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Name: Hal
Status: Other
Grade: N/A
Location: NY
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?

Hi Hal!

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 1980.

"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) (last reviewed March 10, 2006)

Thanks for asking NEWTON!

(Dr. Mabel M. Rodrigues)

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