Sea Level Monument and Elevation
Country: United States
Date: May 2006
When I look at a topographic map
it shows elevations. I assume that this is the elevation
above sea level. Is there a 00.00 monument, or is it assumed? and
Since the east coast and west coast of the United States have two
different oceans with two different levels, how is that handled?
Yours is indeed a very interesting question and you
are correct in saying that in different latitudes
(as Pacific Ocean and Atlantic Ocean) the sea level is
different. Actually in the Panama Canal gates were
built to allow ships to go from one side (higher) to
the other (lower).
Sea level is a relative and variable measure related
to the local place.
An accurate measurement of sea level is very hard to
determine but still it is an important measurement for
two main reasons namely:
1)Through an accurate sea level measurement, it is
possible to determine the height of everything on land
accurately. For example: heights of mountains and
depths of canyons.
2) By knowing sea level, it is possible to determine
if the oceans are rising or falling over time. The
concern is that global warming and other weather
changes caused by man might be leading to an overall
rise in sea level
carrying danger to near the coast cities.
The problem with measuring the sea level is that there
are so many factors that are determinant over it.
The Earth belongs to a solar system very complex and
with many different gravitational forces.
These forces operate and change the globe water levels
at any given point, and are mainly:
- The tides, caused by the moon
- Large and small waves caused by wind and the tides
- High- and low-pressure areas in the atmosphere,
which change the surface level of the ocean
- Temperature changes in the ocean, which change the
density and volume of the water
- Rainfall and river water flowing into the ocean
The sea level changes by the second (waves), by the
hour (tides) and by the week (planetary and solar
orbit changes). To get around this, scientists try
using tide gauges. A tide gauge is a large (1 foot [30
cm] or more in diameter), long pipe with a small hole
below the water line. This pipe is often called a
stilling well. Even though waves are changing the
water level outside the gauge constantly, they have
little effect inside the gauge. The sea level can be
read relatively accurately inside this pipe. If read
on a regular basis over a time span of years and then
averaged, one can get a measurement of sea level.
By other side there are in U.S.A. government agencies
that define and determine data (geodetic datum) as:
1. "A set of constants specifying the coordinate
system used for geodetic control, i.e., for
calculating the coordinates of points on the Earth."
2. "The datum, as defined in (1), together with the
coordinate system and the set of all points and lines
whose coordinates, lengths, and directions have been
determined by measurement or calculation."
The North American Datum of 1927 (NAD 27) is "The
horizontal control datum for the United States that
(was) defined by (a) location and azimuth on the
Clarke spheroid of 1866, with origin at (the survey
station) Meades Ranch." ... The geoidal height at
Meades Ranch (was) assumed to be zero. "Geodetic
positions on the North American Datum of 1927 were
derived from the (coordinates of and an azimuth at
Meades Ranch) through a readjustment of the
triangulation of the entire network in which Laplace
azimuths were introduced, and the Bowie method was
used." (Geodetic Glossary, pp. 57)
The North American Datum of 1983 (NAD 83) is "The
horizontal control datum for the United States,
Canada, Mexico, and Central America, based on a
geocentric origin and the Geodetic Reference System
"This datum, designated as NAD 83, is the new geodetic
reference system. ... NAD 83 is based on the
adjustment of 250,000 points including 600 satellite
Doppler stations which constrain the system to a
geocentric origin." (Geodetic Glossary, pp 57)
http://www.ngs.noaa.gov/faq.shtml (last reviewed
March 10, 2006)
Thanks for asking NEWTON!
(Dr. Mabel M. Rodrigues)
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Update: June 2012