U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
Datums Page                                                        Page  1 of  5

                                                                                
Station ID: 8724246                         PUBLICATION DATE:  03/09/2004
Name:       BIG SPANISH KEY                                   
            FL
NOAA Chart: 11448                           Latitude:   24° 47.3' N (  24.78830)
USGS Quad:  CONTENT KEYS                    Longitude:  81° 24.7' W ( -81.41170)


To reach the tidal bench marks from the U.S. Highway 1 bridge over Bahia Honda,
proceed by boat NW on Bahia Honda and Big Spanish Channel for 19 km (12 statute
mi) to channel marker 36 and the SW shore of Big Spanish Key.  The bench marks
are located on the SW side of the Key.  The tide gage and staff were located
offshore.


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


                 PRIMARY BENCH MARK STAMPING:  NO 1 1973
                         DESIGNATION:          872 4246 TIDAL 1

MONUMENTATION:           Tidal Station disk                    VM#:         6855
AGENCY:                  National Ocean Survey (NOS)           IDB PID#:        
SETTING CLASSIFICATION:  Copper-clad steel rod                 OPUS PID:        
LATITUDE:   24° 47.3' N (  24.78830)          LONGITUDE:  81° 24.7' W ( -81.41170)


The primary bench mark is a disk located near the center of the stretch
constituting the SW shore of Big Spanish Key, 10.76 m (35.3 ft) NW of bench mark
NO 3 1973, 7 m (24 ft) NE of the SW side of the key, 0.30 m (1.0 ft) west of an
old witness post, and 0.30 m (1.0 ft) SW of a new witness post.  The bench mark
is set at ground level, crimped to the top of a copper-clad steel rod driven an
unspecified depth to refusal.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  2 of  5

                                                                                
Station ID: 8724246                         PUBLICATION DATE:  03/09/2004
Name:       BIG SPANISH KEY                                   
            FL
NOAA Chart: 11448                           Latitude:   24° 47.3' N (  24.78830)
USGS Quad:  CONTENT KEYS                    Longitude:  81° 24.7' W ( -81.41170)


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


                         BENCH MARK STAMPING:  NO 2 1973
                         DESIGNATION:          872 4246 TIDAL 2

MONUMENTATION:           Tidal Station disk                    VM#:         6856
AGENCY:                  National Ocean Survey (NOS)           IDB PID#:        
SETTING CLASSIFICATION:  Copper-clad steel rod                 OPUS PID:        
LATITUDE:   24° 47.3' N (  24.78830)          LONGITUDE:  81° 24.7' W ( -81.41170)


The bench mark is a disk located 20 m (67 ft) NE of the SW side of the Key,
14.78 m (48.5 ft) ESE of bench mark NO 1 1973, 1.98 m (6.5 ft) NNW of a witness
sign nailed to a tree, and 0.30 m (1.0 ft) SW of a witness post.  The bench mark
is set at ground level, crimped to the top of a copper-clad steel rod driven an
unspecified depth to refusal.



                         BENCH MARK STAMPING:  NO 3 1973
                         DESIGNATION:          872 4246 TIDAL 3

MONUMENTATION:           Tidal Station disk                    VM#:         6857
AGENCY:                  National Ocean Survey (NOS)           IDB PID#:        
SETTING CLASSIFICATION:  Copper-clad steel rod                 OPUS PID:        
LATITUDE:   24° 47.3' N (  24.78830)          LONGITUDE:  81° 24.7' W ( -81.41170)


The bench mark is a disk located near the center of the stretch constituting the
SW shore of Big Spanish Key, 11.28 m (37.0 ft) SW of bench mark NO 2 1973, 9 m
(30 ft) NE of the SW side of the Key, 1.92 m (6.3 ft) north of a witness sign
nailed to a tree, and 0.30 m (1.0 ft) SW of a witness post.  The bench mark is
set at ground level, crimped to the top of a copper-clad steel rod driven an
unspecified depth to refusal.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  3 of  5

