Ellipsoids Datums And Projections

The Earth's shape is closest to an ellipsoid. It is wider in the middle and smaller at the poles. 

The Earth is not a perfect ellipsoid. And hard to measure. Over time due to the changing shape of the Earth and newer more accurate technology we have created and use different Ellipsoid maps. 

   A datum is a reference point. It is used to show positions on maps to it's real position on the Earth. Horizontal Datums measure latitude and longitude. Vertical datum are used to measure depth.  Because the Earth is not a perfect ellipsoid Datums provide a more accurate picture of the area.

    A map projection is a way to show the shape of the Earth on a flat surface. Map projections have distortions such as size, shape, angle, and distance. It is important to pick the right map to show what you need.

   The biggest problem I had when using Google Earth had to do with my own error in numbers.