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                 'tTT  Hf E """ N   A     I 0N  A 
                        REVIE W

                    Vol. XLVIII, Nï¿½ j, January 1971


















                                COASTAL BOUNDARY SURVEYS

                                 by Captain John O. PHILLIPS, USESSA
                            Associate Director, Geodesy and Photogrammetry
                                     U.S. Coast and Geodetic Survey





                    This paper was first presented at the Symposium on Coastal Geodesy, 20-24 July
                1970, Munich, and is here reproduced by kind permission of the International Union of
                Geodesy and Geophysics.




                                             INTRODUCTION


                    New emphasis is now being given the coastal zone as the nations enter
    'r,~  ~into a new phase of world interest in the sea. Accelerated development and
                growth of the use of the sea are indicative of expanded exploitation for
                the benefit of commerce, industry, recreation, and settlement.  Some day
                aquiculture may well rival and surpass agriculture in importance as the
                population growth forces increased dependence upon the marine environ-
* I ~ment for survival.
                     Increased international effort must be made if our technology is to be
    ';>~        ~ used effectively in making intelligent use of our oceanic resources. One of
                the basic problems now being encountered is the determination of the
                extent of offshore waters over which a maritime nation has sovereignty.
    -~I ~        Ownership of rights to the ocean floor, state-federal jurisdiction, the extent
                of fishing rights, and other factors are pressing problems.
                     The Geneva Conventions clarify some of the legal questions involved,
                but many remain unresolved.  The federal-state contention in the U.S.A.
                over lands and minerals in the complex coastal areas is far from settled.
     '*.-I f   ~ The era of submarine law is here, and with it is a dire need for determining
                precise jurisdictional boundaries.
      ";j~     ~      Determination of location and form of the baseline and other tidal
    ~:.q         boundaries involves two fundamental surveying procedures: (1) establish-
    :   ~        n ment of tidal datum planes which provides the vertical component and
                 (2) the horizontal delineation of the shoreline at the accepted tidal datum
    :-~ ~        plane elevation.
                     The first is performed with tide gages, and the second through aerial

                                                                                    9











 130                 INTERNATIONAL IIYDROGRAPIFC REVIEW

 photomapping where the state of the art in photogrammetry provides tile
 best alternative for the accomplishment of the boundary mapping task.
     International law defines the boundaries, but they must be determined
 and then located and mapped for practical purposes. Furthermore, precision
 in offshore boundary delimitation is necessary in order to resolve legal
 problems of jurisdiction. This means that the boundaries must be position-
 ed in terms of geographic coordinates.
     Hydrographic offices and mapping organizations have a historic
responsibility for mapping the coastal frontiers and have accumulated a
vast store of pertinent data. Nautical charts and topographic and hydro-
graphic surveys, from which the charts have been constructed, will supply
a large part of the information needed in meeting modern problems in
seaward boundary demarcation. These archives include a wealth of accu-
mulated tidal information, tidal datum planes, geodetic data, and aerial
photography.
    The mapping becomes increasingly important as the value of onshore
and offshore coastal properties increases. It is indeed a formidable task to
acquire the great number of qualitative and quantitative measurements
and observations necessary to produce the required coastal zone graphics.
Existing charts and survey data are of inestimable value in portraying tile
coastal zone for boundary demarcation as well as engineering structures
and alongshore economic development.



                        OPERATIONAL ASPECTS


    Boundary "demarcation", or the laying out of a boundary on the ground
and with an appropriately detailed pictorial representation, is strictly an
engineering and cartographic problem. The field work involved encompasses
ground control; projection of the boundary line; mapping and boundary
strip; and placement of survey monuments. Problems usually encountered
in demarcating a dry land boundary are considerably more complicated
in the demarcation of a submerged land boundary.
    In mapping the shoreline for nautical charts, the general practice of
the surveyor or the field inspector of aerial photographs is to identify the
mean high-water line by examining the water marks and other features
of the shoreline rather than leveling from tidal bench marks.
    In many of the states of the U.S.A., the riparian boundary of private
property facing tidal waters is the mean high-water line; in others, it is the
mean  low-water line.  The  demarcation  and mapping  of those  tidal
boundaries are important to property owners. In a more modern context,
we must deal with the delineation of the seaward extensions of national
boundaries. The problem  is to either physically define the boundary by
bottom markers or to (devise a method by which a buoy, ship, or structure
can be placed on a boundary station as it is legally defined.

























