Superseded Bench Mark Sheet for 8724507, CHANNEL KEY FL

Present Epoch

Printable version

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
Datums Page                                                        Page  1 of  4

                                                   PUBLICATION DATE:  09/14/1987
Station ID: 8724507                                SUPERCEDED DATE:   04/21/2003
Name:       CHANNEL KEY, FLORIDA BAY                          
            FLORIDA
NOAA Chart: 11441                                  Latitude:         24° 36.2' N
USGS Quad:  BOCA CHICA KEY                         Longitude:        81° 43.5' W


To reach the tidal bench marks from the west end of the bridge between Stock
Island and Boca Chica Key, proceed NNW by boat for approximately 1.9 statute
miles (3.1 km) to the west entrance of a small channel, locally known as Channel
Grassy, dividing Channel Key.  The bench marks are located in the NW section of
the southern portion of the key.  The tide gage and staff were located at the
western entrance to Channel Grassy.


                         T I D A L   B E N C H   M A R K S


                 PRIMARY BENCH MARK STAMPING:  CHANNEL KEY 2 1934

MONUMENTATION:           Survey Disk                               VM#:    11306
AGENCY:                                                            PID#:  AA1096
SETTING CLASSIFICATION:  Concrete Monument


The bench mark is set in a 1-foot (0.3 m) square concrete monument on a pile on
the west side of the key, about 246 feet (75 m) SE of the entrance to Channel
Grassy, about 82 feet (25 m) NE of the SW side of the key, 0.5 foot (0.2 m) west
of a witness post, and 1 foot (0.3 m) above the ground.



                         BENCH MARK STAMPING:  BM NO 2 1973

MONUMENTATION:           Survey Disk                               VM#:    11020
AGENCY:                                                            PID:         
SETTING CLASSIFICATION:  Concrete Monument


The bench mark is set on top of a 0.5-foot (0.2 m) square concrete monument on
the west side of the key, about 225 feet (69 m) SSE of the entrance to Channel
Grassy, 55.2 feet (16.8 m) WSW of Bench Mark CHANNEL KEY 2 RM 2 1934, 36.0 feet
(11.0 m) NNW of Bench Mark CHANNEL KEY 2 RM 1 1934, and 0.5 foot (0.2 m) NW of a
witness post.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  2 of  4

                                                   PUBLICATION DATE:  09/14/1987
Station ID: 8724507                                SUPERCEDED DATE:   04/21/2003
Name:       CHANNEL KEY, FLORIDA BAY                          
            FLORIDA
NOAA Chart: 11441                                  Latitude:         24° 36.2' N
USGS Quad:  BOCA CHICA KEY                         Longitude:        81° 43.5' W


                         T I D A L   B E N C H   M A R K S


                         BENCH MARK STAMPING:  CHANNEL KEY 2 RM 1 1934

MONUMENTATION:           Survey Disk                               VM#:    11307
AGENCY:                                                            PID#:  AE4421
SETTING CLASSIFICATION:  Concrete Monument


The bench mark is set in a 1-foot (0.3 m) square concrete monument on a pile on
the west side of the key, about 295 feet (90 m) SE of the entrance to Channel
Grassy, about 92 feet (28 m) NE of the SW side of the key, 46.7 feet (14.2 m)
ESE of Bench Mark CHANNEL KEY 2 1934, 0.8 foot (0.2 m) NW of a witness post, and
about 1 foot (0.3 m) above the ground.



                         BENCH MARK STAMPING:  CHANNEL KEY 2 RM 2 1934

MONUMENTATION:           Survey Disk                               VM#:    11308
AGENCY:                                                            PID#:  AE4422
SETTING CLASSIFICATION:  Concrete Monument


The bench mark is set in a 1-foot (0.3 m) square concrete monument on a pile on
the west side of the key, about 295 feet (90 m) SSE of the entrance to Channel
Grassy, 57.6 feet (17.6 m) SW of Bench Mark CHANNEL KEY 2 1934, 13 feet (4 m)
east of the west side of the key, 0.5 foot (0.2 m) NNW of a witness post, and
about 1 foot (0.3 m) above the ground.



                         BENCH MARK STAMPING:  NO 1 1973

MONUMENTATION:           Survey Disk                               VM#:    11309
AGENCY:                                                            PID:         
SETTING CLASSIFICATION:  Concrete/Rod Monument


The bench mark is set in concrete atop a rod driven 8 feet (2 m) to refusal on
the west side of the key, 94 feet (29 m) south of Channel Grassy, 55.7 feet
(17.0 m) WNW of Bench Mark CHANNEL KEY 2 1934, 0.6 foot (0.2 m) NNW of a witness
post, and 0.5 foot (0.2 m) above the ground.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  3 of  4

                                                   PUBLICATION DATE:  09/14/1987
Station ID: 8724507                                SUPERCEDED DATE:   04/21/2003
Name:       CHANNEL KEY, FLORIDA BAY                          
            FLORIDA
NOAA Chart: 11441                                  Latitude:         24° 36.2' N
USGS Quad:  BOCA CHICA KEY                         Longitude:        81° 43.5' W


                            T I D A L   D A T U M S 


Tidal datums at CHANNEL KEY, FLORIDA BAY based on:

     LENGTH OF SERIES:      4 MONTHS
     TIME PERIOD:           JANUARY-APRIL 1974
     TIDAL EPOCH:           1960-1978
     CONTROL TIDE STATION:  8724211 BIG PINE KEY VIADUCT


Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in FEET:

     HIGHEST OBSERVED WATER LEVEL (12/16/1973)         =   2.56
     MEAN HIGHER HIGH WATER                     MHHW   =   1.37
     MEAN HIGH WATER                            MHW    =   1.08
     MEAN TIDE LEVEL                            MTL    =   0.62
     MEAN LOW WATER                             MLW    =   0.17
     MEAN LOWER LOW WATER                       MLLW   =   0.00
     LOWEST  OBSERVED WATER LEVEL (12/23/1973)         =  -0.55

North American Vertical Datum (NAVD88)

Bench Mark Elevation Information                In FEET above:

     Stamping or Designation                    MLLW        MHW

     CHANNEL KEY 2 1934                          1.70      0.62
     BM NO 2 1973                                1.55      0.47
     CHANNEL KEY 2 RM 1 1934                     1.65      0.57
     CHANNEL KEY 2 RM 2 1934                     1.85      0.77
     NO 1 1973                                   1.13      0.05


                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  4 of  4

                                                   PUBLICATION DATE:  09/14/1987
Station ID: 8724507                                SUPERCEDED DATE:   04/21/2003
Name:       CHANNEL KEY, FLORIDA BAY                          
            FLORIDA
NOAA Chart: 11441                                  Latitude:         24° 36.2' N
USGS Quad:  BOCA CHICA KEY                         Longitude:        81° 43.5' 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.

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