This lesson has introduced several important spatial referencing concepts. Cartesian and polar coordinate systems, which are based on orthogonal axes, were discussed, as well as the most widely used spherical system based on latitude and longitude. We have also covered the concepts related to the shape of the Earth, including geodetic datums, geometric Earth models, reference ellipsoids and Earth surfaces. It is important to be aware of the fact that a variety of reference systems is in use to describe locations on the Earth.

"Whether you treat the earth as a sphere or a spheroid, you must transform its three-dimensional surface to create a flat map sheet. This mathematical transformation is commonly referred to as a map projection. One easy way to understand how map projections alter spatial properties is to visualize shining a light through the earth onto a surface, called the projection surface" (ESRI, 2003). In this lesson we have had a detailed look at different types of map projections. We have discussed cylindrical, azimuthal, and conic projections. The concepts of tangent vs. secant projections have been explained as well as conformal and equal area projection properties. You should now be able to recognize and distinguish the characteristics of the main types of map projections.

This lesson concentrated on the problem of map projections having different sets of projection parameters. Each set is related to a particular projection and it is very important to have a sound understanding of this relationship between map projections and their specific parameters. In the second part of this lesson we had a detailed look on the Universal Transverse Mercator system as an example of a grid system that is widely used in GIS operations.