement
and, further, to redeterm ine sea-level elevations through the Great Lakes
region. The Canadian Departm ents o f Transport and Resources and
Developm ent also participated in this decision. As a result, a new datum
called International Great Lakes Datum (1955) [IG L D (1955)], was
established w hich was referred to mean w ater level at Father Point, Quebec,
on the Gulf o f St. Law rence (Th e Coordinating Committee on Great Lakes
Basic Hydraulic and H ydrologie Data <r>), 1961). Elevations based on the
new datum are dynam ic elevations (i.e., they are measures o f the w ork
done in liftin g a pound mass) and represent the elevations which existed
in the year 1955. D ynam ic elevations avoid the difficulties of orthom etric
elevations, w hich do not portray equipotential surfaces, and instrumental
elevations, w hich depend on the route o f levelling followed. The dynam ic
elevations wTere computed from data obtained by instrumental levels along
the connecting rivers and w ater transfers across the lakes. Elevations of
benchm arks were changed, and physical positions o f the 1933 United States
planes w ere changed only to the slight extent necessary to provide rounded
values (as was done in establishing the 1909 planes).
T
Defin ition
of
able
2
1955 l o w
water datu m
Controlling Benchmark and
Elevation Feet IGLD
(1955)
Distance o f Low
Water Datum
Below
Benchmark,
Feet
Lake
Master Gage Location
Superior
MichiganHuron
Erie
Point Iroquois, Mich.
BM Lighthouse
620.62
20.62
Harbor Beach, Mich.
Cleveland, Ohio
Oswego, N.Y.
BM Huron
BM Doorstep
BM A
581.90
580.49
250.67
5.10
11.89
7.87
Ontario
Table 1 shows the elevations o f the 1955 planes (L o w W a ter Datum)
based on the levels o f 1903 in order to provide a comparison o f the positions
o f the planes w ith the positions o f the earlier planes. Table 2 shows the
m aster gage locations, identification and elevations (levels o f 1955) of
con trollin g benchmarks, and locations o f the 1955 planes w ith respect to
the benchmarks. L o w W a te r Datum elevations fo r the Great Lakes now
in use b y the United States and Canada (although the two countries use
different master gage sites) are, in feet IG LD (1955), Lake Superior 600.0,
L a k e M ichigan-Huron 576.8, Lake Erie 568.6, and Lake Ontario 242.8.
Th e L o w W a ter Datum elevation fo r L ake St. Clair is 571.7 feet, IG LD
( 1 9 55).
(5)
T h is com m ittee was form ed in 1953 by U.S. Arm y Corps o f Engineers and the
Canadian Departm ents o f Transport, Mines and Technical Surveys, and Resources and
D evelopm ent to provide a basis fo r acceptance o f identical basic data, prin cipally lake
levels and connecting riv e r flow s, by both countries. The com m ittee is advisory to the
ap propriate operating agencies o f the tw o countries.
L evellin g along the connecting rivers and collection o f w ater level
data on the lakes are now underway to provide a basis fo r determ ining
dynam ic elevations o f benchmarks existing in the year 1970.
Responsibility for Datum Planes
Because the Corps o f Engineers has had United States responsibility
for charting o f the Great Lakes and navigation im provem ents o f the lakes,
the Corps has assumed the concom itant responsibility o f establishing the
United States datum planes. Under a reorganization plan effective 3 Oct­
ober 1970, responsibility fo r charting o f the Great Lakes was transferred
to the National Oceanic and Atm ospheric Adm inistration, D epartm ent of
Commerce, w hich now shares responsibility fo r datum planes. H ow ever,
w hatever agencies are involved, recom m endations fo r datum plane changes
would appropriately come from the Coordinating Com m ittee on Great
Lakes Basic H ydraulic and H ydrologie Data as a result o f an internationally
coordinated study.
THEORY AN D SELECTION OF D ATUM PLANES
General Considerations
Ideally, the datum planes should be selected fro m know ledge o f lake
levels over an extensive future period, say 50 years, but the only certain
knowledge is o f levels o f the past. A n y estimate o f the future levels which
the datum planes must accomm odate can be little m ore than an assumption
that past levels w ill recur w ith possible m odification s to account for
a rtificial changes such as diversion changes, etc. (see b elow ). T h e expecta­
tion o f recurrence, of course, can contain no hope o f sequential repetition
and can on ly be a prediction that statistical characteristics, principally
duration (exceedance frequencies), o f past levels w ill be duplicated in the
future.
