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National Ocean Service
Datums Page Page 1 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
To reach the tidal bench marks from Chignik via boat, proceed 6.0 km (3.2 nm) NE
out of Anchorage Bay, then 15 km (8 nm) ESE to Jack Point, then 98 km (53 nm)
ESE to the NE corner of the northern bight in the embayment on the NW side of
Chowiet Island. This site is 1.2 km (0.6 nm) NW of the 1925 NOS historic tide
station location. The tidal bench marks are located along the beach.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 8519 B 2001
DESIGNATION: 945 8519 B
MONUMENTATION: Tidal Station disk VM#: 16600
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock outcrop OPUS PID#: BBFM57
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The primary bench mark is a disk set at the base of a rock outcrop with broken
boulders protruding seaward, northwesterly of a boulder beach with the remains
of a ship wreck, 1200 m (3937 ft) NE of bench mark 1 1925, 74.2 m (243.4 ft) ESE
of the highest point on a group of rock islands awash at high tide, 16.14 m
(53.0 ft) south of bench mark 8519 C 2001, 12.84 m (42.1 ft) NW of bench mark
8519 A 2001, and 8.4 m (27.6 ft) east of the most seaward point of the outcrop.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 8519 A 2001
DESIGNATION: 945 8519 A
MONUMENTATION: Tidal Station disk VM#: 16599
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock ledge OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set in a rock ledge protruding seaward, northwesterly
of a boulder beach with the remains of a metal ship wreck, 86.70 m (284.4 ft)
ESE of the highest point on a group of rocks awash at high tide, 83.03 m (272.4
ft) SSW of bench mark 8519 E 2001, 18.90 m (62.0 ft) ESE of the furthest seaward
point of the ledge, 12.84 m (42.1 ft) SE of bench mark 8519 B 2001, 1.48 m (4.9
ft) south of a vertical rock face, 0.92 m (3.0 ft) north of the edge of the rock
ledge, and 2.0 m (6.6 ft) below the vegetation line.
BENCH MARK STAMPING: 8519 C 2001
DESIGNATION: 945 8519 C
MONUMENTATION: Tidal Station disk VM#: 16601
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock ledge OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set in a rock ledge, north of a crevice in a rock face,
northwesterly of a boulder beach with the remains of a ship wreck, 18.55 (60.9
ft) SSW of bench mark 8519 D 2001, 16.14 m (53.0 ft) north of bench mark 8519 B
2001, 15.00 m (49.2 ft) north of a rock point, and 1.70 m (5.6 ft) north of the
edge of the rock ledge.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 8519 D 2001
DESIGNATION: 945 8519 D
MONUMENTATION: Tidal Station disk VM#: 16602
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock ledge OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set in a rock ledge south of a boulder beach, 42.49 m
(139.4 ft) SSW of bench mark 8519 E 2001, 23.40 m (76.8 ft) south of a rock
point proturding into the water, 18.55 m (60.8 ft) NNE of bench mark 8519 C
2001, and 15.4 m (50.5 ft) SW of a column of rock at the base of a small ravine.
BENCH MARK STAMPING: 8519 E 2001
DESIGNATION: 945 8519 E
MONUMENTATION: Tidal Station disk VM#: 16603
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Rock ledge OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set at the point of a rock ridge bearing east/west with
a boulder beach on either side, 83.03 m (272.4 ft) NNE of bench mark 8519 A
2001, 42.49 m (139.4 ft) NNE of bench mark 8519 D 2001, 39.80 m (130.6 ft) south
of the next rock point, and 4 m (13 ft) above the boulder beach.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING:
DESIGNATION: 945 8519 BM 1
ALIAS: BM 1
MONUMENTATION: Bench Mark disk VM#: 16606
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#:
SETTING CLASSIFICATION: Rock outcrop OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set in a crack in a rock outcrop, 13.4 m (44.0 ft) west
of the east end of the rock outcrop, 3.0 m (9.8 ft) east of the west edge of the
rock outcrop, 2.25 m (7.4 ft) WNW of old steel eyebolt, 2.1 m (6.9 ft) north of
the south edge of the rock outcrop, 0.3 m (1 ft) lower than the highest point of
the rock outcrop, and awash at high tide.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING:
DESIGNATION: 945 8519 BM 2
ALIAS: BM 2
MONUMENTATION: Bench Mark disk VM#: 16607
AGENCY: US Coast and Geodetic Survey
(USC&GS) IDB PID#:
SETTING CLASSIFICATION: Rock outcrop OPUS PID:
LATITUDE: 56° 3.1' N ( 56.05170) LONGITUDE: 156° 41.9' W (-156.69800)
The bench mark is a disk set in an east-west crack on top of a rock outcrop, 25
m (82 ft) north of the base of a rock wall, 2.45 m (8.0 ft) west of the east
edge of the rock outcrop, 1.92 m (6.3 ft) east of the west edge of the rock
outcrop, 1.45 m (4.8 ft) south of the north edge of the rock outcrop, 0.65 m
(2.1 ft) north of the south edge of the rock outcrop, and awash at higher high
tide.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 6 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
T I D A L D A T U M S
Tidal datums at CHOWIET ISLAND, SEMIDI ISLAND based on:
LENGTH OF SERIES: 1 Month
TIME PERIOD: July 2001 - July 2001
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 9459450 SAND POINT, POPOF ISLAND
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 2.729
MEAN HIGH WATER MHW = 2.488
MEAN TIDE LEVEL MTL = 1.460
MEAN SEA LEVEL MSL = 1.432
MEAN LOW WATER MLW = 0.432
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
8519 B 2001 9.278 6.790
8519 A 2001 9.858 7.370
8519 C 2001 7.187 4.699
8519 D 2001 6.406 3.918
8519 E 2001 6.146 3.658
945 8519 BM 1 3.670 1.182
945 8519 BM 2 3.208 0.720
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 7 of 7
Station ID: 9458519 PUBLICATION DATE: 04/21/2003
Name: CHOWIET ISLAND, SEMIDI ISLAND
AK
NOAA Chart: 16566 Latitude: 56° 3.1' N ( 56.05170)
USGS Quad: CHIGNIK (A-1) Longitude: 156° 41.9' W (-156.69800)
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.