The shape of earth is mainly represented by an oblate spheroid or ellipsoid or a geoid. Imagine the earth as an orange (not a perfect shape). To project the earth on a plane and get a 2D projection of the world, the orange has to be peeled and unfolded.
When peeling the orange and laying the result on a table (projecting), there will be cracks and distortions. Different ways of projecting the orange will lead to different types of distortions at different places of the earth.
Choosing the coordinate system (datum and 2D projection):
- The distortions are locally minimized.
- The correct measurement unit is selected (meters, feet).
- The results are aligned with GIS data from other sources.