                                 II
                                 r                                         Coast



     ...., ..  Offshore                          Foreshore                     Backshore


                                 I    .  I                                                       I 





High water line                       X                               } 





                              tow water line' ',2</7//                  '-                       T~I Edge of marine cliff 

                                                                        J Inland limit. of' debris                                              c

                                                              /  Shoreline (MHW)


                                 I Baseline (MLW)





                               Fro. 1. - Diagram defining the limits of coastal features.











    ,:~~    ~     132                   INTERNATIONAL HiYDROGRAI'IIIC REVIEW

a*                               ?~'!~ States                      /7
               j~~~~~~~~~~~~~~~~~~~~~~~~
                              ..  Federal
                                   ~  ~~~~~Inland /rrn t/o r;y]~ii::iii? 
                                 Federal          Waters
                              o "Sovereign' rights



                                   24 milesZ      - 

                                      2/


                        Area >     2
                                 2



:1~
CIA
                          XLow water    ...
                                 l ine.                                    2. ,.
                 ifast ;?) ilk ~~~~~~~~~~~~200 m




     X~~~~ FIr. 2. - Generalized application of the principles of seaward boundaries.

                     Rapid developments now occurring in the coastal zone and on the
    -:  ~  ~      Continental Shelf of the U.S.A. make it increasingly imperative that the
    X~~ ~         U.S.A. accelerate its traditional shore and sea boundary program specifically
                 for boundary purposes.  Since the outer seaward boundaries for most of
                 its coastal states extend only three nautical miles from the ordinary low-
    .~4 ~         water line, accurate positioning can be obtained  by  onshore  geodetic
    Ail>    ~     operations; that is, by triangulation, electronic distance measurements, or
                 a combination of both. Accuracy obtainable should be to within less than
                 0.3 m which is probably far better than bottom markers can be placed or
                 a ship can be maneuvered directly over a bottom marker or into a
                 predefined position.
                     As economic factors involving the Continental Shelf require seaward
                 extensions well beyond the 3-mile limit and the line of sight from shore,
    ~:~~        ~ other methods for positioning must be considered. Some existing electronic
    -5~ ~         positioning systems now in use provide control for distances as far out as
                 1 000 nautical miles. It is realized, of course, that the accuracy at these
                 longer ranges decreases considerably.
                     For the recovery of a previously located offshore boundary point, there
                 are certain applications of untderwater acoustic positioning which depend











                                            COASTAL BOUNDARY SURVEYS                          133

                  on a series of arrays of active bottom markers. This method of positioning
                  will permit high relative accuracies of a few metres in limited areas
                  anywhere in the ocean, but the absolute positioning of this system will
                  have to be provided by other means such as the Navigation Satellite System
                  or some long-range electronic fixing system.
                       Mean sea level takes on a new significance when we deal in the offshore
                  area. Mean sea level is defined by geodesists as the equipotential surface
                  which the oceans would assume if the only forces acting upon them were
                  the earth's gravitational forces and the centrifugal forces set up by the
                  earth's rotation. There are other forces, however, with which we must deal,
                  such as the tidal forces of the sun and the moon, and the meteorological

    '~:;  ~time to time and place to place.




                                                      low water line
                                                      Headlandr








                                          Territorial   na Waters

. :                                         Marine -
          I~~~~~~~~~~~~~~~~~






          X                               ~~~~~~       ~        ~      ~~~~A- Belt  ,_v


                  :2*@~~~~~ Ioz--                                    ~~~~~~~~~~~~ f Ordinary
                                                                             high water line
                                               .~ {. |\ Headland




                       1Fin. 3. -  The 3-mile territorial zone and its relation to inland waters.
              .~~~~~~~~~~~                                   'Wtrs.








