U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 5
Station ID: 8724302 PUBLICATION DATE: 05/28/2004
Name: KNOCKEMDOWN KEY
FL
NOAA Chart: 11445 Latitude: 24° 42.9' N ( 24.71500)
USGS Quad: SUMMERLAND KEY Longitude: 81° 28.7' W ( -81.47830)
To reach the tidal bench marks from the intersection of Blimp Road and U.S.
Highway 1 on Cudjoe Key, proceed north on Blimp Road for 2.7 km (1.7 mi) to the
road's end and the public boat ramp, then proceed by boat NE for 5 km (3 statute
mi) to the NE point of Knockemdown Key. The bench marks are located on the NE
point of the key. The tide gage and staff were located offshore of the point.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: NO 1 1974
DESIGNATION: 872 4302 TIDAL 1
MONUMENTATION: Tidal Station disk VM#: 6914
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 42.9' N ( 24.71500) LONGITUDE: 81° 28.7' W ( -81.47830)
The primary bench mark is a disk located on a sand berm, 24 m (80 ft) west of
the eastern outer edge of the mangrove, and 6 m (19 ft) WSW of a witness sign
nailed to a fence post. The bench mark is set 9 cm (0.3 ft) above ground level,
crimped to the top of a copper-clad steel rod driven an unspecified depth to
refusal, and encased in a 0.3 m (1 ft) concrete monument.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 8724302 PUBLICATION DATE: 05/28/2004
Name: KNOCKEMDOWN KEY
FL
NOAA Chart: 11445 Latitude: 24° 42.9' N ( 24.71500)
USGS Quad: SUMMERLAND KEY Longitude: 81° 28.7' W ( -81.47830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: NO 2 1974
DESIGNATION: 872 4302 TIDAL 2
MONUMENTATION: Tidal Station disk VM#: 6915
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 42.9' N ( 24.71500) LONGITUDE: 81° 28.7' W ( -81.47830)
The bench mark is a disk located on a sand berm, 15 m (50 ft) WNW of a witness
sign nailed to a fence post, and 10.45 m (34.3 ft) NW of bench mark NO 1 1974.
The bench mark is set 9 cm (0.3 ft) above ground level, crimped to the top of a
copper-clad steel rod driven an unspecified depth to refusal, and encased in a
0.3 m (1 ft) concrete monument.
BENCH MARK STAMPING: NO 3 1974
DESIGNATION: 872 4302 TIDAL 3
MONUMENTATION: Tidal Station disk VM#: 6916
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 42.9' N ( 24.71500) LONGITUDE: 81° 28.7' W ( -81.47830)
The bench mark is a disk located on a sand berm, 13 m (43 ft) SE of a witness
sign nailed to a tree, and 8.02 m (26.3 ft) NW of bench mark NO 2 1974. The
bench mark is set 9 cm (0.3 ft) above ground level, crimped to the top of a
copper-clad steel rod driven an unspecified depth to refusal, and encased in a
0.3 m (1 ft) concrete monument.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 8724302 PUBLICATION DATE: 05/28/2004
Name: KNOCKEMDOWN KEY
FL
NOAA Chart: 11445 Latitude: 24° 42.9' N ( 24.71500)
USGS Quad: SUMMERLAND KEY Longitude: 81° 28.7' W ( -81.47830)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: NO 4 1974
DESIGNATION: 872 4302 TIDAL 4
MONUMENTATION: Tidal Station disk VM#: 6917
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 42.9' N ( 24.71500) LONGITUDE: 81° 28.7' W ( -81.47830)
The bench mark is a disk located on a sand berm, 31 m (102 ft) ENE of a witness
sign nailed to a tree, and 11.64 m (38.2 ft) NW of bench mark NO 3 1974. The
bench mark is set 9 cm (0.3 ft) above ground level, crimped to the top of a
copper-clad steel rod driven an unspecified depth to refusal, and encased in a
0.3 m (1 ft) concrete monument.
BENCH MARK STAMPING: NO 5 1974
DESIGNATION: 872 4302 TIDAL 5
MONUMENTATION: Tidal Station disk VM#: 6918
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 42.9' N ( 24.71500) LONGITUDE: 81° 28.7' W ( -81.47830)
The bench mark is a disk located on a sand berm, 18 m (60 ft) north of a witness
sign nailed to a tree, and 17.83 m (58.5 ft) NNW of bench mark NO 4 1974. The
bench mark is set 9 cm (0.3 ft) above ground level, crimped to the top of a
copper-clad steel rod driven an unspecified depth to refusal, and encased in a
0.3 m (1 ft) concrete monument.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 8724302 PUBLICATION DATE: 05/28/2004
Name: KNOCKEMDOWN KEY
FL
NOAA Chart: 11445 Latitude: 24° 42.9' N ( 24.71500)
USGS Quad: SUMMERLAND KEY Longitude: 81° 28.7' W ( -81.47830)
T I D A L D A T U M S
Tidal datums at KNOCKEMDOWN KEY based on:
LENGTH OF SERIES: 1 MONTH
TIME PERIOD: September 1974 - September 1974
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8724580 KEY WEST
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 0.706
MEAN HIGH WATER MHW = 0.623
MEAN TIDE LEVEL MTL = 0.355
MEAN SEA LEVEL MSL = 0.316
MEAN LOW WATER MLW = 0.087
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
NO 1 1974 0.712 0.089
NO 2 1974 0.994 0.371
NO 3 1974 1.072 0.449
NO 4 1974 0.901 0.278
NO 5 1974 0.898 0.275
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 8724302 PUBLICATION DATE: 05/28/2004
Name: KNOCKEMDOWN KEY
FL
NOAA Chart: 11445 Latitude: 24° 42.9' N ( 24.71500)
USGS Quad: SUMMERLAND KEY Longitude: 81° 28.7' W ( -81.47830)
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.