U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Datums Page Page 1 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
To reach the tidal bench marks from the bridge linking Big Pine Key and No Name
Key, proceed NNW by boat for 2.6 km (1.6 statute mi) to the east side of
Porpoise Key. The bench marks are located on the SE end of the Key in an open
grassy field. The tide gage and staff were located offshore, east of the
eastern shoreline.
T I D A L B E N C H M A R K S
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: PORPOISE 1935
DESIGNATION: PORPOISE KEY
MONUMENTATION: Triangulation Station disk VM#: 8496
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#: AA1531
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 24° 43.2' N ( 24.72000) LONGITUDE: 81° 21.1' W ( -81.35170)
The primary bench mark is a disk set flush in a concrete monument, 100 m (328
ft) north of the southern shore of the key, 80 m (262 ft) NE of the SW
shoreline, 29 m (95 ft) west of the eastern shoreline, 0.30 m (1.0 ft) west of a
witness post, and 0.21 m (0.7 ft) above ground level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: PORPOISE 1935 RM 1
DESIGNATION: PORPOISE KEY RM 1
MONUMENTATION: Reference Mark disk VM#: 8497
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#: AE4395
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 24° 43.1' N ( 24.71830) LONGITUDE: 81° 21.1' W ( -81.35170)
The bench mark is a disk set flush in a concrete monument, 23.59 m (77.4 ft) WSW
of bench mark PORPOISE 1935, 3.20 m (10.5 ft) west of a witness sign nailed to a
tree, 0.30 m (1.0 ft) west of a witness post, and 0.21 m (0.7 ft) above ground
level.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: PORPOISE 1935 RM 2
DESIGNATION: PORPOISE KEY RM 2
MONUMENTATION: Reference Mark disk VM#: 8498
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#: AE4396
SETTING CLASSIFICATION: Concrete monument OPUS PID:
LATITUDE: 24° 43.1' N ( 24.71830) LONGITUDE: 81° 21.1' W ( -81.35170)
The bench mark is a disk set flush in a concrete monument, 33.41 m (109.6 ft)
NNW of bench mark PORPOISE 1935 RM 1, 27.49 m (90.2 ft) NNW of bench mark
PORPOISE 1935, 0.30 m (1.0 ft) east of a witness post, and 0.21 m (0.7 ft) above
ground level.
BENCH MARK STAMPING: NO 3 1974
DESIGNATION: 872 4196 TIDAL 3
MONUMENTATION: Tidal Station disk VM#: 8499
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 43.1' N ( 24.71830) LONGITUDE: 81° 21.1' W ( -81.35170)
The bench mark is a disk located on the east edge of the clearing, 128 m (420
ft) north of the southern tip of the Key, 34 m (113 ft) NNE of bench mark
PORPOISE 1935, 30 m (99 ft) west of the outer eastern mangrove edge, 24.20 m
(79.4 ft) east of bench mark PORPOISE 1935 RM 2, 0.40 m (1.3 ft) north of a wood
fence post, and 0.24 m (0.8 ft) west of a witness post. The bench mark is set
level with 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 5 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: NO 4 1974
DESIGNATION: 872 4196 TIDAL 4
MONUMENTATION: Tidal Station disk VM#: 8500
AGENCY: National Ocean Survey (NOS) IDB PID#:
SETTING CLASSIFICATION: Copper-clad steel rod OPUS PID:
LATITUDE: 24° 43.1' N ( 24.71830) LONGITUDE: 81° 21.1' W ( -81.35170)
The bench mark is a disk located 157 m (516 ft) north of the southern tip of the
Key, 28.93 m (94.9 ft) north of bench mark PORPOISE 1935 RM 2, and 0.46 m (1.5
ft) south of a witness post. The bench mark is set 3 cm (0.1 ft) above ground
level, 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 6 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
T I D A L D A T U M S
Tidal datums at PORPOISE KEY based on:
LENGTH OF SERIES: 1 MONTH
TIME PERIOD: August 1974 - August 1974
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8724211 BIG PINE KEY VIADUCT
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 0.408
MEAN HIGH WATER MHW = 0.332
MEAN SEA LEVEL MSL = 0.213
MEAN TIDE LEVEL MTL = 0.202
MEAN LOW WATER MLW = 0.072
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
PORPOISE 1935 0.643 0.311
PORPOISE 1935 RM 1 0.639 0.307
PORPOISE 1935 RM 2 0.760 0.428
NO 3 1974 0.429 0.097
NO 4 1974 0.442 0.110
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 7 of 7
Station ID: 8724196 PUBLICATION DATE: 03/09/2004
Name: PORPOISE KEY
FL
NOAA Chart: 11445 Latitude: 24° 43.1' N ( 24.71830)
USGS Quad: BIG PINE KEY Longitude: 81° 21.1' W ( -81.35170)
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.