U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 4
PUBLICATION DATE: 06/04/1986
Station ID: 8724397 SUPERCEDED DATE: 04/21/2003
Name: JOHNSTON KEY, TURKEY BASIN
FLORIDA
NOAA Chart: 11448 Latitude: 24° 42.6' N
USGS Quad: SUGARLOAF KEY Longitude: 81° 35.6' W
To reach the tidal bench marks from the bridge between Cudjoe Key and Sugarloaf
Key, proceed NWW by boat for 6.0 statute miles (9.7 km) to the SW tip of
Johnston Key. The bench marks are located on the SW tip of the key. The tide
gage and staff were located just west of the SW tip of the key.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: JOHNSTON KEY 2 RM 1 1934-35
MONUMENTATION: Survey Disk VM#: 7059
AGENCY: PID#: AE4416
SETTING CLASSIFICATION: Concrete Monument
The bench mark is set in the top of a 0.6-foot (0.2 m) square concrete monument,
141 feet (43 m) SE of the approximate high water line, 79.56 feet (24.25 m) east
of Bench Mark JOHNSTON KEY 2 1934-35, and 0.2 foot (0.1 m) above the ground.
BENCH MARK STAMPING: JOHNSTON KEY 2 1934-35
MONUMENTATION: Survey Disk VM#: 7058
AGENCY: PID#: AA1037
SETTING CLASSIFICATION: Concrete Monument
The bench mark is set in the top of a 0.7-foot (0.2 m) square concrete monument,
about 446 feet (136 m) NE of the most SW tip of the key, 164 feet (50 m) NW of
the mangroves on the SE shore, 98 feet (30 m) east of the mangroves on the west
shore, 3.5 feet (1.1 m) NNW of a witness post, and 0.2 foot (0.1 m) above the
ground.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 4
PUBLICATION DATE: 06/04/1986
Station ID: 8724397 SUPERCEDED DATE: 04/21/2003
Name: JOHNSTON KEY, TURKEY BASIN
FLORIDA
NOAA Chart: 11448 Latitude: 24° 42.6' N
USGS Quad: SUGARLOAF KEY Longitude: 81° 35.6' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: JOHNSTON KEY 2 RM 2 1934-35
MONUMENTATION: Survey Disk VM#: 7060
AGENCY: PID#: AE4417
SETTING CLASSIFICATION: Concrete Monument
The bench mark is set in the top of a 0.6-foot (0.2 m) square concrete monument,
98 feet (30 m) NE of the outer edge of mangroves, 67.91 feet (20.70 m) SSW of
Bench Mark JOHNSTON KEY 1934-35, and 0.2 foot (0.1 m) above the ground.
BENCH MARK STAMPING: NO 1 1973
MONUMENTATION: Survey Disk VM#: 7061
AGENCY: PID:
SETTING CLASSIFICATION: Rock Outcrop
The bench mark is set in rock outcrop, 361 feet (110 m) NE of the most SW tip of
the key, 120.0 feet (36.6 m) west of Bench Mark JOHNSTON KEY 2 RM 2 1934-35, 41
feet (12 m) SSW of Bench Mark NO 2 1973, 20.0 feet (6.1 m) ESE of the outer edge
of mangroves on the west side of the key, 1.3 feet (0.4 m) SE of an 1.5-foot
(0.5 m) high copper-clad rod driven into the ground.
BENCH MARK STAMPING: NO 2 1973
MONUMENTATION: Survey Disk VM#: 7062
AGENCY: PID:
SETTING CLASSIFICATION: Rock Outcrop
The bench mark is set in rock outcrop, 400 feet (122 m) NE of the most SW tip of
the key, 120.5 feet (36.7 m) WNW of Bench Mark JOHNSTON KEY 2 RM 2 1934-35, 41.0
feet (12.5 m) NE of Bench Mark NO 1 1973, 11.0 feet (3.4 m) ESE of the outer
edge of mangroves on the west side of the key.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 4
PUBLICATION DATE: 06/04/1986
Station ID: 8724397 SUPERCEDED DATE: 04/21/2003
Name: JOHNSTON KEY, TURKEY BASIN
FLORIDA
NOAA Chart: 11448 Latitude: 24° 42.6' N
USGS Quad: SUGARLOAF KEY Longitude: 81° 35.6' W
T I D A L D A T U M S
Tidal datums at JOHNSTON KEY, TURKEY BASIN based on:
LENGTH OF SERIES: 3 MONTHS
TIME PERIOD: DECEMBER 1973-FEBRUARY 1974
TIDAL EPOCH: 1960-1978
CONTROL TIDE STATION: 8724580 KEY WEST
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in FEET:
HIGHEST OBSERVED WATER LEVEL (12/16/1973) = 3.24
MEAN HIGHER HIGH WATER MHHW = 2.01
MEAN HIGH WATER MHW = 1.71
MEAN TIDE LEVEL MTL = 0.92
MEAN LOW WATER MLW = 0.12
MEAN LOWER LOW WATER MLLW = 0.00
LOWEST OBSERVED WATER LEVEL (12/23/1973) = -0.33
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In FEET above:
Stamping or Designation MLLW MHW
JOHNSTON KEY 2 RM 1 1934-35 1.87 0.16
JOHNSTON KEY 2 1934-35 1.97 0.26
JOHNSTON KEY 2 RM 2 1934-35 1.79 0.08
NO 1 1973 0.95 -0.76
NO 2 1973 0.85 -0.86
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 4
PUBLICATION DATE: 06/04/1986
Station ID: 8724397 SUPERCEDED DATE: 04/21/2003
Name: JOHNSTON KEY, TURKEY BASIN
FLORIDA
NOAA Chart: 11448 Latitude: 24° 42.6' N
USGS Quad: SUGARLOAF KEY Longitude: 81° 35.6' 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.