U.S. DEPARTMENT OF COMMERCE
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National Ocean Service
Datums Page Page 1 of 5
Station ID: 9462721 PUBLICATION DATE: 12/06/2011
Name: TRIDENT BAY, AKUN ISLAND
AK
NOAA Chart: 16531 Latitude: 54° 8.4' N ( 54.14000)
USGS Quad: UNIMAK (A-5) Longitude: 165° 31.6' W (-165.52700)
To reach the bench marks from the Dutch Harbor City Dock, proceed NNE 14.0 km
(8.7 mi) to Priest Rock, then proceed east for 33.6 km (20.9 mi) crossing Akutan
Pass to Battery Point on the southern point of Akutan Island, then proceed 26.7
km (16.6 mi) ENE thru Avatanak Strait to the west end of Poa Island, then
proceed NW 1.6 km (1.0 mi) to the west end of an unnamed island that is 0.5 km
(0.3 mi) long, also referred to as Bird Rock Island. The bench marks are on
either side of a 3 m (10 ft) channel between an islet and the western end of
Bird Rock Island. The tide gauge and staff are located on the SE side of the
islet.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 1 1934
DESIGNATION: 946 2721 TIDAL 1
MONUMENTATION: Bench Mark disk VM#: 20181
AGENCY: US Coast and Geodetic Survey (USCGS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID:
LATITUDE: 54° 8.3' N ( 54.13889) LONGITUDE: 165° 31.5' W (-165.52581)
The primary bench mark is a disk set in a bedrock outcrop located on the east
side of an islet midway along the channel between the islet and the west end of
an unnamed island, 37.67 m (123.6 ft) SSW of 946 2721 TIDAL 3, 12.15 m (39.9 ft)
SW from the face of a 10.00 m (32.8 ft) high rock cliff at the south end of a
boulder beach, 8.14 m (26.7 ft) south from the south end of the rock in the
north end of the channel, and 1.86 m (6.1 ft) east of the west wall of the
islet.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 9462721 PUBLICATION DATE: 12/06/2011
Name: TRIDENT BAY, AKUN ISLAND
AK
NOAA Chart: 16531 Latitude: 54° 8.4' N ( 54.14000)
USGS Quad: UNIMAK (A-5) Longitude: 165° 31.6' W (-165.52700)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 3 1934
DESIGNATION: 946 2721 TIDAL 3
MONUMENTATION: Bench Mark disk VM#: 20183
AGENCY: US Coast and Geodetic Survey (USCGS) IDB PID#:
SETTING CLASSIFICATION: Boulder OPUS PID:
LATITUDE: 54° 8.4' N ( 54.13922) LONGITUDE: 165° 31.5' W (-165.52564)
The bench mark is a disk set on top of a 2 m (6 ft) high boulder at the north
end of a boulder beach on the west end of an unnamed island, 37.67 m (123.6 ft)
NE of 946 2721 TIDAL 1, 31.46 m (103.2 ft) NE of bench mark 946 2721 B, 19.17 m
(62.9 ft) NNW of bench mark 946 2721 C, 13.34 m (43.8 ft) NW of the bedrock
outcrop in the center of the beach and 3.41 m (11.2 ft) SSW of the south side of
the rock wall at the northern end of the beach.
BENCH MARK STAMPING: 2721 A 2011
DESIGNATION: 946 2721 A
MONUMENTATION: Tidal Station disk VM#: 20184
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID#: BBCL02
LATITUDE: 54° 8.3' N ( 54.13883) LONGITUDE: 165° 31.6' W (-165.52614)
The bench mark is a disk set in a bedrock outcrop at the south end of an islet
which lies off the west end of an unnamed island, 4.95 m (16.2 ft) SW of a rock
high point adjacent a 5.00 m (16.4 ft) deep cut in the islet, 2.20 m (7.2 ft) SE
of the highest point at the south end of the islet, and 0.40 m (1.3 ft) SE of
the east rock face of the southerly high point.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 9462721 PUBLICATION DATE: 12/06/2011
Name: TRIDENT BAY, AKUN ISLAND
AK
NOAA Chart: 16531 Latitude: 54° 8.4' N ( 54.14000)
USGS Quad: UNIMAK (A-5) Longitude: 165° 31.6' W (-165.52700)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 2721 B 2011
DESIGNATION: 946 2721 B
MONUMENTATION: Tidal Station disk VM#: 20185
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID:
LATITUDE: 54° 8.3' N ( 54.13908) LONGITUDE: 165° 31.6' W (-165.52597)
The bench mark is a disk set in a bedrock outcrop on the north end of an islet
which lies off the west end of an unnamed island, 31.46 m (103.2 ft) SW of 946
2721 TIDAL 3, 25.17 m (82.6 ft) west of bench mark 946 2721 C, and 6.49 m (21.3
ft) west of the north end of the rock in the north end of the channel.
BENCH MARK STAMPING: 2721 C 2011
DESIGNATION: 946 2721 C
MONUMENTATION: Tidal Station disk VM#: 20186
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock outcrop OPUS PID:
LATITUDE: 54° 8.3' N ( 54.13908) LONGITUDE: 165° 31.5' W (-165.52564)
The bench mark is disk set in a 5 m (16 ft) long rock outcrop in the southern
half a boulder beach on the west end of an unnamed island, 19.17 m (62.9 ft) SE
of bench mark 946 2721 TIDAL 3, 18.67 m (61.2 ft) NE of the north end of the
rock in the channel between the island and the islet, 11.43 m (37.5 ft) NE of
the cliff face at the south end of the boulder beach, 6.95 m (22.8 ft) south
from the seaward end of a rock outcrop in the center of the beach, 1.86 m (6.0
ft) NW of the vegetation line, and 0.50 m (1.6 ft) above the beach.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 9462721 PUBLICATION DATE: 12/06/2011
Name: TRIDENT BAY, AKUN ISLAND
AK
NOAA Chart: 16531 Latitude: 54° 8.4' N ( 54.14000)
USGS Quad: UNIMAK (A-5) Longitude: 165° 31.6' W (-165.52700)
T I D A L D A T U M S
Tidal datums at TRIDENT BAY, AKUN ISLAND based on:
LENGTH OF SERIES: 1 MONTH
TIME PERIOD: August 2011 - August 2011
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 9462620 UNALASKA, DUTCH HARBOR
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 1.259
MEAN HIGH WATER MHW = 1.136
MEAN SEA LEVEL MSL = 0.674
MEAN TIDE LEVEL MTL = 0.653
MEAN LOW WATER MLW = 0.170
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
1 1934 1.999 0.863
3 1934 2.215 1.079
2721 A 2011 4.893 3.757
2721 B 2011 1.907 0.771
2721 C 2011 1.945 0.809
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 9462721 PUBLICATION DATE: 12/06/2011
Name: TRIDENT BAY, AKUN ISLAND
AK
NOAA Chart: 16531 Latitude: 54° 8.4' N ( 54.14000)
USGS Quad: UNIMAK (A-5) Longitude: 165° 31.6' W (-165.52700)
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.