U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
Datums Page                                                        Page  1 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


To reach the tidal bench marks from No Name Key, proceed north by boat 3.9 km
(2.4 statute mi) to the west point of Water Key.  The bench marks are located on
the west point in the center clearing surrounded by mangrove bushes.  The tide
gage and staff were located north of the west point of the key.


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


                 PRIMARY BENCH MARK STAMPING:  NO 1 1935
                         DESIGNATION:          872 4189 TIDAL 1

MONUMENTATION:           Tidal Station disk                    VM#:         6409
AGENCY:                  US Coast and Geodetic Survey
                         (USC&GS)                              IDB PID#:        
SETTING CLASSIFICATION:  Concrete monument                     OPUS PID:        
LATITUDE:   24° 44.4' N (  24.74000)          LONGITUDE:  81° 20.5' W ( -81.34170)


The primary bench mark is a disk set in a concrete monument located at the
northwest tip of the west end of the key, 30 m (98 ft) NNW of bench mark NO 5
1974, 0.30 m (1.0 ft) SE of a wooden witness post with a sign attached, and 0.09
m (0.3 ft) above ground level.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  2 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


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


                         BENCH MARK STAMPING:  GRASSY 1935
                         DESIGNATION:          GRASSY

MONUMENTATION:           Gravity Station disk                  VM#:         6406
AGENCY:                  US Coast and Geodetic Survey
                         (USC&GS)                              IDB PID#:  AA1529
SETTING CLASSIFICATION:  Set in concrete in a clay tile pipe   OPUS PID:        
LATITUDE:   24° 44.3' N (  24.73830)          LONGITUDE:  81° 20.5' W ( -81.34170)


The bench mark is a disk set in concrete in the top of a 0.15 m (0.5 ft)
diameter terra cotta pipe, 100 m (328 ft) NE of the extreme southerwesten point,
30 m (98 ft) NW of the south side of the key, 0.18 m (0.6 ft) east of a witness
post, and 0.09 m (0.3 ft) above ground level.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  3 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


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


                         BENCH MARK STAMPING:  GRASSY NO 1 1935
                         DESIGNATION:          GRASSY RM 1

MONUMENTATION:           Reference Mark disk                   VM#:         6407
AGENCY:                  US Coast and Geodetic Survey
                         (USC&GS)                              IDB PID#:  AE4393
SETTING CLASSIFICATION:  Concrete monument                     OPUS PID:        
LATITUDE:   24° 44.4' N (  24.74000)          LONGITUDE:  81° 20.5' W ( -81.34170)


The bench mark is a disk set in a concrete monument, 18.32 m (60.1 ft) NE of
bench mark GRASSY NO 2 1935, 12.59 m (41.3 ft) north of bench mark GRASSY 1935,
0.30 m (1.0 ft) south of a witness post, and 0.03 m (0.1 ft) above ground level.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  4 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


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


                         BENCH MARK STAMPING:  GRASSY NO 2 1935
                         DESIGNATION:          GRASSY RM 2

MONUMENTATION:           Reference Mark disk                   VM#:         6408
AGENCY:                  US Coast and Geodetic Survey
                         (USC&GS)                              IDB PID#:  AE4394
SETTING CLASSIFICATION:  Concrete monument                     OPUS PID:        
LATITUDE:   24° 44.4' N (  24.74000)          LONGITUDE:  81° 20.5' W ( -81.34170)


The bench mark is a disk set in a concrete monument, 13.26 m (43.5 ft) west of
bench mark GRASSY 1935, 0.24 m (0.8 ft) WSW of a witness post, and 3 cm (0.1 ft)
above ground level.



                         BENCH MARK STAMPING:  NO 5 1974
                         DESIGNATION:          872 4189 TIDAL 5

MONUMENTATION:           Tidal Station disk                    VM#:         6410
AGENCY:                  National Ocean Survey (NOS)           IDB PID#:        
SETTING CLASSIFICATION:  Copper-clad steel rod                 OPUS PID:        
LATITUDE:   24° 44.4' N (  24.74000)          LONGITUDE:  81° 20.5' W ( -81.34170)


The bench mark is a disk located 56 m (185 ft) NNW of bench mark GRASSY 1935, 30
m (100 ft) SE of bench mark NO 1 1935, 4.42 m (14.5 ft) NNE of a witness sign
nailed to a tree, and 0.30 m (1.0 ft) SE of a witness post.  The mark is set
0.09 m (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 concrete.

                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  5 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


                            T I D A L   D A T U M S 


Tidal datums at WATER KEY, BIG SPANISH CHANNEL 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   =  0.444
     MEAN HIGH WATER                            MHW    =  0.372
     MEAN TIDE LEVEL                            MTL    =  0.223
     MEAN SEA LEVEL                             MSL    =  0.222
     MEAN LOW WATER                             MLW    =  0.074
     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 1935                                   0.467    0.095
     GRASSY 1935                                 0.642    0.270
     GRASSY NO 1 1935                            0.622    0.250
     GRASSY NO 2 1935                            0.551    0.179
     NO 5 1974                                   0.571    0.199


                          U.S. DEPARTMENT OF COMMERCE
                National Oceanic and Atmospheric Administration
                            National Ocean Service
                                                                   Page  6 of  6

                                                                                
Station ID: 8724189                         PUBLICATION DATE:  09/15/2004
Name:       WATER KEY, BIG SPANISH CHANNEL                    
            FL
NOAA Chart: 11448                           Latitude:   24° 44.4' N (  24.74000)
USGS Quad:  BIG PINE KEY                    Longitude:  81° 20.5' W ( -81.34170)


                             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.