In the period 1933-1968 the frequencies w ith w hich recorded m onthly
average lake levels exceeded the 1933 L o w W a te r D atum planes during the
navigation season April-N ovem ber w ere L a k e Superior (at Marquette,
Michigan) 93 per cent, Lake M ichigan-Huron (at H arbor Beach, M ichigan)
83 per cent, Lake Erie (at Cleveland, O hio) 95 per cent, and L a k e O ntario
(at Oswego, N ew Y o r k ) 94 per cent. Figure 2 shows the relation to L o w
W a te r Datum o f the average recorded lake levels fo r the navigation season
1933-1968.
I f occurrences o f m onthly average levels above L o w W a te r Datum
w ere independent events, the frequencies (probabilities) o f tw o or m ore
lakes being sim ultaneously above datum would be the product o f the
individual frequencies. Th is product fo r all fou r lakes is 69 per cent.
JUL
AUG
MONTH
F ig . 2.
A verage levels o f the Great Lakes, A p ril-N ovem ber, 1933-1968, referred to
L ow W a ter Datums.
The recorded frequency for the fo u r lakes, w hich is 77 per cent, is higher
as w ould be expected.
In using past levels to indicate future levels, a difficulty results
because changes of w ater level regim e often occur. Even if nature cooper­
ated by providing an identical future sequence o f m eteorological events to
that of the 1933-1968 period, the levels resulting therefrom w ould not be
statistically the same as the comparison period because of systematic
changes. These changes include dredging o f the connecting channels for
the 25-foot project completed in 1937 and the 27-foot project completed in
1962 (particularly the St. Clair and Detroit Rivers), initiation o f diversion
from the A lbany River basin into Lake Superior in 1939, m odification of
the Lake Superior regulation plan in 1955, and initiation of the regulation
o f Lake Ontario in 1960.
It is possible to adjust recorded levels to a fixed set o f conditions,
expected to apply in the future, in order to obtain a better estimate of
future levels. Such adjustments o f recorded levels have been made for
various hydraulic studies.
Recorded water levels for the period 1900-1967 adjusted to selected fixed
conditions (as developed by the Regulation Subcommittee of the Interna­
tional Great Lakes Levels Board) show that the frequencies of exceedance
o f the m onthly average levels of the present L o w W ater Datums are 84 per
cent for Lake Superior, 96 per cent for Lake Michigan-Huron, 99 per cent
for Lake Erie and 99 per cent for Lake Ontario. Figure 3 shows the
relation o f the average o f the recorded adjusted m onthly average levels
during the navigation seasons to the L o w W a te r Datums.
F ig . 3. —
A dju sted average levels o f the Great Lakes, A p ril-N ovem b er, 1900-1967,
referred to Low W a te r Datums.
---------------P resent datums.
-------------- Datums equal to the low est m on th ly level o f the to tal period.
Th e recorded adjusted levels described above contain no adjustment
for differential gage site movement. T h e selection o f datum planes could
be affected if the definin g benchmarks at the master gage sites are rising
or subsiding w ith respect to the lake outlets. H ow ever, the regulation o f
Lakes Superior and Ontario tends to elim inate such effect o f site m ovem ent
on those lakes. Th e rates o f m ovem ent of the definin g benchmarks at
Harbor Beach, M ichigan (about 60 m iles from the outlet of Lake MichiganH uron), and Cleveland, Ohio (about 170 miles fro m the outlet o f Lake
E rie), w ith respect to the lake outlets are both less than 0.5 foot per hundred
years (both outlets rising). These changes are of no practical significance
in the selection o f datum planes.
Selection of Planes for Charting
A m ariner using a navigation chart is confronted w ith a series o f
numbers representing w ater depths that w ill be present when the w ater
surface is at a specified elevation. On Lake Survey charts o f the Great
Lakes, the specified elevation on each lake is the 1955 L o w W a te r Datum
described above. Th e Great Lakes are essentially nontidal, but their w ater
surface elevations change continuously and sometim es rapidly. Trends o f
several years’ duration m ay cause the w ater to rise or fa ll a foot or two,
seasonal variations w ithin a given year o f the same m agnitude are super­
posed on the long-term trends, and short-period fluctuations lasting a few
hours and having a magnitude of several feet may occur locally on a given
lake. It is therefore impossible to select L o w W ater Datums which coincide
w ith the water surface other than as rare occurrences. The selection must
be made on the basis o f statistical criteria, such as exceedance frequencies
and level averages, obtained most practically from monthly average levels.
There is no practical w ay to provide fo r localized short period level
fluctuations in the selection o f datum planes. Monthly average levels
generally reflect a negligible effect of short-period fluctuations at any
location. But it is possible for w ind tide to affect the monthly average level
at a given location by a few 7 inches.
If every m ariner were fu lly aware of the nature of fluctuations of lake
level in affecting water depths and had full knowledge o f the existing lake
level, there would be little basis for preferring one datum over another
w ithin reasonable lim its (a very low datum plane would tend to show some
land areas where usually there is water, and a very high datum plane
would show large depths in areas where usually the depths are sm all).
