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

                                                   PUBLICATION DATE:  05/09/2001
Station ID: 8444162                                SUPERCEDED DATE:   04/21/2003
Name:       BOSTON LIGHT, BOSTON HARBOR                       
            MASSACHUSETTS
NOAA Chart: 13270                                  Latitude:         42° 19.7' N
USGS Quad:  HULL                                   Longitude:        70° 53.5' W


To reach the tidal bench marks from Hull, MA, go 2 km (1 nm) north of Hull, MA,
to Little Brewster Island which is near the mouth of Boston Harbor and what is
known as Nantasket Roads.  The tidal bench marks are located on the island in
the vicinity, and the Boston Light gauge is located at the end of the boat
landing area for the island.


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


                 PRIMARY BENCH MARK STAMPING:   
                         DESIGNATION:          844 4162 NO 8
                         ALIAS:                NO STAMPING 1

MONUMENTATION:           Tidal Station disk                        VM#:    16256
AGENCY:                  US Coast and Geodetic Survey (USC&GS)     PID:         
SETTING CLASSIFICATION:  Rock outcrop


The primary bench mark is a disk set flush in a rock outcrop on the western side
of the island in front of and to the south of the westernmost house on island,
2.74 m (9.0 ft) north of the main sidewalk which leads to the lighthouse and
dock, 1.37 m (4.5 ft) east of the centerline of a narrow sidewalk leading to
house.



                         BENCH MARK STAMPING:  USE 1885
                         DESIGNATION:          844 4162 USE

MONUMENTATION:           Chiseled Cross                            VM#:    16257
AGENCY:                  US Army Corps of Engineers (USE)          PID:         
SETTING CLASSIFICATION:  See descriptive text


The bench is a chisled x on top of a granite monument, 5.09 m (16.7 ft) west of
the SW corner of the light keepers house and near a flagpole, 5.09 m (16.7 ft)
WNW of the top step leading to the house, 1.64 m (5.4 ft) NE of a white
flagpole, and 0.09 m (0.3 ft) above ground.

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

                                                   PUBLICATION DATE:  05/09/2001
Station ID: 8444162                                SUPERCEDED DATE:   04/21/2003
Name:       BOSTON LIGHT, BOSTON HARBOR                       
            MASSACHUSETTS
NOAA Chart: 13270                                  Latitude:         42° 19.7' N
USGS Quad:  HULL                                   Longitude:        70° 53.5' W


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


                         BENCH MARK STAMPING:  10 1926
                         DESIGNATION:          844 4162 NO 10

MONUMENTATION:           Bench Mark disk                           VM#:    16258
AGENCY:                  US Coast and Geodetic Survey (USC&GS)     PID:         
SETTING CLASSIFICATION:  Rock outcrop


The bench mark is a disk set in top of a rock ledge on the NW part of the
island, 17.83 m (58.5 ft) NW of the NW corner of the westernmost house on the
island, 14.63 m (48.0 ft) north of the centerline of the north concrete
adandoned pier.



                         BENCH MARK STAMPING:  9 1926
                         DESIGNATION:          844 4162 NO 9

MONUMENTATION:           Bench Mark disk                           VM#:    16259
AGENCY:                  US Coast and Geodetic Survey (USC&GS)     PID:         
SETTING CLASSIFICATION:  Rock outcrop


The bench mark is a disk set in top of a rock outcrop on the western part of the
island, 11.13 m (36.5 ft) south of a narrow sidewalk at the north end of the
rock outcrop, 7.32 m (24.0 ft) east of the SE corner of the westernmost house on
the island, 3.96 m (13.0 ft) NW of the centerline of the main sidewalk between
lighthouse and boathouse.

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

                                                   PUBLICATION DATE:  05/09/2001
Station ID: 8444162                                SUPERCEDED DATE:   04/21/2003
Name:       BOSTON LIGHT, BOSTON HARBOR                       
            MASSACHUSETTS
NOAA Chart: 13270                                  Latitude:         42° 19.7' N
USGS Quad:  HULL                                   Longitude:        70° 53.5' W


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


                         BENCH MARK STAMPING:  LITTLE BREWSTER 1981
                         DESIGNATION:          844 4162 LITTLE BREWSTER

MONUMENTATION:           Tidal Station disk                        VM#:    16260
AGENCY:                  National Ocean Survey (NOS)               PID:         
SETTING CLASSIFICATION:  Rock outcrop


The bench mark is a disk set flush in a rock outcrop on the hill near the
lighthouse, 25.51 m (83.7 ft) north of the north wall of the lighthouse, 17.31 m
(56.8 ft) WNW of the west corner of a utility garage, 10.91 m (35.8 ft) NNE of
the north side of the most southern sidewalk.

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

                                                   PUBLICATION DATE:  05/09/2001
Station ID: 8444162                                SUPERCEDED DATE:   04/21/2003
Name:       BOSTON LIGHT, BOSTON HARBOR                       
            MASSACHUSETTS
NOAA Chart: 13270                                  Latitude:         42° 19.7' N
USGS Quad:  HULL                                   Longitude:        70° 53.5' W


                            T I D A L   D A T U M S 


Tidal datums at BOSTON LIGHT, BOSTON HARBOR based on:

     LENGTH OF SERIES:      4 MONTHS
     TIME PERIOD:           September 2000 - December 2000
     TIDAL EPOCH:           1960-1978
     CONTROL TIDE STATION:  8443970 BOSTON, BOSTON HARBOR


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

     MEAN HIGHER HIGH WATER                     MHHW   =  2.998
     MEAN HIGH WATER                            MHW    =  2.866
     MEAN SEA LEVEL                             MSL    =  1.495
     MEAN TIDE LEVEL                            MTL    =  1.480
     MEAN LOW WATER                             MLW    =  0.094
     MEAN LOWER LOW WATER                       MLLW   =  0.000



Bench Mark Elevation Information                In METERS above:

     Stamping or Designation                    MLLW        MHW

     844 4162 NO 8                               7.773    4.907
     USE 1885                                    7.632    4.766
     10 1926                                     4.843    1.977
     9 1926                                      7.037    4.171
     LITTLE BREWSTER 1981                        8.393    5.527


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

                                                   PUBLICATION DATE:  05/09/2001
Station ID: 8444162                                SUPERCEDED DATE:   04/21/2003
Name:       BOSTON LIGHT, BOSTON HARBOR                       
            MASSACHUSETTS
NOAA Chart: 13270                                  Latitude:         42° 19.7' N
USGS Quad:  HULL                                   Longitude:        70° 53.5' W


                             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 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.