                      These forces produce an ever-changing surface of the sea which is
                  difficult to relate to the ideal equipotential surface of the geodesists' mean
                  sea level. It is true that the differences between the outer surface of the
                  sea and the equipotential surface are of a rather small order; perhaps only
                  a few metres at most.  Still some oceanographers are interested in what
                  they call the slope of the sea and its position above or below the equi-
                  potential surface at any given time at a given place.




-3
                  I'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~- :,









                      134    ~~~INTERNATIONAL t!YDROGRAPIIIC REVIEW




                                       DEFINITIONS OF DATUMS


                     In general, aerial photography for mapping the coastline is other than
                tide-controlled photography, but tide-controlled photography for the loca-
                 tion of both the mean high-water and the low-water lines is now coming into
                greater use. The topographer or field inspector of aerial photographs ordi-
                narily judges whether the elevations of the tops of detached rocks lying
                close to shore are between mean low water or mean high water by their
                appearance and with his knowledge of the stage of the tide from the pre-
                dicted tide tables. The low-water line and the elevation of off-lying rocks,
                ledges, and shoals that are awash are of paramount importance in the
                establishment of sea and shore boundaries. (See pages 131 and 140).
                     Much of the low-water line for nautical charts is mapped by the hydro-
                grapher. Soundings are made over this line at higher stages of the tide and
                then reduced in accordance with the tide records. The mean low-water
                contour is then drawn through the zero soundings. The hydrographer also
                positions and determines the elevation of many of these off-lying features.
   :~   ~~~He usually records the approximate elevation of rocks lying far enough
                offshore to be a factor in navigation.
                     U.S.A. practice in charting rocks awash 'at low tide stages along the
   iji   ~~Atlantic and Gulf coasts is to show a rock awash or by symbol on the nau-
                tical chart for land features that are bare anywhere between 0.3 mn below
                mean low water or 0.3 mn above mean high water. This practice is followed
   :~  ~~~~for the safety of navigation and does not bear any direct relation to boundary
   ::~ ~~~matters, although it becomes a factor in laying out tile coastal baseline for
   ~" ~~~boundary extension.
                     On the other hand, a low-tide elevation in terms of the Genev-a Conven-
   ï¿½.7  ~~~tion is a land feature that is bare at any stage of the tide between the low-
   .~  ~~~water datum and the plane of mean high water. Occasionally, the question
                arises as to whether a rock awash shown on a chart may possibly be covered
                at low water or may.be just bare at high water making it an island. Such
   ï¿½ !~  ~~uncertainties can usually be resolved by existing survey r~ecords. On occasion,
   =:~   ~~however, it is necessary to make a nmore definitive field survey in an area
   :7.~ ~~where a significant econonie factor is involved.
-,I

























                     Although U.S.A. nautical charts, and the surveys from which those
                charts were made, supply some of the infornmation for establishing sea
   i ~~~~boundaries, they do not supply all of it. It is necessary to map the normal
                baseline in many places specifically for boundary pnrposes, either because
                of a'need for greater boundary accuracy or because the coastline has chang-
                ed. Let us then consider the accuracy requirements and means for this
                miappingy.
                     The survey or mapping of the 'normal baseline" involves two principal
                components : a vertical component and a horizontal component. We must
                first-determine the elevation to recoverable bench marks on the gronnd;
                this is the vertical component. Once the elevation of the tidal datum has
A










