U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
Datums Page                                                        Page  1 of  5

                                                                                
Station ID: 8417881                         PUBLICATION DATE:  06/09/2021
Name:       GREAT CHEBEAGUE ISLAND                            
            ME
NOAA Chart: 13290                           Latitude:   43° 43.3' N (  43.72170)
USGS Quad:  PORTLAND EAST                   Longitude:  70°  8.5' W ( -70.14170)


To reach the tidal bench marks proceed by boat to Indian Point, located on Great
Chebeague Island, which is in the SW section of Casco Bay.  The bench marks are
located in the vicinity of the pier at Indian Point, and extend east towards the
dirt road leading to the pier.  The staff was located on the NW side of the
pier.  The tide gage was mounted in a shed east of the pier.  The orifice and
tubing extended offshore (west of the pier).


                         T I D A L   B E N C H   M A R K S


                 PRIMARY BENCH MARK STAMPING:   
                         DESIGNATION:          841 7881 S TIDAL

MONUMENTATION:           Triangulation Station disk            VM#:         1894
AGENCY:                  US Army Corps of Engineers (USE)      IDB PID:         
SETTING CLASSIFICATION:  Rock outcropping                      OPUS PID:        
LATITUDE:   43° 43.3' N (  43.72170)          LONGITUDE:  70°  8.5' W ( -70.14170)


The primary bench mark is a disk set flush in a rock outcropping, east of the
pier, 44.5 m (146 ft) NW of a 15.2 m (50 ft) tall oak tree, the only tree on
Indian Point, 13.6 m (44.5 ft) east of the NE concrete leg of the pier, and 11.4
m (37.5 ft) north of the NE corner of a shed, the only building on Indian Point.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  2 of  5

                                                                                
Station ID: 8417881                         PUBLICATION DATE:  06/09/2021
Name:       GREAT CHEBEAGUE ISLAND                            
            ME
NOAA Chart: 13290                           Latitude:   43° 43.3' N (  43.72170)
USGS Quad:  PORTLAND EAST                   Longitude:  70°  8.5' W ( -70.14170)


                         T I D A L   B E N C H   M A R K S


                         BENCH MARK STAMPING:  841 7881 B 1979
                         DESIGNATION:          841 7881 B TIDAL

MONUMENTATION:           Tidal Station disk                    VM#:         1895
AGENCY:                  National Ocean Survey (NOS)           IDB PID:         
SETTING CLASSIFICATION:  Rock outcrop                          OPUS PID:        
LATITUDE:   43° 43.3' N (  43.72170)          LONGITUDE:  70°  8.5' W ( -70.14170)


The bench mark is a disk set flush in a rock outcrop, south of the pier, 48.5 m
(159 ft) west of a 15.2 m (50 ft) tall oak tree, the only tree on Indian Point,
27.0 m (88.5 ft) south of the SE corner of the SE concrete leg of the pier, and
14.5 m (47.5 ft) SW of the SW corner of a shed, the only building on Indian
Point.



                         BENCH MARK STAMPING:  841 7881 C 1979
                         DESIGNATION:          841 7881 C TIDAL

MONUMENTATION:           Tidal Station disk                    VM#:         1896
AGENCY:                  National Ocean Survey (NOS)           IDB PID:         
SETTING CLASSIFICATION:  Rock outcrop                          OPUS PID:        
LATITUDE:   43° 43.3' N (  43.72170)          LONGITUDE:  70°  8.5' W ( -70.14170)


The bench mark is a disk set flush in a rock crop NE of the pier, 42.7 m (140
ft) NNE of a 15.2 m (50 ft) tall oak tree, the only tree on Indian Point, 31.9 m
(104.5 ft) NE of the NE corner of a shed, the only building on Indian Point, and
21.5 m (70.5 ft) NE of bench mark 841 7881 S.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  3 of  5

                                                                                
Station ID: 8417881                         PUBLICATION DATE:  06/09/2021
Name:       GREAT CHEBEAGUE ISLAND                            
            ME
NOAA Chart: 13290                           Latitude:   43° 43.3' N (  43.72170)
USGS Quad:  PORTLAND EAST                   Longitude:  70°  8.5' W ( -70.14170)


                         T I D A L   B E N C H   M A R K S


                         BENCH MARK STAMPING:  841 7881 D 1979
                         DESIGNATION:          841 7881 D TIDAL

MONUMENTATION:           Tidal Station disk                    VM#:         1897
AGENCY:                  National Ocean Survey (NOS)           IDB PID:         
SETTING CLASSIFICATION:  Rock outcrop                          OPUS PID:        
LATITUDE:   43° 43.3' N (  43.72170)          LONGITUDE:  70°  8.5' W ( -70.14170)


The bench mark is a disk set flush in a rock outcrop NE of the pier, 73.2 m (240
ft) NNW of a 15.2 m (50 ft) tall oak tree, the only tree on Indian Point, 61.0 m
(200 ft) NNE of the NE corner of a shed, the only building on Indian Point, and
31.1 m (102 ft) north of bench mark 841 7881 C 1979.



