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

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."

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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