Maps are usually seen in a flat, two-dimensional medium such as a drawing on paper or an image on a computer screen. Since the surface of the Earth is curved, or three-dimensional, the surface must somehow be transformed from three dimensions to two in order to display a map of the Earth's surface. Projections are a mathematical process by which the surface is transformed from three dimensions to two.
One of the simplest forms of projection is analogous to shining a light through a translucent globe onto a piece of paper and tracing the outlines. Other forms of projection may involve dozens of complex mathematical equations. Since no two-dimensional representation of a three-dimensional surface can be accurate in every regard, different projections have been developed to suit different purposes. Some projections are accurate in terms of area but not in scale, some are accurate in terms of scale but not in shape, and so on. The selection of an appropriate projection for a map depends on which characteristics of a map are most important or most desirable for a given project or audience. Didger supports several of the projections that are used most often in modern cartography and related fields.
There are many excellent textbooks and publications on this subject, and we will not attempt to explain projections in full detail here. If you need or want more information, you might consider reading the references that provide good introductory discussions of map projections.
When you do not use projections, land areas can become distorted in shape and size, so some areas might appear relatively larger or smaller than they actually are in relation to other land areas and visual representation of data can become somewhat misleading. However, these problems only become significant when you are plotting large land masses, such as an entire continent. For most Didger applications, such as plotting a single state or a group of states, this problem is minimal.
Available Projections
See Also
Characteristics of Projections
Latitude/Longitude Coordinates
Latitude/Longitude in Decimal Degrees
Using Scaling to Minimize Distortion in Latitude/Longitude Projects