…from Geoweb Guru…
“This two part article is intended as an introduction to map projections and coordinate systems. … They start a new series of articles which I shall be publishing over the next few weeks. I shall look at different projections and coordinate systems, the inadequacies of the current choices, and how to do something about it.
“Most users of online maps ignore the issues of different coordinate systems and projections. Any concern is usually limited to whether their system of choice will read the coordinates they have. As both are typically the same, the issue rarely surfaces. However, over the last two millenia, cartographers have produced hundreds if not thousands of different projections and coordinate systems for a wide range of different applications and locations.
“Most online map services use geographic longitude,latitude coordinates on the WGS84 datum using a Mercator Projection. Although the coordinate system and datum are both logical choices for global mapping, As you shall see, Mercator is a poor choice of projection for most map applications.
“So what are these different datums, coordinate systems, and projections? Why should we care?”