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Q: "SEA LEVEL" CORRELATION TO "ZERO ELEVATION" ( Answered,   1 Comment )
Question  
Subject: "SEA LEVEL" CORRELATION TO "ZERO ELEVATION"
Category: Science
Asked by: johnnyd-ga
List Price: $50.00
Posted: 26 Sep 2003 12:04 PDT
Expires: 26 Oct 2003 11:04 PST
Question ID: 260547
IS ZERO ELEVATION AS ESTABLISHED BY THE U.S. COASTAL AND GEODETIC
SURVEY THE SAME AS SEA LEVEL? AND THEN WHAT IS THE "ZERO ELEVATION" OF
THE CITY OF OCEAN RIDGE FLORIDA?
Answer  
Subject: Re: "SEA LEVEL" CORRELATION TO "ZERO ELEVATION"
Answered By: googlenut-ga on 26 Sep 2003 23:35 PDT
 
Hello johnnyd,

The mean sea level (MSL) is the zero elevation for a local or regional
area.

===========================================

The National Imagery and Mapping Agency (NIMA) ( http://www.nima.mil/
) provides the following explanation:

“The zero surface to which elevations or heights are referred is
called a vertical datum. Traditionally, surveyors and mapmakers have
tried to simplify the task by using the average (or mean) sea level as
the definition of zero elevation, because the sea surface is available
worldwide. The mean sea level (MSL) is determined by continuously
measuring the rise and fall of the ocean at "tide gauge stations" on
seacoasts for a period of about 19 years. This averages out the highs
and lows of the tides caused by the changing effects of the
gravitational forces from the sun and moon which produce the tides.
The mean sea level (MSL) then is defined as the zero elevation for a
local or regional area. But what do you do for Mt. Elbert? Where is
the MSL in Colorado? There is no tangible surface of the ocean from
which to measure height.

It turns out that MSL is a close approximation to another surface,
defined by gravity, called the geoid, which is the true zero surface
for measuring elevations. Because we cannot directly see the geoid
surface, we cannot actually measure the heights above or below the
geoid surface. We must infer where this surface is by making gravity
measurements and by modeling it mathematically. For practical
purposes, we assume that at the coastline the geoid and the MSL
surfaces are essentially the same. Nevertheless, as we move inland we
measure heights relative to the zero height at the coast, which in
effect means relative to MSL.”

---

“How Good is MSL?

All of our maps say "elevations are based on mean sea level," so we're
all using the same standard, right? Unfortunately for mapmakers and
surveyors, sea level is not a simple surface. Because the Earth is
ellipsoidal, you might expect the ocean to form a smooth, even
"ellipsoidal" surface around the Earth, but it doesn't. Variations in
the topography and the different densities within the Earth's crust
produce slight variations in the gravity field, described by the dips
and peaks of the geoid. Since the sea surface conforms to that gravity
field, MSL also has slight hills and valleys in it, similar to the
topographic surface although much smoother. Depending on where you
are, sea level may be closer to or farther from the center of the
Earth than at some other location. That's why locally-defined vertical
datums differ from each other. Zero elevation (MSL) as defined by
China differs from zero elevation (MSL) as defined by Chile.

By definition, the geoid describes this irregular gravity shape.
Because there is no way to accurately measure the geoid, it has been
roughly approximated in the past by MSL. Technically, the geoid can
differ from MSL by several meters due to other ocean effects beyond
the scope of this pamphlet.”


National Imagery and Mapping Agency (NIMA)
Geospatial Sciences Division
Vertical Datums, Elevations, and Heights
http://164.214.2.59/GandG/datums/vertdatum.htm


About NIMA
http://www.nima.mil/cda/article/0,2311,3104_10573_112428,00.html
“The National Imagery and Mapping Agency was established October 1,
1996, by the National Imagery and Mapping Agency Act of 1996. The
creation of NIMA followed more than a year of study, debate and
planning by the defense, intelligence and policy-making communities as
well as the Congress, and continuing consultations with customer
organizations. Because it has responsibilities to customers outside
DoD, NIMA has also been designated a part of the U.S. Intelligence
Community.”