Under such conditions the best choice might be a datum equal to the
average, or perhaps median level, in order to m inim ize deviations of
existing level from datum. How ever, many and possibly most m ariners do
not have this knowledge, and the chartm aker must protect all mariners by
recording nearly the least depths ever expected to occur. Such a policy
assures that a mariner, even w ith complete lack o f knowledge of the current
lake level and how it affects the charted depths, w ill rarely incur a disaster
as a result of actual depths being less than charted depths during the
normal navigation season.
A L o w W ater Datum fo r charting must therefore be selected to be
equal to a level exceeded most o f the time. T o select a datum equal to the
lowest level expected to occur has been generally considered im practical.
The recorded adjusted levels o f Lakes Erie and Ontario, April-N ovem ber
1900-1967, as described above, provide an indication of the differences
between datums and m inimum levels. In this set o f levels the amounts by
which the L o w W a te r Datums exceed the m inimum m onthly average levels
are Lake Superior 1.64 foot, Lake Michigan-Huron 0.86 foot, Lake Erie
0.43 foot, and Lake Ontario 0.95 foot.
In terms of marine safety it is difficult to draw a dividing line between
safe and unsafe datums. There is little or no basis, for example, to decide
that a datum which the level exceeds 90 per cent o f the time is safe while
a datum w ith only an 80 per cent exceedance frequency is not safe. W h ile
the use o f datums equal to the lowest recorded adjusted level m ight be
feasible, such a change, or any lowering, o f the present datums is not
warranted in order to im prove m arine safety because there is no evidence
that the present datums are unsafe.
Another consideration in the selection o f datum planes as references
for charted depths is com patibility, i.e., the state o f having the planes of
each lake bear the same relation to the w ater levels. A m ariner on Lake
Erie one day and Lake Huron the next, or perhaps dividing his sailing
time between these two lakes over the course of a month or longer, would
certainly find some convenience in having the actual depths exceed the
charted depths by the same amount on each lake. But the levels o f the
lakes do not va ry w ith the same periods or amplitudes, and it is therefore
impossible to assure com patibility at a given tim e by selection o f datum
planes. Further, the quantitative definition o f com patibility is uncertain.
H aving the levels o f both lakes w ithin the same 0.2 foot range w ith respect
to L o w W a te r Datum w ould be satisfactory to almost every m ariner, w ith in
the same 0.5 foot range would probably be satisfactory to many, and
w ithin the same 1 . 0 foot range would perhaps be satisfactory to relatively
few.
Although com patibility cannot be assured at a given time, statistical
com patibility can be im proved by selecting datum planes so as to provide
the m axim um probability that during a given tim e the planes w ill be
compatible. L a k e level exceedance frequencies (o f L o w W a te r D atum ) in
a high range, say 80 per cent to 1 0 0 per cent, do not provide as good a
criterion o f com patibility as do average levels (referred to L o w W a te r
Datum ). But the tw o statistics tend to va ry in the same way, and both are
changed, o f course, by a change o f the datum planes. Figure 3 shows the
relative positions w ith respect to lake levels of the present L o w W a te r
Datums and o f L o w W a te r Datums equal to m inim um m onthly average
levels, also based on the recorded adjusted m onthly average levels, A p rilNovem ber, 1900-1967.
Selection of Planes for Harbor and River Deepening
Despite the considerable com plexity involved in the use o f datum
planes as references fo r dredging projects, the selection o f planes fo r this
purpose involves only one simple criterion. T h e criterion is that the
selected planes should result in project depths (defin ed as the dredged
depths below L o w W a te r Datum and equal to the safe draft plus allow an­
ces) that are sligh tly greater than the d ra ft of the deepest draft vessel that
w ill use the im proved channels. Differences between project depths and
safe vessel drafts represent allowances fo r clearance, squat, exposure, and
nature of the channel bottom. Datum planes w hich do not satisfy this
criterion w ould be unacceptably confusing and possibly not acceptable as
charting planes. A detailed discussion o f project depths is beyond the scope
o f this paper.
Priority of Criteria
In the above discussion, separate criteria are indicated fo r charting
planes and fo r dredging planes except that the unsuitability fo r charting
of certain dredging planes must be noted. H ow ever, there is no basis fo r
questioning the w isdom o f having the same planes fo r both charting and
dredging, and the selected planes must therefore satisfy criteria fo r both.
Th e positions o f the charting planes have a tangible, though poorly de­
finable, effect on the usefulness and sa fety o f lake charts. Th e positions
o f the dredging planes have no equivalent tangible effect. Th erefore
charting considerations should have p riority in the selection o f planes.
