U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 5
Station ID: 8724139 PUBLICATION DATE: 02/26/2004
Name: HORESHOE KEYS
FLORIDA
NOAA Chart: 11442 Latitude: 24° 46.0' N
USGS Quad: HORSESHOE KEY Longitude: 81° 17.0' W
To reach the tidal bench marks by boat from the west end of the Seven Mile
Bridge on Little Duck Key, proceed NW for 11.1 km (6.9 statute mi) to the
southernmost key of the Horseshoe Keys group. The bench marks are located in
the area around the mangroves. The tide gage and staff were located on the
Government Wildlife Refuge pier.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: HORSESHOE KEY 2 1909 1935
DESIGNATION: HORSESHOE KEY 2 RESET
MONUMENTATION: Triangulation Station disk VM#: 6372
AGENCY: US Coast and Geodetic Survey (USC&GS) PID#: AA1571
SETTING CLASSIFICATION: Concrete monument
The primary bench mark is a disk set in the top of a concrete monument in a
clearing in the center of the key, near the SE end of Flat Key, 85.50 m (280.5
ft) north of the south end of the Key, 12.07 m (39.6 ft) from the west side of
the key, and 0.2 m (0.7 ft) south of a witness post.
BENCH MARK STAMPING: BM 1 1974
DESIGNATION: 872 4139 TIDAL 1
MONUMENTATION: Tidal Station disk VM#: 6370
AGENCY: National Ocean Survey (NOS) PID:
SETTING CLASSIFICATION: Copper-clad steel rod
The bench mark is a disk located 41.45 m (136.0 ft) west of the outer edge of
the mangroves, 28.65 m (94.0 ft) north of bench mark BM 2 1974, 24.66 m (80.9
ft) ESE of the tide station, 19 m (61 ft) WSW of reference mark HORSESHOE KEY 2
NO 2 1909 1935, and 0.18 m (0.6 ft) WSW of a witness post. The bench mark is set
3 cm (0.1 ft) above the ground, crimped to the top of a copper-clad steel rod
driven an unspecified depth to refusal and encased in concrete.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 8724139 PUBLICATION DATE: 02/26/2004
Name: HORESHOE KEYS
FLORIDA
NOAA Chart: 11442 Latitude: 24° 46.0' N
USGS Quad: HORSESHOE KEY Longitude: 81° 17.0' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: BM 2 1974
DESIGNATION: 872 4139 TIDAL 2
MONUMENTATION: Tidal Station disk VM#: 6371
AGENCY: National Ocean Survey (NOS) PID:
SETTING CLASSIFICATION: Copper-clad steel rod
The bench mark is a disk located 28.65 m (94.0 ft) south of bench mark BM 1
1974, 17.86 m (58.6 ft) south of reference mark HORSESHOE KEY 2 NO 2 1909 1935,
and 0.27 m (0.9 ft) north of a witness post. The bench mark is crimped to the
top of a copper-clad rod driven an unspecified depth to refusal and encased in
concrete.
BENCH MARK STAMPING: HORSESHOE KEY 2 NO 1 1909 1935
DESIGNATION: HORSESHOE KEY 2 RM 1
MONUMENTATION: Reference Mark disk VM#: 6373
AGENCY: US Coast and Geodetic Survey (USC&GS) PID#: AE4389
SETTING CLASSIFICATION: Concrete post
The bench mark is a disk set in the top of a concrete post located in a clearing
in the south part of Horseshoe Key, 95.55 m (313.5 ft) ENE of the south end of
the Key, 19.66 m (64.5 ft) SSE of bench mark BM 1 1974, 13.01 m (42.7 ft) ENE of
the triangulation station, 0.27 m (0.9 ft) south of a witness post, and 0.03 m
(0.1 ft) above the ground.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 8724139 PUBLICATION DATE: 02/26/2004
Name: HORESHOE KEYS
FLORIDA
NOAA Chart: 11442 Latitude: 24° 46.0' N
USGS Quad: HORSESHOE KEY Longitude: 81° 17.0' W
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: HORSESHOE KEY 2 NO 2 1909 1935
DESIGNATION: HORSESHOE KEY 2 RM 2
MONUMENTATION: Reference Mark disk VM#: 6374
AGENCY: US Coast and Geodetic Survey (USC&GS) PID#: AE4390
SETTING CLASSIFICATION: Concrete monument
The bench mark is a disk set in the top of a concrete monument, 75.44 m (247.5
ft) north of the south end of the Key, 21.12 m (69.3 ft) SSE of NO 1 1974, 14.39
m (47.2 ft) SSE of the triangulation station, 0.18 m (0.6 ft) NNW of a witness
post.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 8724139 PUBLICATION DATE: 02/26/2004
Name: HORESHOE KEYS
FLORIDA
NOAA Chart: 11442 Latitude: 24° 46.0' N
USGS Quad: HORSESHOE KEY Longitude: 81° 17.0' W
T I D A L D A T U M S
Tidal datums at HORESHOE KEYS based on:
LENGTH OF SERIES: 1 MONTH
TIME PERIOD: June 1974 - June 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) = 0.462
MEAN HIGH WATER (MHW) = 0.398
MEAN TIDE LEVEL (MTL) = 0.236
MEAN SEA LEVEL (MSL) = 0.216
MEAN LOW WATER (MLW) = 0.074
MEAN LOWER LOW WATER (MLLW) = 0.000
National Geodetic Vertical Datum (NGVD 29)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
HORSESHOE KEY 2 1909 1935 0.588 0.190
BM 1 1974 0.611 0.213
BM 2 1974 0.559 0.161
HORSESHOE KEY 2 NO 1 1909 1935 0.619 0.221
HORSESHOE KEY 2 NO 2 1909 1935 0.648 0.250
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 8724139 PUBLICATION DATE: 02/26/2004
Name: HORESHOE KEYS
FLORIDA
NOAA Chart: 11442 Latitude: 24° 46.0' N
USGS Quad: HORSESHOE KEY Longitude: 81° 17.0' 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 (NAVD 88).
NGVD 29 is a fixed datum adopted as a national standard geodetic reference for
heights but is now considered superseded. NGVD 29 is sometimes referred to as
Sea Level Datum of 1929 or as Mean Sea Level on some early issues of Geological
Survey Topographic Quads. NGVD 29 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 NGVD 29 are available from the National
Geodetic Survey (NGS) data base via the World Wide Web at
National Geodetic Survey.
NAVD 88 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. NAVD 88 replaces NGVD 29 as the national
standard geodetic reference for heights. Bench mark elevations relative to
NAVD 88 are available from NGS through the World Wide Web at
National Geodetic Survey.
NGVD 29 and NAVD 88 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 NAVD 88 elevation is shown on the Elevations of Tidal Datums Table Referred
to MLLW only when two or more of the bench marks listed have NAVD 88 elevations.
The NAVD 88 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, NAVD 88 bench mark elevations computed indirectly from the tidal
datums elevation table may differ slightly from NAVD 88 elevations listed for
each bench mark in the NGS database.