===========================================

Other references:


U.S. Geological Survey Manual
800.2 - National Mapping Program
http://www.usgs.gov/usgs-manual/800/800-2.html
“Vertical Datums. The vertical datum provides a surface of reference
for determining point elevations and for compiling contours. The mean
sea level surface, measured by tide gauges, is generally the zero
point for vertical datums. Vertical datums used on Geological Survey
maps of the United States include:

Mean Sea Level 

Mean Sea Level of 1929 

National Geodetic Vertical Datum of 1929”


GEODESY FOR THE LAYMAN
National Geodetic Survey
http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/geo4lay.pdf
“In the European area, there are fewer vertical datum problems than in
Asia and Africa. Extensive leveling work has been done in Europe and
practically all of it has been referred to the same mean sea level
surface. However, in Asia and Africa the situation has been different.
In places there is precise leveling information available based on
mean sea level. In other areas the zero elevation is an assumed
elevation which sometimes has no connection to any sea level surface.
China has been an extreme example of this situation where nearly all
of the provinces have had an independent zero reference. There is very
little reliable, recent, vertical data available for much of the area
of Africa and Asia including China.

The mean sea level surface in the United States was determined using
21 tidal stations in this country and five in Canada. This vertical
datum has been extended over most of the continent by first-order
differential leveling. Concurrent with the new adjustment of the
horizontal network, mentioned previously, is the readjustment of the
vertical network. Countries of North and Central America are involved.
In the conterminous United States 110,000 kilometers of the basic
network are being releveled.”


National Geodetic Survey's Geodetic Control
Data Sheets for the U.S.
http://oceanservice.noaa.gov/mapfinder/products/geodetic/fgdc_geodetic.pdf
“A set of control stations established by geodetic methods. Horizontal
Geodetic Control: These data of geodetic control consist first of
distances, directions, and angles between control stations. These are
converted to geodetic coordinates (i.e., geodetic latitudes and
longitudes) and azimuths. The former, in turn, may be converted into
other kinds of coordinates such as plane coordinates in a State Plane
Coordinate System. This is the form in which they are usually used in
the United States for local surveys. Vertical Geodetic Control: These
control points are those whose elevations (i.e., orthometric heights)
have been accurately determined, can be identified with physical
points on the Earth, and can be used to provide elevations for other
surveys. Elevations are referred, by definition, to the geoid.
However, horizontal surfaces through selected points on mean sea level
have been used for reference, as have non-horizontal surfaces defined
by a combination of leveling surveys and points on (local) mean sea
level.”


The National Geodetic Survey was formerly known as the Coastal And
Geodetic Survey:

History of the National Geodetic Survey
http://geodesy.noaa.gov/INFO/NGShistory.html
“The National Geodetic Survey, our Nation's first civilian scientific
agency, was established by President Thomas Jefferson in 1807 as the
Survey of the Coast. Its mission soon included surveys of the interior
as the nation grew westward. In 1878 the agency was reorganized and
given a new name, the Coast and Geodetic Survey (C&GS), which it
maintained until 1970.

In 1970 a reorganization created the National Oceanic and Atmospheric
Administration (NOAA) and the National Ocean Service (NOS) was created
as a line office of NOAA. To acknowledge the geodetic portion of NOAA
mission, the part of NOS responsible for geodetic functions was named
the National Geodetic Survey.”


The Office of Coast Survey, National Ocean Service
National Oceanic and Atmospheric Administration, (NOAA)
Bathy/Topo/Shoreline
VDatum Transformation Tool 
http://chartmaker.ncd.noaa.gov/bathytopo/vdatum.htm
“Vertical datums come in three categories: those based on a form of
Mean Sea Level (MSL), called Orthometric Datums, those based on
tidally-derived surfaces of high or low water, called Tidal Datums,
and three-dimensional (3-D) datums realized through space-based
systems such as Global Positioning System (GPS).