                                                                                
Station ID: 8724246                         PUBLICATION DATE:  03/09/2004
Name:       BIG SPANISH KEY                                   
            FL
NOAA Chart: 11448                           Latitude:   24° 47.3' N (  24.78830)
USGS Quad:  CONTENT KEYS                    Longitude:  81° 24.7' W ( -81.41170)


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


                         BENCH MARK STAMPING:  4246 D 1984
                         DESIGNATION:          872 4246 D

MONUMENTATION:           Tidal Station disk                    VM#:         6858
AGENCY:                  National Ocean Service (NOS)          IDB PID#:        
SETTING CLASSIFICATION:  Stainless steel rod                   OPUS PID:        
LATITUDE:   24° 47.3' N (  24.78830)          LONGITUDE:  81° 24.7' W ( -81.41170)


The bench mark is a disk located near the center of the SW shore of the Key,
13.87 m (45.5 ft) NW of bench mark NO 1 1973, 7 m (24 ft) NE of the outer edge
of the mangroves, and 0.30 m (1.0 ft) SW of a witness post.  The bench mark is
set 0.73 m (2.4 ft) above ground level, crimped to a stainless steel rod driven
an unspecified depth to refusal, and encased in a 6-inch PVC pipe which projects
0.85 m (2.8 ft) above ground level.



                         BENCH MARK STAMPING:  4246 E 1984
                         DESIGNATION:          872 4246 E

MONUMENTATION:           Tidal Station disk                    VM#:         6859
AGENCY:                  National Ocean Service (NOS)          IDB PID#:        
SETTING CLASSIFICATION:  Stainless steel rod                   OPUS PID:        
LATITUDE:   24° 47.3' N (  24.78830)          LONGITUDE:  81° 24.7' W ( -81.41170)


The bench mark is a disk located near the center of the SW shore of the Key,
16.00 m (52.5 ft) NW of bench mark NO 3 1973, 8.72 m (28.6 ft) SSE of bench mark
4246 D 1984, 6.49 m (21.3 ft) WNW of bench mark NO 1 1973, 5 m (15 ft) NE of the
outer edge of the mangroves, and 0.30 m (1.0 ft) SW of a witness post.  The
bench mark is set 0.76 m (2.5 ft) above ground level, crimped to a stainless
steel rod driven an unspecified depth to refusal, and encased in a 6-inch PVC
pipe which projects 0.82 m (2.7 ft) above ground level.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  4 of  5

                                                                                
Station ID: 8724246                         PUBLICATION DATE:  03/09/2004
Name:       BIG SPANISH KEY                                   
            FL
NOAA Chart: 11448                           Latitude:   24° 47.3' N (  24.78830)
USGS Quad:  CONTENT KEYS                    Longitude:  81° 24.7' W ( -81.41170)


                            T I D A L   D A T U M S 


Tidal datums at BIG SPANISH KEY based on:

     LENGTH OF SERIES:      1 MONTH
     TIME PERIOD:           January 1974 - January 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   =  0.980
     MEAN HIGH WATER                            MHW    =  0.901
     MEAN TIDE LEVEL                            MTL    =  0.502
     MEAN SEA LEVEL                             MSL    =  0.482
     MEAN LOW WATER                             MLW    =  0.103
     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 1973                                   0.426   -0.475
     NO 2 1973                                   0.520   -0.381
     NO 3 1973                                   0.562   -0.339
     4246 D 1984                                 1.045    0.144
     4246 E 1984                                 1.074    0.173


                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  5 of  5

                                                                                
Station ID: 8724246                         PUBLICATION DATE:  03/09/2004
Name:       BIG SPANISH KEY                                   
            FL
NOAA Chart: 11448                           Latitude:   24° 47.3' N (  24.78830)
USGS Quad:  CONTENT KEYS                    Longitude:  81° 24.7' W ( -81.41170)


                             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.