U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 4
Station ID: 8631542 PUBLICATION DATE: 04/29/2003
Name: SAND SHOAL INLET, COBB ISLAND
VA
NOAA Chart: 12224 Latitude: 37° 18.1' N ( 37.30170)
USGS Quad: COBB ISLAND Longitude: 75° 46.7' W ( -75.77830)
To reach the bench marks from the intersection of US Route 13 and Virginia State
Road 639 proceed east along State Road 639 for 4.5 km (2.4 nm) to the public
boat ramp at Oyster Virginia, then proceed by boat east along Sand Shoal Channel
for 13.9 km (7.5 nm) to the Old Coast Guard Station on the southeastern end of
Cobb Island. The tide gage and staff were located 260 m (850 ft) south of the
active Coast Guard Base, west of the Old Coast Guard House on an old dilapidated
wooden pier.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 1542 C 1979
DESIGNATION: 863 1542 C
MONUMENTATION: Tidal Station disk VM#: 3346
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Galvanized steel rod OPUS PID:
LATITUDE: 37° 18.0' N ( 37.30000) LONGITUDE: 75° 46.0' W ( -75.76670)
The primary bench mark is a disk located 34.38 m (112.8 ft) NE of the NW corner
of an old coast guard house, 31.76 m (104.2 ft) NE of bench mark 1542 D 1979,
30.57 m (100.3 ft) north of the NE corner of the old coast guard house, and 25 m
(82 ft) northwest of a power pole. The bench mark is set 9 cm (0.3 ft) above
ground level, crimped to the top of a galvanized steel rod driven 14.6 m (48 ft)
to refusal and encased in a PVC pipe.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 4
Station ID: 8631542 PUBLICATION DATE: 04/29/2003
Name: SAND SHOAL INLET, COBB ISLAND
VA
NOAA Chart: 12224 Latitude: 37° 18.1' N ( 37.30170)
USGS Quad: COBB ISLAND Longitude: 75° 46.7' W ( -75.77830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 1542 D 1979
DESIGNATION: 863 1542 D
MONUMENTATION: Tidal Station disk VM#: 3347
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Galvanized steel rod OPUS PID:
LATITUDE: 37° 18.0' N ( 37.30000) LONGITUDE: 75° 46.0' W ( -75.76670)
The bench mark is a disk located 46.85 m (153.7 ft) SW of bench mark 1542 E
1979, 16.34 m (53.6 ft) NE of the SE corner of the old coast guard house, 12.13
m (39.8 ft) SE of the NE corner of the old coast guard house, and 7.25 m (23.8
ft) south of a power pole. The bench mark is set 0.12 m (0.4 ft) above ground
level, crimped to the top of a galvanized steel rod driven 14.6 m (48 ft) to
refusal and encased in a PVC pipe.
BENCH MARK STAMPING: 1542 E 1979
DESIGNATION: 863 1542 E
MONUMENTATION: Tidal Station disk VM#: 3348
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Galvanized steel rod OPUS PID:
LATITUDE: 37° 18.0' N ( 37.30000) LONGITUDE: 75° 46.0' W ( -75.76670)
The bench mark is a disk located 52.73 m (173.0 ft) east of bench mark 1542 C
1979, 29.29 m (96.1 ft) SE of wooden post, 27.86 m (91.4 ft) SE of an iron post,
16.88 m (55.4 ft) NW of a wooden post, and 0.79 m (2.6 ft) SW of a power pole.
The bench mark is crimped to the top of galvanized steel rod driven 14.6 m (48
ft) to refusal and encased in a PVC pipe.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 4
Station ID: 8631542 PUBLICATION DATE: 04/29/2003
Name: SAND SHOAL INLET, COBB ISLAND
VA
NOAA Chart: 12224 Latitude: 37° 18.1' N ( 37.30170)
USGS Quad: COBB ISLAND Longitude: 75° 46.7' W ( -75.77830)
T I D A L D A T U M S
Tidal datums at SAND SHOAL INLET, COBB ISLAND based on:
LENGTH OF SERIES: 36 MONTHS
TIME PERIOD: October 1978 - September 1981
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8638610 SEWELLS POINT, HAMPTON ROADS
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 1.369
MEAN HIGH WATER MHW = 1.268
MEAN TIDE LEVEL MTL = 0.658
MEAN SEA LEVEL MSL = 0.650
MEAN LOW WATER MLW = 0.049
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
1542 C 1979 1.264 -0.004
1542 D 1979 1.263 -0.005
1542 E 1979 1.161 -0.107
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 4
Station ID: 8631542 PUBLICATION DATE: 04/29/2003
Name: SAND SHOAL INLET, COBB ISLAND
VA
NOAA Chart: 12224 Latitude: 37° 18.1' N ( 37.30170)
USGS Quad: COBB ISLAND Longitude: 75° 46.7' W ( -75.77830)
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.