U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
To reach the tidal bench marks from the intersection of Gulf and Velasco
Boulevards in Freeport, proceed east on Gulf Boulevard (becomes County Road 229)
for 3.2 km (2.0 mi) to the Dow Chemical Company gate where permission to enter
and an escort must be obtained, continue on County Road 229 for 1.0 km (0.6 mi)
to the west levee road of the Dow Barge Canal, go SE and south on the levee road
for 1.0 km (0.6 mi) to the skimmer gate. The bench marks are located on the
west side of the canal between the levee and the shoreline. The tide gage is
located on the east side of the skimmer gate.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 2440 A 1980
DESIGNATION: 877 2440 A
MONUMENTATION: Tidal Station disk VM#: 864
AGENCY: National Ocean Survey (NOS) PID#: AI1121
SETTING CLASSIFICATION: Stainless steel rod
The primary bench mark is a disk located in the Dow Chemical Company's plant
area near the skimmer gate, 32 m (105 ft) SW of the SW corner of the steel
walkway to the skimmer gate, 26.52 m (87.0 ft) west of the west shoreline of the
barge canal, 26.52 m (87.0 ft) south of the centerline of the overhead pipeline
over the skimmer gate, and 17 m (56 ft) east of the centerline of the levee
road. The bench mark is 9 cm (0.3 ft) below ground level, crimped to a
stainless steel rod driven 13.7 m (45 ft) to refusal, and encased in a 4-inch
PVC pipe and a concrete kickblock.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: NO 8 1973
DESIGNATION: 877 2441 TIDAL 8
MONUMENTATION: Tidal Station disk VM#: 869
AGENCY: National Ocean Survey (NOS) PID#: AM0028
SETTING CLASSIFICATION: Concrete monument
The bench mark is a disk set in the top of a concrete post located 0.3 km (0.2
mi) south of the junction of the levee road and a road south to the Dow Chemical
Company skimmer gate, caretaker house and a tide gage, 43.59 m (143.0 ft) west
of the west edge of the canal, 29.84 m (97.9 ft) east of the centerline of the
levee road, 11.89 m (39.0 ft) west of a fence line, 6.40 m (21.0 ft) east of the
center line of a shell road, 1.10 m (3.6 ft) north of power pole RPL 432/227,
and 0.91 m (3.0 ft) west of a witness post.
BENCH MARK STAMPING: 2440 B 1980
DESIGNATION: 877 2440 B
MONUMENTATION: Tidal Station disk VM#: 871
AGENCY: National Ocean Survey (NOS) PID:
SETTING CLASSIFICATION: Stainless steel rod
The bench mark is a disk located in the Dow Chemical Company's plant area near
the skimmer gate, 51.82 m (170.0 ft) east of the NE face of styrene tank No.
V-1, 21.80 m (71.5 ft) north of a red valve for tanks, 16.37 m (53.7 ft) WNW of
a 4-inch steel concrete post, and 6.07 m (19.9 ft) east of the centerline of the
levee road. The bench mark is 0.4 foot (0.1 m) below ground level, crimped to a
stainless steel rod driven 13.7 m (45 ft) to refusal, and encased in a 4-inch
PVC pipe and a concrete kickblock.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 2440 C 1980
DESIGNATION: 877 2440 C
MONUMENTATION: Tidal Station disk VM#: 872
AGENCY: National Ocean Survey (NOS) PID:
SETTING CLASSIFICATION: Stainless steel rod
The bench mark is a disk located in the Dow Chemical Company's plant area west
of the skimmer gate, 43 m (141 ft) west of the west shoreline of the barge
canal, 15.42 m (50.6 ft) east of the centerline of the levee road, 10.36 m (34.0
ft) NW of utility pole No. 91, and 0.40 m (1.2 ft) west of a steel witness post.
The bench mark is 9 cm (0.3 ft) below ground level, crimped to a stainless
steel rod driven 15.24 m (50.0 ft) to refusal, and encased in a 4-inch PVC pipe
and a concrete kickblock.
BENCH MARK STAMPING: 2440 D 1980
DESIGNATION: 877 2440 D 1980
MONUMENTATION: Tidal Station disk VM#: 873
AGENCY: National Ocean Survey (NOS) PID:
SETTING CLASSIFICATION: Stainless steel rod
The bench mark is a disk located in the Dow Chemical Company's plant area and
west of the skimmer gate, 23.16 m (76.0 ft) south of the SE corner of a chain
link fence compound, 23.16 m (76.0 ft) east of the centerline of the levee road,
and 17.37 m (57.0 ft) SW of utility pole No. 230 with telephone booth. The
bench mark is 9 cm (0.3 ft) below ground level, crimped to a stainless steel rod
driven 13.7 m (45 ft) to refusal, and encased in a 4-inch PVC pipe and a
concrete kickblock.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING:
DESIGNATION: 877 2440 SRM
ALIAS:
MONUMENTATION: Bolt VM#: 14017
AGENCY: National Ocean Service PID:
SETTING CLASSIFICATION: Skimmer gate fender
The bench mark is the head of a galvanized steel lag bolt in the top of the
fender on the east side of the skimmer gate.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
T I D A L D A T U M S
Tidal datums at FREEPORT, DOW BARGE CANAL based on:
LENGTH OF SERIES: 5 YEARS
TIME PERIOD: January 1990 - December 1994
TIDAL EPOCH: 1960-1978
CONTROL TIDE STATION:
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
HIGHEST OBSERVED WATER LEVEL (09/11/1998) = 1.822
MEAN HIGHER HIGH WATER MHHW = 0.561
MEAN HIGH WATER MHW = 0.507
MEAN SEA LEVEL MSL = 0.312
MEAN TIDE LEVEL MTL = 0.305
MEAN LOW WATER MLW = 0.102
MEAN LOWER LOW WATER MLLW = 0.000
LOWEST OBSERVED WATER LEVEL (02/25/1965) = -1.273
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
2440 A 1980 3.662 3.155
NO 8 1973 2.861 2.354
2440 B 1980 5.109 4.602
2440 C 1980 4.023 3.516
2440 D 1980 3.051 2.544
877 2440 SRM 2.393 1.886
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 6 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' W
Foot Notes:
Freeport has anomalous relative sea level trends compared to most other
geographic regions in the United States. This is due to a general subsidence of
the land in the area, which has been occurring at a rapid rate. Because of the
magnitude of the sea level trends in these areas, NOAA has adopted a procedure
for computing accepted tidal datums for the National Water Level Observation
Network (NWLON) using the last several years of sea level data rather than the
19-year tidal epoch. The tide ranges are still based on the 1960-1978 National
Tidal Datum Epoch (NTDE) and are applied to the 5 year (1990 - 1994) mean tide
level (MTL) to compute other tidal datums. The adoption of this procedure was
necessary to ensure that these tidal datums accurately represent the existing
stand of sea level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 7 of 7
PUBLICATION DATE: 06/30/2000
Station ID: 8772440 SUPERCEDED DATE: 04/21/2003
Name: FREEPORT, DOW BARGE CANAL
TEXAS
NOAA Chart: 11321 Latitude: 28° 56.9' N
USGS Quad: FREEPORT Longitude: 95° 18.5' 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.