                        COASTAL BOUNDARY SURVEYS                        133

been established and referred to recoverable bench marks, the local sur-
veyor can begin to map the normal baseline. This is the horizontal
component.
    After the tidal bench marks are recovered, a closed line or loop of
differential levels is run from the bench marks to that part of the shore
where the boundary is to be located. Levels are run along the shoreline and
points are selected at intervals along the shore in such a manner that the
ground at each point is at the elevation of the tidal datum. Thus, the
boundary is demarcated.
    The surveyor then measures the horizontal distances and directions,
or bearings, between each of the points and between those points and other
features in the area - and/or between the points and horizontal control
stations - so that the boundary may be compiled on a plat, or on a map,
to an exact scale ratio and in true relation to other boundaries of the pro-
perty and/or to the international geodetic datum. Tidal bench marks in the
U.S.A. are set near the tide stations. It might be quite laborious for a
surveyor if he had to run his levels all the way from the tidal bench marks
to the place of his boundary survey where that place may be a number
of miles from the tide station. Usually, this is not necessary due to the
connections already made to many of the tidal bench marks to the primary
level network of the nation. Thus, the surveyor usually can start his survey
from the nearest bench mark of that network.
    The primary level network is a U.S.A. system of bench marks establish-
ed by the Coast and Geodetic Survey. These bench marks are connected
by precise leveling and all are adjusted to one reference surface called the
Sea Level Datum of 1929. The last continental adjustment of this network
was made in 1929. At that time, the network was connected to some 28
primary tide stations distributed around the coast of the United States to
determine the reference surface. Thus, the Sea Level Datum of 1929 is not
a tidal datum, but is based on tidal datums at the 28 tide stations. The Sea
Level Datum of 1929 will not always be at exactly the same elevation as
mean sea level (a tidal datum) determined at any one tide station. For the
purpose of boundary surveys, however, the primarylevel network is impor-
tant only as an accurate network of bench marks when a tide station, or
the tidal bench marks for that station, have been connected to the primary
network. Every bench mark in the primary network then has a known
elevation in relation to the tidal datums of that particular tide station.
    In selecting a chart compilation scale for the mapping of the normal
baseline, a scale of 1/20 000 is usually considered adequate, although some
areas may require mapping at the larger scale of 1/10 000. The scale 1/20 000
is more than adequate for most practical purposes when one considers the
methods that must be used in mapping anything as difficult to reach and
to define exactly as the mean low-water line. These lines, marked by the
intersection of the surface of the sea with the shore at a specific tide stage,
are ordinarily not well defined on the ground because the water surface is
seldom calm.
    Because the shoreline along tidal waters changes continfially in shape
and horizontal position due to the rise and fall of the tide, the vertical datum









136                  INTERNATIONAL flYDROGRAPHIC REVIEW

has special significance. Accuracy is controlled by the number of tide obser-
vations that are made and the length of the period of the tides are observed.
In this connection, the slope of the foreshore (the area between high water
and low water) must be considered. The minimum degree of slope within
an area to be mapped is the deciding factor. For example, in Louisiana, there
are places where the foreshore is over 1 km wide with a tide range of only
about 0.4 m. In this situation, the tides were observed for something over
a year to establish the datum within about 0.05 m vertically. On much of the
outer coasts of the U.S.A., however, the beaches are much steeper than
this. If the foreshore, for example, has a slope of 5.0 degrees then an error
in the tidal datum of 0.06 min will cause a horizontal displacement of the
normal baseline of about 0.7 m.



                   PHOTOGRAMMETRIC TECHNIQUES


    The Coast and Geodetic Survey has found that tide-controlled infrared
aerial photography provides an excellent means of mapping the normal base-
line. After the tidal datum has been established, observers at the tide stations
establish radio communication with the photographic aircraft and can
notify the flight crew when the water surface is at the datum elevation.
Infrared photographs are then taken at the correct water level (as, for
example, mean low water or mean lower low water). These show the line of
intersection of the water with the land very clearly and this line is mapped
by photogrammetric methods. The negative scale is 1/30 000.
    Infrared photography utilizes the "near" infrared portion of the elec-
tromagnetic spectrum (wave length range of 700 to 900 millimicrons). It has
a special characteristic that water areas are rendered black. This is due to an
increase in absorption at 700 millimicrons. Thus, an infrared photograph
shows the water as black, or very dark, in contrast to the shore and provides
a sharp, well-defined line of contrast between land and water.
    The techniques of obtaining good quality infrared photography are
somewhat different than for panchromatic photography. Special attention
must be given to the selection of the camera, the storage of unexposed film,
filter, exposure, and processing, but these techniques have been quite well
worked out at the Coast and Geodetic Survey and are not too difficult.
    Although infrared photography is unsurpassed for mapping a shoreline
contour, such as the mean low-water line or the mean high-water line, it is
not very suitable for mapping very small, detached features such as small
pinnacle rocks. These may be missed on infrared photography because this
photography does not permit any water penetration. If the existence of such
features is expected, color photography should be used to inventory and map
them.
    For example, at a mapping scale of 1/20 000, the infrared photographs
might be taken at any scale between 1/20 000 and 1/40 000. The larger-scale
photographs will, of course, show more detail, and this is necessary on some











