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National Ocean Service
Datums Page Page 1 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
To reach the tidal bench marks from the boat ramp in Chassahowitzka, proceed by
boat on the Chassahowitzka River for approximately 6 km (3 nm) to Johns Island
on the south side of the river about 0.3 km (0.2 nm) east of the mouth of the
river. The bench marks are located around the island. The tide gage and staff
were located on a platform in the river.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 7235 B 1978
DESIGNATION: 872 7235 B TIDAL
MONUMENTATION: Tidal Station disk VM#: 7751
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Stainless steel rod OPUS PID:
LATITUDE: 28° 41.5' N ( 28.69170) LONGITUDE: 82° 38.3' W ( -82.63830)
The primary bench mark is a disk located 18 m (60 ft) west of bench mark 7235 A
1978, 7 m (24 ft) south of water tank supports, 1.07 m (3.5 ft) north of the
dock, 1 m (4 ft) north of the water's edge, and 0.15 m (0.5 ft) north of a
witness post. The bench mark is set 15 cm (0.5 ft) above ground level, crimped
to the top of a stainless steel rod driven 5.5 m (18 ft) to refusal, encased in
PVC pipe and a concrete kickblock.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 7235 A 1978
DESIGNATION: 872 7235 A TIDAL
MONUMENTATION: Tidal Station disk VM#: 7750
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 28° 41.5' N ( 28.69170) LONGITUDE: 82° 38.3' W ( -82.63830)
The bench mark is a disk set in top of a concrete monument, 13 m (43 ft) north
of the canal, 7 m (23 ft) SW of the SW corner of the middle house of three, 7 m
(22 ft) south of the SE corner of the westernmost house, 5 m (15 ft) east of the
sawgrass edge, and 0.91 m (3.0 ft) north of a palm tree with a witness sign
attached.
BENCH MARK STAMPING: 7235 C 1978
DESIGNATION: 872 7235 C TIDAL
MONUMENTATION: Tidal Station disk VM#: 7752
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 28° 41.5' N ( 28.69170) LONGITUDE: 82° 38.3' W ( -82.63830)
The bench mark is a disk set in top of a concrete monument, 30 m (100 ft) NE of
bench mark 7235 A 1978, 26 m (85 ft) north of bench mark 7235 B 1978, 6 m (20
ft) south of the water's edge, 5 m (18 ft) north of the NW corner of the
easternmost house, and 0.3 m (1 ft) south of a witness post.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: JOHNS 1934 1940
DESIGNATION: JOHNS
MONUMENTATION: Triangulation Station disk VM#: 7753
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#:
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 28° 41.5' N ( 28.69170) LONGITUDE: 82° 38.3' W ( -82.63830)
The bench mark is a disk set in top of a concrete monument located on the south
side of the Chassahowitzka River, on the NW end of Johns Island, 11 m (35 ft)
south of the water's edge, and 0.3 m (1 ft) north of a witness post.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: JOHNS 1934 NO 1
DESIGNATION: JOHNS RM 1
MONUMENTATION: Reference Mark disk VM#: 7754
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#:
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 28° 41.5' N ( 28.69170) LONGITUDE: 82° 38.3' W ( -82.63830)
The bench mark is a disk set in the top of a concrete monument, 22.04 m (72.3
ft) NE of the triangulation station JOHNS 1934 1940, 3 m (10 ft) south of the
water's edge, 0.61 m (2.0 ft) from the base of a leaning sugarberry tree, and
0.3 m (1 ft) north of a witness post.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
T I D A L D A T U M S
Tidal datums at JOHNS ISLAND, CHASSAHOWITZKA BAY based on:
LENGTH OF SERIES: 5 MONTHS
TIME PERIOD: July 1978 - November 1978
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8726724 CLEARWATER BEACH, GULF OF MEXICO
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 0.723
MEAN HIGH WATER MHW = 0.621
MEAN TIDE LEVEL MTL = 0.347
MEAN SEA LEVEL MSL = 0.333
MEAN LOW WATER MLW = 0.074
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
7235 B 1978 1.127 0.506
7235 A 1978 0.915 0.294
7235 C 1978 1.413 0.792
JOHNS 1934 1940 1.655 1.034
JOHNS 1934 NO 1 1.569 0.948
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 6 of 6
Station ID: 8727235 PUBLICATION DATE: 01/06/2004
Name: JOHNS ISLAND, CHASSAHOWITZKA BAY
FL
NOAA Chart: 11409 Latitude: 28° 41.5' N ( 28.69170)
USGS Quad: CHASSAHOWITZKA BAY Longitude: 82° 38.3' W ( -82.63830)
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.