Topographic maps from the USGS generally have elevations referenced to
an orthometric datum, either the North American Vertical Datum 1988
(NAVD 88) or to the older National Geodetic Vertical Datum 1929 (NGVD
29). The NAVD 88 was affirmed as the official vertical datum for the
United States (by a notice in the Federal Register, Vol. 58, No. 120
page 34245) on June 24, 1993. Nautical charts have depths referenced
to different tidal datums, which can vary from chart to chart. In the
United States, mean lower low water (MLLW) is the official NOAA chart
datum. To support harbor and river navigation, bridge clearances are
referenced to a mean high water (MHW), and not MLLW.”


Mean Sea Level, GPS, and the Geoid
http://www.esri.com/news/arcuser/0703/geoid1of3.html
“MSL is defined as the zero elevation for a local area. The zero
surface referenced by elevation is called a vertical datum.
Unfortunately for mapmakers, sea level is not a simple surface. Since
the sea surface conforms to the earth's gravitational field, MSL also
has slight hills and valleys that are similar to the land surface but
much smoother. However, zero elevation as defined by Spain is not the
same zero elevation defined by Canada, which is why locally defined
vertical datums differ from each other.”


===========================================


According to TopoZone.com, the elevation of Ocean Ridge, Florida is 8
feet.
http://www.topozone.com/findresults.asp?place=ocean+ridge&statefips=12&placesearch=Search
http://www.topozone.com/map.asp?lat=26.52667&lon=-80.04861


TopoZone doesn’t specifically state that the elevation of Ocean Ridge
is 8 feet above mean sea level however, TopoZone creates its maps in
association with the USGS, which does use mean sea level as its
vertical datum, as stated in the reference listed previously.


About TopoZone
http://www.topozone.com/about.asp
“The TopoZone is a production of Maps a la carte, Inc. We're bringing
the power of digital topographic mapping to the Web for the first
time.”

“Maps a la carte, Inc. is a USGS Digital Cartographic Business Partner
and a member of the International Map Trade Association.”


Other references:

City-data.com
http://www.city-data.com/city/Ocean-Ridge-Florida.html
“Elevation: 8 feet”


I hope you have found this information helpful.  If you have any
questions, please request clarification prior to rating the answer.

Googlenut


Search Strategy:

Google Search Terms:

"ocean ridge florida" "sea level" OR elevation
://www.google.com/search?q=%22OCEAN+RIDGE+FLORIDA%22+%22sea+level%22+OR+elevation&hl=en&lr=&ie=ISO-8859-1&safe=off

"zero elevation" "sea level"
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22zero+elevation%22+%22sea+level%22&btnG=Google+Search

topography "united states" online
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&safe=off&q=topography+%22united+states%22+online&btnG=Google+Search

"vertical datum" msl
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22vertical+datum%22+msl&btnG=Google+Search


Searched U.S. Geological Survey (USGS), National Oceanic & Atmospheric
Administration (NOAA),  National Geodetic Survey (NGS) websites for
“sea level”, “zero elevation”, “elevation”, “mean sea level”, “msl”
and other related terms.
Comments  
Subject: Re: "SEA LEVEL" CORRELATION TO "ZERO ELEVATION"
From: mooseo-ga on 01 Apr 2004 09:24 PST
 
Just as a follow up:

The tidal datum gets re-evaluated about every 20 years, and a new
datum was just issued in 2003 which will replace the 1988 one.  This
is necessary for a number of reasons, one being rising sea levels. 
The 20 year period is long enough to take into account a complete 18.6
year lunar cycle which has an effect on tides.  Oceanic events such as
El Nino also affect the height of the tides... since we had 2 large El
Nino events during the 20 years that make up the most recent tidal
datum epoch, MSL for many places in the US went up a few centimeters.
In some other locations with lots of tectonic activity (such as the
Pacific Northwest), the ground is actually rising out of the ocean a
bit, causing sea levels to appear to fall.

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