A lthough a priority o f criteria can be assigned, planes selected on the
basis of charting alone are very likely to be suitable for dredging and
conversely. Th is Elysian situation results because the percentages o f time
the actual depths are intended to exceed charted depths and actual existing
drafts are intended to exceed design drafts are similar.
SUITABILITY OF PRESENT LA K E PLANES
Present datum planes (1933 planes) w ere the first datum planes o f the
Great Lakes selected to satisfy the requirements of both charting and
dredging. In the selection o f these planes there is no record o f specific lake
level statistics being used. But the 1933 criteria w ere in effect based on
the past levels o f Lakes Ontario, Erie, and Michigan-Huron and on the
expected regulation o f Lake Superior, and produced planes that are today
consistent w ith the criteria given herein.
Because the criteria fo r the selection o f planes are not precise, a
considerable range o f datums, individu ally and collectively is acceptable.
W ith the possible exception o f La k e Superior as described below, the
present L o w W a te r Datums sa tisfy the criteria and are suitable (0). This
is true w hether the basis for evaluation is the 1933-1968 w ater level regim e
o f record, the 1900-1967 recorded adjusted water level regim e described
herein, or the present w ater level regime.
W ith respect to the selection of planes fo r charting and based on
recorded adjusted levels, A pril-N ovem ber 1900-1967, both the frequencies
w ith w hich the levels exceed the datums and the average levels referred to
the datums show the L o w WTater Datum o f Lake Superior to be discordant
w ith the datums o f the other three lakes. I f the conditions on w hich the
recorded adjusted levels used for Figure 3 are based occur and are expected
to continue for some time, the L o w W a te r Datum o f Lake Superior should
be lowered as much as one fo ot (and the project depths reduced accord­
in gly). But a change of datum now would be premature because a study
o f regulating all the Great Lakes being made at the direction o f the Inter­
national Joint Commission m ay result in further regulation o f the lakes
and consequent change of w ater level regim es and because continuance
o f the present datum cannot be expected to have any very serious effects.
The study o f regulating the Great Lakes noted above could result in
a need fo r changed datums on any o f the lakes.
(6)
In contrast, the depth o f dredging required to provide a specified ben efit to
n avigation has little fle x ib ility . Because w ater level regim e changes can occur and
because dredging w ill become much m ore expensive if spoil is to be disposed onshore
instead o f offshore (Corps o f Engineers, 1969), periodic reanalysis, say every 10 years,
o f project depths m ay be warranted.
CONCLUSIONS
Th e p rim ary requirem ents for datum planes o f the Great Lakes — as
reference planes fo r charted depths and dredging projects — have been
satisfied by conceptual and practical developm ents occurring over a period
o f m any years. Present planes, adopted in 1933, are a culm ination of these
developments. Th e planes o f 1933 satisfy stated criteria for the selection
o f datum planes and are therefore satisfactory. T h e L o w W a te r Datum
o f La k e Superior, w hich is com paratively higher than the datums o f Lakes
M ichigan-Huron, Erie and Ontario, m ay need to be low ered in the future.
ACKNOWLEDGEMENTS
T h e author is gratefu l to Mr. Louis D. K i r s h n e r , form er Technical
D irector o f the Lake Survey, for his m any helpful comm ents and suggest­
ions.
Th e views expressed in this paper do not necessarily represent the
official views o f any office o f the National Ocean Survey or the N ational
Oceanic and Atm ospheric Adm inistration.
REFERENCES
[1 ] Chief o f Engineers, U.S. Arm y, 1876 Annual
P rin tin g Office, W ashington, 1876.
Report,
Governm ent
[2 ] Chief o f Engineers, U.S. Arm y, 1909 Annual
Prin tin g Office, W ashington, 1909.
Report,
Governm ent
[3 ] Coordinating Committee on Great Lakes Basic H ydrau lic and H y d ro ­
logie Data, Establishment o f International Great Lakes D atum
(1955), 1961.
[4 ]
[5 ]
[6 ]
[7 ]
Corps o f Engineers, U. S. A rm y, B uffalo D istrict, P ilo t Progra m
Sum m ary: Dredging and W a te r Q uality Problem s in the Great
Lakes, 1969.
Sherman. Datum Planes on the Great Lakes,
report o f the U.S. Lake Survey, File 3-2869, 1939.
M oore,
unpublished
Sherman. L o w W a ter Datum, unpublished report o f the
U.S. Lake Survey, F ile 3-3139, 1949.
Moore,
R opes,
G.E. V ertical Control on the Great Lakes, J o u r n a l o f S u r v e y i n g
ASCE, A pril, 1965.
and M a p p i n g D ivision,
[8 ]
United States Deep W aterw ays Commission, 1896 Report, Governm ent
P rin tin g Office, W ashington, 1897.