                         COASTAL iBOUNDARY SURVEYS                        137

types of shoreline. The smaller-scale photographs will make the photogram-
metric plotting easier and will require less ground control.
    Normally, color photographs are taken simultaneously with a different
camera in the same airplane. Also, another set of color photographs is taken
at a smaller scale (for example, 1/50 000 to 1/80 000) to use for analytic
aerotriangulation so as to reduce the amount of geodetic control surveys.
    The first step in mapping by means of tide-controlled photography is
to recover, or establish, the tide stations and to lay out, on the flight maps,
the section of coastline controlled by each tide station. Along a generally
straight coastline, one tide station will serve to control many miles; for
example four tide stations probably will be adequate to control the photo-
graphy for the entire outer coast of the State of Texas -  a distance of
600 km; on the other hand, it was necessary to use eight tide stations for
tide-controlled photography of the outer shoreline of the Mississippi Delta
(300 km); and a few years ago, nine stations were used for tide-controlled
photography of the shoreline around Nantucket Sound, Massachusetts (160
kin).
    Adequate tidal datums have already been established for much of the
outer coast of the contiguous United States.
    Many of the tide stations established in the past have been connected to
tidal bench marks or to the primary level network. Speaking very generally,
the datum thus established would be correct within -  0.06 m, and in the
vicinity of primary tide stations, it would be even more accurate.
    Tide-controlled aerial photography must be done on days when the tide
reaches the proper level during daylight hours and when the sky is cloud'-
free or nearly so. These are rather difficult conditions.to meet, and conse-
quently, tide-controlled photography is more time-consuming and more
expensive than normal aerial photography. A special problem is encountered
in areas where most of the low tides occur at night; and in some areas low
water may occur in daylight hours only during the months of November to
late January. In such instances, the tide-controlled photography has to be
taken in the winter. Also, the cloud cover has to be considered because cloud-
free photographic weather is essential. In other applications, a strong wind
holding for some days could change the tides and prevent them from falling
as low as mean low water.
    Moreover, the period of photography is very short for any one tide (for
example, 15 minutes to 1 hour). The tidal datum of mean low water is a
mean of all the low waters. On any given day, the low tide will either not get
as low as the datum or it will. go below the datum, In other words, the low
tide rarely goes exactly to the datum level and stops there for a time. As a
consequence of this, mean low-water photography is taken when the falling.
tide crosses the datum level, or when the tide, having fallen below the datum
level, crosses that level again on the rise, and these periods are very short.
    It is thus very difficult to obtain photographs of all the shoreline at
exactly the datum level. Usually, it is advisable to take several sets of photo-
graphs and endeavor to have these slightly above the datum level and slightly
below the datum lyevl. Then if the photography is not at exactly the datum











