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National Ocean Service
Datums Page Page 1 of 5
Station ID: 8458694 PUBLICATION DATE: 04/01/2004
Name: WATCH HILL POINT
RI
NOAA Chart: 13214 Latitude: 41° 18.3' N ( 41.30500)
USGS Quad: WATCH HILL Longitude: 71° 51.6' W ( -71.86000)
To reach the tidal bench marks from the city of Watch Hill proceed south 0.8 km
(0.5 mi) to the abandoned U.S. Coast Guard Watch Hill Station at Watch Hill
Point. The bench marks are located in the area around the Coast Guard Station
and Watch Hill Point Lighthouse. The gage and staff were located on a piling in
a small cove NW of the Coast Guard Station. NOTE: These bench mark
descriptions are based on September 10, 1962 recovery information.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: BM NO 2 1939
DESIGNATION: 845 8694 TIDAL 2
MONUMENTATION: Bench Mark disk VM#: 2219
AGENCY: US Coast & Geodetic Survey
(USC&GS) IDB PID#: LW0809
SETTING CLASSIFICATION: Step OPUS PID:
LATITUDE: 41° 18.3' N ( 41.30500) LONGITUDE: 71° 51.5' W ( -71.85830)
The primary bench mark is a disk set in the concrete base of the wooden steps at
the NW entrance door to the main Coast Guard Station Building, 5.30 m (17.4 ft)
NNE of the west corner of the building, and 0.12 m (0.4 ft) NE of the NE edge of
the bottom step.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 8458694 PUBLICATION DATE: 04/01/2004
Name: WATCH HILL POINT
RI
NOAA Chart: 13214 Latitude: 41° 18.3' N ( 41.30500)
USGS Quad: WATCH HILL Longitude: 71° 51.6' W ( -71.86000)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: BM NO 1 1939
DESIGNATION: 845 8694 TIDAL 1
MONUMENTATION: Bench Mark disk VM#: 2218
AGENCY: US Coast & Geodetic Survey
(USC&GS) IDB PID#: LW0810
SETTING CLASSIFICATION: Concrete apron OPUS PID#: LW0810
LATITUDE: 41° 18.3' N ( 41.30500) LONGITUDE: 71° 51.5' W ( -71.85830)
The bench mark is a disk set in the concrete apron in the south corner of the
Coast Guard station garage at the gate entrance, 1 m (2 ft) SW of the SW edge of
the SW door of the garage, and 0.12 m (0.4 ft) NE of the SW edge of the apron.
BENCH MARK STAMPING: BM NO 3 1939
DESIGNATION: 845 8694 TIDAL 3
MONUMENTATION: Tidal Station disk VM#: 2220
AGENCY: US Coast & Geodetic Survey
(USC&GS) IDB PID#: LW0808
SETTING CLASSIFICATION: Concrete sea wall OPUS PID:
LATITUDE: 41° 18.2' N ( 41.30330) LONGITUDE: 71° 51.5' W ( -71.85830)
The bench mark is a disk set in the top of a concrete sea wall, 10.91 m (35.8
ft) NW of the north corner of the main building for the Watch Hill Point
lighthouse, 8 m (25 ft) north of a bend in the sea wall, and 4.48 m (14.7 ft) SW
of the top of the sea wall where the wall goes down to a lower level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 8458694 PUBLICATION DATE: 04/01/2004
Name: WATCH HILL POINT
RI
NOAA Chart: 13214 Latitude: 41° 18.3' N ( 41.30500)
USGS Quad: WATCH HILL Longitude: 71° 51.6' W ( -71.86000)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: BM NO 4 1961
DESIGNATION: 845 8694 TIDAL 4
MONUMENTATION: Bench Mark disk VM#: 2221
AGENCY: US Coast & Geodetic Survey
(USC&GS) IDB PID#:
SETTING CLASSIFICATION: Concrete sea wall OPUS PID:
LATITUDE: 41° 18.3' N ( 41.30500) LONGITUDE: 71° 51.4' W ( -71.85670)
The bench mark is a disk set in the western breakwater at the Coast Guard
Station, 88.79 m (291.3 ft) north of bench mark BM NO 3 1939, 75.71 m (248.4 ft)
SW of bench mark BM NO 2 1939, 11 m (35 ft) east of the approximate high water
mark, and 2.62 m (8.6 ft) south of the north end of the breakwater.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 8458694 PUBLICATION DATE: 04/01/2004
Name: WATCH HILL POINT
RI
NOAA Chart: 13214 Latitude: 41° 18.3' N ( 41.30500)
USGS Quad: WATCH HILL Longitude: 71° 51.6' W ( -71.86000)
T I D A L D A T U M S
Tidal datums at WATCH HILL POINT based on:
LENGTH OF SERIES: 1 MONTH
TIME PERIOD: August 1962 - August 1962
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8461490 NEW LONDON, THAMES RIVER
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 0.914
MEAN HIGH WATER MHW = 0.831
North American Vertical Datum NAVD88 = 0.553
MEAN SEA LEVEL MSL = 0.457
MEAN TIDE LEVEL MTL = 0.438
MEAN LOW WATER MLW = 0.045
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
BM NO 2 1939 6.271 5.440
BM NO 1 1939 3.262 2.431
BM NO 3 1939 4.450 3.619
BM NO 4 1961 3.862 3.031
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 8458694 PUBLICATION DATE: 04/01/2004
Name: WATCH HILL POINT
RI
NOAA Chart: 13214 Latitude: 41° 18.3' N ( 41.30500)
USGS Quad: WATCH HILL Longitude: 71° 51.6' W ( -71.86000)
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.