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National Ocean Service
Datums Page Page 1 of 5
Station ID: 9454671 PUBLICATION DATE: 06/08/2011
Name: POINT HELEN, KNIGHT ISLAND
AK
NOAA Chart: 16701 Latitude: 60° 9.2' N ( 60.15330)
USGS Quad: Seward A-3 Longitude: 147° 46.0' W (-147.76700)
To reach the tidal bench marks, go to the southern end of Knight Island, Prince
William Sound along a cobble beach west of Point Helen Light.
T I D A L B E N C H M A R K S
PRIMARY BENCH MARK STAMPING: 4671 A 1996
MONUMENTATION: Survey disk VM#: 13295
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: In bedrock outcrop OPUS PID:
LATITUDE: 60° 9.2' N ( 60.15330) LONGITUDE: 147° 46.0' W (-147.76700)
The bench mark is a disk set in a bedrock outcrop at the base of a cliff on the
west side of the cobble beach, 2.0 m (6.6 ft) from the treeline and 2.0 m (6.6
ft) above the approximate high water line.
BENCH MARK STAMPING: 4671 B 1996
MONUMENTATION: Survey disk VM#: 14655
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID:
LATITUDE: 60° 9.2' N ( 60.15330) LONGITUDE: 147° 46.0' W (-147.76700)
The bench mark is a disk set in a bedrock outcrop three-quarters of the way
along the cobble beach from west to east. The outcrop is approximately 1.0 m
(3.3 ft) above the cobble beach and 0.5 m (1.6 ft) above the approximate mean
high water line. The bench mark is 10.0 m (32.8 ft) seaward of treeline and
70.0 m (229.7 ft) NE (080 magnetic) from bench mark 4671 A 1996.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 2 of 5
Station ID: 9454671 PUBLICATION DATE: 06/08/2011
Name: POINT HELEN, KNIGHT ISLAND
AK
NOAA Chart: 16701 Latitude: 60° 9.2' N ( 60.15330)
USGS Quad: Seward A-3 Longitude: 147° 46.0' W (-147.76700)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 4671 C 1996
MONUMENTATION: Survey disk VM#: 14656
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID:
LATITUDE: 60° 9.2' N ( 60.15330) LONGITUDE: 147° 46.0' W (-147.76700)
The bench mark is a disk set in a bedrock outcrop near the eastern end of the
cobble beach, 10.0 m (32.8 ft) seaward from the treeline, 30.0 m (93.4 ft) east
(090 magnetic) from bench mark 4671 B 1996, and 1.0 m (3.3 ft) above the
approximate mean high water line. A rock outcrop 3.0 m (9.8 ft) in height
protrudes inshore of the bench mark.
BENCH MARK STAMPING: 4671 D 1996
MONUMENTATION: Survey disk VM#: 14657
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: Bedrock outcrop OPUS PID:
LATITUDE: 60° 9.2' N ( 60.15330) LONGITUDE: 147° 46.0' W (-147.76700)
The bench mark is a disk set 3.0 m (9.8 ft) from the treeline at the east end of
cobble beach, 5.0 m (16.4 ft) west of a large moss-covered rock outcrop 2.50 m
(8.2 ft) high, approximately 20.0 m (65.6 ft) west of Point Helen Light, and
30.0 m (98.4 ft) NE (060 magnetic) from bench mark 4671 C 1996.
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 3 of 5
Station ID: 9454671 PUBLICATION DATE: 06/08/2011
Name: POINT HELEN, KNIGHT ISLAND
AK
NOAA Chart: 16701 Latitude: 60° 9.2' N ( 60.15330)
USGS Quad: Seward A-3 Longitude: 147° 46.0' W (-147.76700)
T I D A L B E N C H M A R K S
BENCH MARK STAMPING: 4671 E 1996
MONUMENTATION: Survey disk VM#: 14658
AGENCY: National Ocean Service (NOS) IDB PID#:
SETTING CLASSIFICATION: In boulder OPUS PID:
LATITUDE: 60° 9.2' N ( 60.15330) LONGITUDE: 147° 46.0' W (-147.76700)
The bench mark is a disk set in a large 1.50 m x 0.5 m (4.9 ft x 1.6 ft) boulder
at the base of a large rock outcrop in which Point Helen Light is set, 5.0 m
(16.4 ft) west of Point Helen Light, 15.0 m (49.2 ft) seaward of the reeline and
cliff, 18 m (59 ft) SE (120 magnetic) from bench mark 4671 D 1996, and 5.0 m
(16.4 ft) above the approximate mean high water line
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 4 of 5
Station ID: 9454671 PUBLICATION DATE: 06/08/2011
Name: POINT HELEN, KNIGHT ISLAND
AK
NOAA Chart: 16701 Latitude: 60° 9.2' N ( 60.15330)
USGS Quad: Seward A-3 Longitude: 147° 46.0' W (-147.76700)
T I D A L D A T U M S
Tidal datums at POINT HELEN, KNIGHT ISLAND based on:
LENGTH OF SERIES: 48 Days
TIME PERIOD: September 4 1996 - October 21 1996
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 9454240 VALDEZ, PRINCE WILLIAM SOUND
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
MEAN HIGHER HIGH WATER MHHW = 3.546
MEAN HIGH WATER MHW = 3.264
MEAN SEA LEVEL MSL = 1.891
MEAN TIDE LEVEL MTL = 1.861
MEAN LOW WATER MLW = 0.458
MEAN LOWER LOW WATER MLLW = 0.000
North American Vertical Datum (NAVD88)
Bench Mark Elevation Information In METERS above:
Stamping or Designation MLLW MHW
4671 A 1996 5.530 2.266
4671 B 1996 5.325 2.061
4671 C 1996 4.599 1.335
4671 D 1996 4.576 1.312
4671 E 1996 5.171 1.907
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Ocean Service
Page 5 of 5
Station ID: 9454671 PUBLICATION DATE: 06/08/2011
Name: POINT HELEN, KNIGHT ISLAND
AK
NOAA Chart: 16701 Latitude: 60° 9.2' N ( 60.15330)
USGS Quad: Seward A-3 Longitude: 147° 46.0' W (-147.76700)
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.