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Earth and the Terrestrial Coordinate System
Background
Before we begin to examine the nautical chart, we must first understand
some facts about Earth itself.
Facts about Earth
It is not a perfect sphere
The diameter at the Equator equals approximately 6,888 nautical miles.
The polar diameter is approximately 6,865 nautical miles, or 23 miles
less than the diameter at the Equator.
Technically it is classified as an oblate spheroid (a sphere flattened at
the poles.)
Figure 1-4. Earth.
For the purposes of navigation, we assume that we are working with a
perfect sphere.
The differences between the two diameters are small
enough to be considered insignificant.
Nautical charts do NOT take Earth’s oblateness into account.
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