U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 4
PUBLICATION DATE: 02/12/1982
Station ID: 8631542 SUPERCEDED DATE: 04/21/2003
Name: COBB ISLAND, SAND SHOAL INLET
VIRGINIA
NOAA Chart: 12224 Latitude: 37° 18.1' N
USGS Quad: COBB ISLAND, VA Longitude: 75° 46.7' W
To reach the tidal bench marks from the intersection of US Route 13 and Virginia
State Road 639 proceed east along State Road 639 for 2.8 miles (4.5 m) to the
public boat ramp at Oyster Virginia, then proceed by boat east along Sand Shoal
Channel for 7.5 nautical miles (13.9 km) to the Old Coast Guard Station on the
southeastern end of Cobb Island. The tide gage and staff were located 850 feet
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
MONUMENTATION: Survey Disk VM#: 3346
AGENCY: PID:
SETTING CLASSIFICATION: Galvanized Steel Rod
The bench mark is 112.8 feet (34.4 m) northeast of the northwest corner of an
old coast guard house, 104.2 feet (31.8 m) northeast of Bench Mark 1542 D 1979,
100.3 feet (30.6 m) north of the northeast corner of the old coast guard house,
and 82.0 feet (25 m) northwest of a power pole. The bench mark is 0.3 foot (0.1
m) above ground level crimped to the top of a galvanized steel rod driven 48
feet (14.6 m) to refusal and encased in a PVC pipe.
BENCH MARK STAMPING: 1542 D 1979
MONUMENTATION: Survey Disk VM#: 3347
AGENCY: PID:
SETTING CLASSIFICATION: Galvanized Steel Rod
The bench mark is 153.7 feet (46.8 m) southwest of Bench Mark 1542 E 1979, 53.6
feet (16.3 m) northeast of the southeast corner of the old coast guard house,
39.8 feet (12.1 m) southeast of the northeast corner of the old coast guard
house and 23.8 feet (7.3 m) south of a power pole. The bench mark is 0.4 foot
(0.l m) above ground level, crimped to the top of a galvanized steel rod driven
48 feet (14.6 m) 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
PUBLICATION DATE: 02/12/1982
Station ID: 8631542 SUPERCEDED DATE: 04/21/2003
Name: COBB ISLAND, SAND SHOAL INLET
VIRGINIA
NOAA Chart: 12224 Latitude: 37° 18.1' N
USGS Quad: COBB ISLAND, VA Longitude: 75° 46.7' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 1542 E 1979
MONUMENTATION: Survey Disk VM#: 3348
AGENCY: PID:
SETTING CLASSIFICATION: Galvanized Steel Rod
The bench mark is 173.0 feet (52.7 m) east of Bench Mark 1542 C 1979, 96.1 feet
(29.3 m) southeast of wooden post, 91.4 feet (27.9 m) southeast of an iron post,
55.4 feet (11.9 m) northwest of a wooden post and 2.6 feet southwest of a power
pole. The bench mark is crimped to the top of galvanized steel rod driven 48
feet (14.6 m) 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
PUBLICATION DATE: 02/12/1982
Station ID: 8631542 SUPERCEDED DATE: 04/21/2003
Name: COBB ISLAND, SAND SHOAL INLET
VIRGINIA
NOAA Chart: 12224 Latitude: 37° 18.1' N
USGS Quad: COBB ISLAND, VA Longitude: 75° 46.7' W
T I D A L D A T U M S
Tidal datums at COBB ISLAND, SAND SHOAL INLET based on:
LENGTH OF SERIES: 23 MONTHS
TIME PERIOD: AUGUST 1978 - JULY 1980
TIDAL EPOCH: 1960-1978
CONTROL TIDE STATION: 8638610 HAMPTON ROADS, VA
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in FEET:
MEAN HIGHER HIGH WATER MHHW = 4.56
MEAN HIGH WATER MHW = 4.22
MEAN TIDE LEVEL MTL = 2.19
MEAN LOW WATER MLW = 0.16
MEAN LOWER LOW WATER MLLW = 0.00
Bench Mark Elevation Information In FEET above:
Stamping or Designation MLLW MHW
1542 C 1979 4.57 0.35
1542 D 1979 4.57 0.35
1542 E 1979 4.23 0.01
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 4
PUBLICATION DATE: 02/12/1982
Station ID: 8631542 SUPERCEDED DATE: 04/21/2003
Name: COBB ISLAND, SAND SHOAL INLET
VIRGINIA
NOAA Chart: 12224 Latitude: 37° 18.1' N
USGS Quad: COBB ISLAND, VA Longitude: 75° 46.7' 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.