                 138                  INTERNATIONAL IIYDROGRAPHiIC REVIEW

                 level, an accurate interpolation can be made from the photographs taken
                 just below and just above that level.
                     Some field examination of the tide-controlled photographs, or of the
                 maps compiled from those photographs, is necessary: to ensure that no
                 small features, such as pinnacle rocks, have failed to show on the photogra-
                 phy and have, therefore, been missed; to check the elevations of small off-
                 lying rocks or bars whose tops happen to be at, or very close, to the datum
                 level; and to examine any sections of the datum line that may have been
                 difficult to interpret from the photographs. This field inspection should be
                 made with the tide at, or close, to the datum plane.
                     Questions often arise regarding the interpretation of the mean high-
                 water line in marsh or swamp areas on our coastal topographic surveys. In
                 marsh or swamp areas, the nautical charts show the offshore edge of vegeta-
:4                tion (not the actual mean high-water line) as the shoreline, because this is
                 the line that the mariner sees at high water. Consequently, the topographic
                 surveys of the Coast and Geodetic Survey map the'offshore edge of the
                 grass in marsh and the offshore edge of brush and trees in cypress and
                 mangrove swamps; the topographic surveys do not show the actual mean
                 high-water line on the ground in these areas. The mean high-water line on
                 the ground (intersection of plane of mean high water on the ground) often
                 is not along the front or offshore edge of the marsh or at the back limits
                 of the marsh, but meanders around between the front and back limits.
                     The location of a boundary as it exists sometime prior to the time of
                 the survey is often of interest to property owners. Physical features of boun-
                 daries as they existed at some time in the past may be very difficult to
                 determine. The shore often changes because of wave action along the open
                 coast and because of erosion and accretion due to currents. Once a change
                 in the shore has occurred, it is not possible to demarcate or map a tidal
                boundary as it existed before the change because the old boundary (for
                 example, the mean high-water line) no longer exists and cannot be seen. This
                 fact is readily understood if one considers that the boundary is the line of
                 intersection of the surface of the water with the land. Obviously, if the land
                 changes, this line will be in a different horizontal position, and if the land
                 has changed, we have no ready means of tracing out where the line was
                 before the change. Old maps made before the shoreline change are about the
                only means of finding where the boundary was in the past, that is, prior
                to the change. Most places have been mapped several times by C & GS and
                 these repeat surveys serve a useful purpose in this connection.




                                              APPIUCATIONS


                     About ten years ago, our attention was directed to the need for more
                intensive surveying and charting of the coastal zone. The low-water and
                high-water lines on existing surveys and  depicted on  nautical  charts,
                although adequate for navigational purposes, were not, in some instances,

4











                        COASTAL BOUNDARY SURVEYS                         139

in sufficient detail for settling shore and sea boundary disputes because of
the small scale of the coastal charts; they did not provide the necessary
large-scale accuracy required in boundary litigation matters.
    Certain coastal states and concerned agencies of the Federal Govern-
ment requested assistance through the production of special purpose shore
and sea maps. The request was made in recognition of the C & GS as an
authority on tidal datums with long years of experience in charting these
lines. Especially important was the modern use of tide-controlled infrared
aerial photographic techniques.
    In response to one of these requests, the low-water line mapping of
most of the coast of Louisiana was accomplished through a cooperative
project between C & GS, the Bureau of Land Management, and the State
Mineral Board of Louisiana. Through this effort, additional basic tidal data
and up-to-date planimetric maps were provided for revision of C & GS
nautical charts. Also, a special set of 54 maps was produced showing the
mean low-water line and the coordinates of baseline points that were espe-
cially selected by the State of Louisiana and the Bureau of Land Management
for use in establishing an accurate baseline on which to base jurisdictional
boundaries essential in leasing extensive offshore oil and gas fields.
    Recently the State of Florida of U.S.A. requested the determination of
tidal datum planes and the mapping of the mean low-water line and the
mean high-water line. It is estimated that the work will be completed in five
or six years - the first phase is essentially completed.



