MATHEMATICS OF THE EARTH'S SHAPE
The Earth is a somewhat pear shaped ellipsoid, with a slightly larger southern hemisphere than northern hemisphere. No single mathematical formula works for all of the Earth's surface, so different regions are represented by different formulas. Position coordinates determined for one formula (datum) will usually not match the position values determined by another datum.
USGS topo maps specifies the reference datum in the information at the bottom of the map. Most GPS receivers allow the user to select the desired datum from a list of common datums. North American Datum 1927 (NAD 27) and Department of Defense World Geodetic System 1984 (WGS 84) are commonly used in North America. Most USGS 7 1/2 minute series topo maps for regions in the United States use NAD 27. When providing your position coordinates to someone from another agency (such as, a helicopter pilot), be sure to also include the datum as part of the information.
In 1958, a World Geodetic System (WGS) was designed by the DOD as the datum for intercontinental ballistic missiles, and subsequent revisions have become the de facto worldwide standard, with WGS-84 as the most current version.
WGS-84 Ellipsoid -- The reference ellipsoid used by NASA/GSFC is the Defense Mapping Agency (DMA) World Geodetic System 1984 (WGS-84) ellipsoid model. The ellipsoid is defined as:
Flattening (ellipticity): 1/298.257
Semimajor axis: 6378137 m
GPS receivers in the United States provide coordinates relative to the North American Datum (NAD), which was revised in 1983. NAD differs from WGS-84 by up to a meter in some areas. But many paper maps used as sources for computer map vectors were compiled at least 20 years ago using NAD-27 instead of NAD-83. Actual positional differences of 95 meters for identical coordinates have been noted between the two NAD reference systems.
2006-08-29 13:19:00
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answer #1
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answered by anotherthirteen 2
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The Earth is not a perfect sphere. Due to it's rotation, the Earth (like all rotating planets) has a slightly distorted shape. The rotational momentum tends to force the matter to bunch up in the middle. In the case of the Earth, this "middle" is the equator.
2006-08-29 01:12:24
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answer #2
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answered by ptblueghost64 4
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The shape of th earth is hard to describe. It isn't exactly spherical. There are many mountains and other physical features which alter the shape of the earth.
2006-08-29 01:14:00
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answer #3
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answered by Anonymous
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The earth is a sphere shape.
2006-08-29 10:22:50
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answer #4
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answered by Tori 5
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Sphere
2006-08-29 01:14:02
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answer #5
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answered by michelle 3
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Although commonly thought to be a sphere, the earth is actually an OBLATE SPHEROID. It bulges slightly at the equator and is slightly flattened at the poles.
2006-08-29 02:15:28
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answer #6
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answered by genius777 2
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It used to be flat, but seems to have mutated into an almost round shape over the years.
2006-08-29 01:27:36
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answer #7
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answered by Polo 7
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Geoid
2006-08-29 03:21:36
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answer #8
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answered by sweety 2
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Spherical.
2006-08-29 01:12:53
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answer #9
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answered by Anonymous
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almost spherical, but, flatter at the poles & protruded at the equator, which makes the equator bigger than a longitudinal circle (the longitudinal lines R half circle & don't go around)...
2006-08-29 03:24:40
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answer #10
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answered by white skull 3
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