The Earth is Not a Sphere! It's an Ellipsoid

Key points:

• The Earth is not a perfect sphere. Because it is spinning, it "bulges" at the middle. This makes the Earth a "squished sphere," or "oblate spheroid."
• "Datums" include an approximation of this spheroid.
• Over the last several hundred years, datums have become more accurate.
• Different datums can have over 100 meters of error between them.
• We must use the correct "Geographic Transformation" to convert between some datums.
  • Typically, NAD27, WGS84, or NAD83

The Earth is an Ellipsoid

A sphere is a perfectly round 3-dimensional object. Earth is like a big bag of molten lava spinning on its axis. Because of the "bulging" caused by the Earth spinning, the Earth is not completely round, thus, is not a sphere. Instead, we use the term "oblate spheroid," or "ellipsoid."

Notice that the difference between the diamter of the earth at the equator and the poles is about 42 kilometers. This may not seem like a lot givent that the overall diameter is over 12,000 kilometers, but it can impact our cordinate values by over 100 meters.

An Approximation

To measure and specify coordinates on a 3D surface, one first must define the geometry of the surface itself.

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