                 A NEW NAUTICAL CHART PRODUCT


    A noteworthy by-product has evolved from recent seaward boundary
surveys in the form of a nautical chart which includes aerial photographic
imagery of the onshore detail. The new chart therefore can now depict the
onshore information as it actually appears rather than simply with chart
symbols such as swamp, trees, etc. Due to the new techniques that are
applied, the details fall in their correct locations. The compilation of line
drawings is greatly reduced and simplified. The new product is both more
pleasing in appearance, and also more meaningful to the user - yet the cost
need not be any greater.
    The new product has resulted from several factors: (1) techniques had
been developed for the production of accurate orthophotos; (2) several
scales of excellent photographs were already available because they had been
used for aerotriangulation and for shoreline compilation; and (3) a large
number of accurately located points had been determined by the aerotrian-
gulation procedure.
    Perhaps I should define the term orthophoto map. In appearance, it
resembles an aerial photograph;  however,  the planimetric positions of
features are corrected for the tilt of the aerial camera and for the elevation
of the terrain. An orthophoto may be produced photographically from an










                  140                 INTERNATIONAL HYDROGRAPHIC REVIEW

                  aerial photograph with a special line-scanning type of printer but, where
_9                the terrain is flat, an orthophoto may also be produced with a photograrp-
                  metric rectifying printer or by conventional controlled mosaicking. It is
                  significant that an orthophoto map has a planimetric accuracy equal to or
                  better than line-drawn maps.
                      We expect that eventually all nautical charts may include orthophoto
                  imagery although it is now being applied only in a limited way for the
                  seaward boundary plans. Moreover, it is also possible to use full-color
                  orthophotos; it is my expectation that before long our charts will include
                  this color material. The color images aid materially in their identification
                  by the user; for example, trees and green rooftops are various colors, etc.
                  A further improvement is also available in the form of a photographic screen
                  that eliminates the half-tone screen in chart printing _ the photo images
                  themselves perform the printing function of the dots of a screen.




                                                CONCLUSION


                      It becomes evident that as developments in the coastal zone and the
>I                 Continental Shelf accelerate, an increasing demand will result for the U.S.A.
                  to accelerate its traditional shore and sea program specifically for boundary
                  purposes. Studies of sea and shore boundaries forcefully indicate the number
                  of technical questions that arise and the extent of judgement required. The
                  most that can be provided are the principles for the delimitation of sea
                  boundaries; answers cannot be provided to every technical or interpretation
                  problem which will arise in laying out sea boundaries in the presence of
                  an almost infinite variety of physical features. This will require agreement
                  and cooperation between the states and the Federal Government and pro-
                  bably some litigation.




                                      TIDAL DATUMS - DEFINITIONS


                      Several reference planes are derived from tidal data, but only three are
                  of interest here. Tidal datum planes vary somewhat with the type of tide.
                  Along the Atlantic Coast of the U.S.A., where both semi-diurnal and diurnal
                  types of tide occur, tidal datum planes are mean high water and mean low
                  water. Along the Pacific Coast, including Alaska and Hawaii, where tides
                  are chiefly of the mixed type, datum planes are mean high water and mean
                  lower low water. A simple definition of these planes is
                      1. Mean high water at any place is the average height of the high waters
                  at that place over a given period of time.
                      2. Mean low water at any place is the average height of the low waters
                  at that place over a given period of time.











                        COASTAL BOUNDARY SURVEYS                       141

    3. Mean lower low water at any place is the average height of the lower
low waters at that place over a given period of time.
    Although tidal data observed throughout a 19-year period permits the
most accurate determination of a tidal plane, determinations of acceptable
accuracy for many engineering uses can be made from data observed over a
much shorter period of time.



                            BIBLIOGRAPHY


 [1] A study of the U.S. Coast and Geodetic Survey's products and services
        as related to economic activity in the U.S. Continental Shelf regions.
        Battelle Memorial Institute, January 1966.
 [2] California and the use of the ocean. University of California, Institute
        Marine Resources, October 1965.
 [3] Effective use of the Sea. Report of the panel on oceanography of the
        President's Science Advisory Committee, June 1966.
 [4] GRIFFIN, W.L., JONES, B.G. et McALINDEN, J.M.: Establishing tidal
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 [6] JONES, B.G. and SHoFNos, W.: Mapping the low-water line of the
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[7] Our nation and the sea. Report of the Commission on Marine Science,
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 [9] TIsoN, J.C., Jr. : Sea boundaries and nautical charts. Paper presented
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[10] SHALOWITZ, A.L.   : Shore and sea boundaries.   Publication  10-1
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