                         BENCH MARK STAMPING:  841 7881 E 1979
                         DESIGNATION:          841 7881 E TIDAL

MONUMENTATION:           Tidal Station disk                    VM#:         1898
AGENCY:                  National Ocean Survey (NOS)           IDB PID:         
SETTING CLASSIFICATION:  Rock outcrop                          OPUS PID:        
LATITUDE:   43° 43.3' N (  43.72170)          LONGITUDE:  70°  8.5' W ( -70.14170)


The bench mark is a disk set flush in a rock outcrop NE of the pier, 58.8 m (193
ft) NE of the NE corner of a small shed, the only building on Indian Point, 51.2
m (168 ft) north of a 15.2 m (50 ft) tall oak tree, the only tree on Indian
Point, 29.2 m (96 ft) east of bench mark 841 7881 C 1979, and 1.5 m (5.0 ft)
south of a small pier on Indian Point.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  4 of  5

                                                                                
Station ID: 8417881                         PUBLICATION DATE:  06/09/2021
Name:       GREAT CHEBEAGUE ISLAND                            
            ME
NOAA Chart: 13290                           Latitude:   43° 43.3' N (  43.72170)
USGS Quad:  PORTLAND EAST                   Longitude:  70°  8.5' W ( -70.14170)


                            T I D A L   D A T U M S 


Tidal datums at GREAT CHEBEAGUE ISLAND based on:

     LENGTH OF SERIES:      1 MONTH
     TIME PERIOD:           October 1979 - October 1979
     TIDAL EPOCH:           1983-2001
     CONTROL TIDE STATION:  8418150 PORTLAND, CASCO BAY


Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:

     MEAN HIGHER HIGH WATER                     MHHW   =  3.020
     MEAN HIGH WATER                            MHW    =  2.884
     MEAN SEA LEVEL                             MSL    =  1.503
     MEAN TIDE LEVEL                            MTL    =  1.495
     MEAN LOW WATER                             MLW    =  0.106
     MEAN LOWER LOW WATER                       MLLW   =  0.000



Bench Mark Elevation Information                In METERS above:

     Stamping or Designation                    MLLW        MHW

     841 7881 S TIDAL                            4.619    1.735
     841 7881 B 1979                             3.718    0.834
     841 7881 C 1979                             3.603    0.719
     841 7881 D 1979                             4.069    1.185
     841 7881 E 1979                             3.623    0.739


                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  5 of  5

                                                                                
Station ID: 8417881                         PUBLICATION DATE:  06/09/2021
Name:       GREAT CHEBEAGUE ISLAND                            
            ME
NOAA Chart: 13290                           Latitude:   43° 43.3' N (  43.72170)
USGS Quad:  PORTLAND EAST                   Longitude:  70°  8.5' W ( -70.14170)


                             D E F I N I T I O N S


Mean Sea Level (MSL) is a tidal datum determined over a 19-year National Tidal
Datum Epoch.  It pertains to local mean sea level and should not be confused
with the fixed datums of North American Vertical Datum of 1988 (NAVD88).

NAVD88 is a fixed datum derived from a simultaneous, least squares, minimum
constraint adjustment of Canadian/Mexican/United States leveling observations.
Local mean sea level observed at Father Point/Rimouski, Canada was held fixed as
the single initial constraint.  NAVD88 replaces NGVD29 as the national
standard geodetic reference for heights.  Bench mark elevations relative to
NAVD88 are available from NGS through the World Wide Web at 
National Geodetic Survey.

NGVD29 is a fixed datum adopted as a national standard geodetic reference for
heights but is now considered superseded.  NGVD29 is sometimes referred to as
Sea Level Datum of 1929 or as Mean Sea Level on some early issues of Geological
Survey Topographic Quads.  NGVD29 was originally derived from a general
adjustment of the first-order leveling networks of the U.S. and Canada after
holding mean sea level observed at 26 long term tide stations as fixed.
Numerous local and wide-spread adjustments have been made since establishment in
1929.  Bench mark elevations relative to NGVD29 are available from the National
Geodetic Survey (NGS) data base via the World Wide Web at 
National Geodetic Survey.

NAVD88 and NGVD29 are fixed geodetic datums whose elevation relationships to
local MSL and other tidal datums may not be consistent from one location to
another.

The Vertical Mark Number (VM#) and PID# shown on the bench mark sheet are unique
identifiers for bench marks in the tidal and geodetic databases, respectively.
Each bench mark in either database has a single, unique VM# and/or PID# assigned.
Where both VM# and PID# are indicated, both tidal and geodetic elevations are
available for the bench mark listed. The NGS's Integrated Data Base (IDB PID)
provides geodetic datum based on traditionally leveled and bluebooked ties to
the National Spatial Reference System(NSRS) https://www.ngs.noaa.gov/datasheets/.
The NGS Online Positioning User Service (OPUS PID) provides geodetic datum based
on GPS position & tie it to the NSRS https://www.ngs.noaa.gov/OPUS/.

The NAVD88 elevation is shown on the Elevations of Tidal Datums Table Referred
to MLLW only when two or more of the bench marks listed have NAVD88 elevations.
The NAVD88 elevation relationship shown in the table is derived from an average
of several bench mark elevations relative to tide station datum.  As a result of
this averaging, NAVD88 bench mark elevations computed indirectly from the tidal
datums elevation table may differ slightly from NAVD88 elevations listed for
each bench mark in the NGS database.