U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 5
Station ID: 8723921 PUBLICATION DATE: 04/01/2004
Name: GRASSY KEY NORTH SIDE, FLORIDA BAY
FL
NOAA Chart: 11449 Latitude: 24° 46.3' N ( 24.77170)
USGS Quad: GRASSY KEY Longitude: 80° 56.4' W ( -80.94000)
To reach the tidal bench marks from the post office at Layton, proceed 14.0 km
(8.7 mi) SW along U.S. Highway 1 to a paved private drive on the right, continue
0.24 km (0.15 mi) north on the private drive to an abandoned ferry landing on
the Dorset Estate. The bench marks are along the driveway and near the old
ferry landing, on the gulf side of Grassy Key. The tide gage and staff were
located on the pier at the boat basin west of the old ferry landing.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: BM NO 2 1935
DESIGNATION: 872 3921 TIDAL 2
MONUMENTATION: Tidal Station disk VM#: 6266
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#: AA0354
SETTING CLASSIFICATION: Coral outcrop OPUS PID:
LATITUDE: 24° 46.4' N ( 24.77330) LONGITUDE: 80° 56.4' W ( -80.94000)
The primary bench mark is a disk set in a large coral outcrop, 20 m (67 ft) SW
of the extended centerline of the road, 9 m (30 ft) NE of the north corner of an
old 4 m (12 ft) square concrete block building, 3 m (9 ft) SE of the shoreline,
and at road level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 8723921 PUBLICATION DATE: 04/01/2004
Name: GRASSY KEY NORTH SIDE, FLORIDA BAY
FL
NOAA Chart: 11449 Latitude: 24° 46.3' N ( 24.77170)
USGS Quad: GRASSY KEY Longitude: 80° 56.4' W ( -80.94000)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: NO 5 1964
DESIGNATION: 872 3921 NO 5
MONUMENTATION: Bench Mark disk VM#: 6267
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#: AA0356
SETTING CLASSIFICATION: Coral outcrop OPUS PID:
LATITUDE: 24° 46.4' N ( 24.77330) LONGITUDE: 80° 56.4' W ( -80.94000)
The bench mark is a disk set in the top of a coral outcrop, 36 m (117 ft) NE of
the extended centerline of the road near the old ferry landing, 4.11 m (13.5 ft)
west of a small button-wood tree with a triangle blaze cut on its west side, 4 m
(13 ft) NW of the NW side of a 1 m (4 ft) boulder which projects 1 m (2 ft), 3 m
(10 ft) SE of the shoreline, and at ground level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 8723921 PUBLICATION DATE: 04/01/2004
Name: GRASSY KEY NORTH SIDE, FLORIDA BAY
FL
NOAA Chart: 11449 Latitude: 24° 46.3' N ( 24.77170)
USGS Quad: GRASSY KEY Longitude: 80° 56.4' W ( -80.94000)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: H 328 1970
DESIGNATION: H 328
MONUMENTATION: Bench Mark disk VM#: 6269
AGENCY: National Geodetic Survey (NGS) IDB PID#: AA0352
SETTING CLASSIFICATION: Coral outcrop OPUS PID:
LATITUDE: 24° 46.4' N ( 24.77330) LONGITUDE: 80° 56.4' W ( -80.94000)
The bench mark is a disk set in top of a coral outcrop west of the boat basin,
71 m (234 ft) SW of the extended centerline of the paved drive near the old
ferry landing, 29.05 m (95.3 ft) west of the NW corner of a house, 9 m (31 ft)
west of the centerline of a driveway, 4.11 m (13.5 ft) SW of the centerline of
the boat ramp, 3.35 m (11.0 ft) east of the east side of a 1 m (3 ft) high wall
at the shoreline, and 0.30 m (1.0 ft) below driveway level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 8723921 PUBLICATION DATE: 04/01/2004
Name: GRASSY KEY NORTH SIDE, FLORIDA BAY
FL
NOAA Chart: 11449 Latitude: 24° 46.3' N ( 24.77170)
USGS Quad: GRASSY KEY Longitude: 80° 56.4' W ( -80.94000)
T I D A L D A T U M S
Tidal datums at GRASSY KEY NORTH SIDE, FLORIDA BAY based on:
LENGTH OF SERIES: 1 YEAR
TIME PERIOD: January 1971 - December 1971
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8724580 KEY WEST
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
HIGHEST OBSERVED WATER LEVEL (02/19/1972) = 0.702
North American Vertical Datum NAVD88 = 0.422
MEAN HIGHER HIGH WATER MHHW = 0.388
MEAN HIGH WATER MHW = 0.337
MEAN TIDE LEVEL MTL = 0.207
MEAN SEA LEVEL MSL = 0.185
MEAN LOW WATER MLW = 0.076
MEAN LOWER LOW WATER MLLW = 0.000
LOWEST OBSERVED WATER LEVEL (02/04/1971) = -0.355
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
BM NO 2 1935 0.644 0.307
NO 5 1964 0.695 0.358
H 328 1970 1.275 0.938
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 8723921 PUBLICATION DATE: 04/01/2004
Name: GRASSY KEY NORTH SIDE, FLORIDA BAY
FL
NOAA Chart: 11449 Latitude: 24° 46.3' N ( 24.77170)
USGS Quad: GRASSY KEY Longitude: 80° 56.4' W ( -80.94000)
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.