North American Overlay Mapping
All of the maps in the North American Overlay Mapping System use an Equidistant Cylindrical projection. This projection is best visualised by imagining a sphere 'unwrapped' onto a cylinder, making a complete transformation to a flat surface. In a cylindrical projection, distortion increases away from the Equator, and is extreme in polar regions. Areas and shapes of large areas are distorted, but angles and shapes within any small area are essentially true. A cylindrical projection that we are all familiar with is the Mercator projection, used for navigation, or maps of equatorial regions. However, unlike the Equidistant Cylindrical projection, the lattitudes in a Mercator map are not equally spaced.
Throughout the 47 small-scale maps of the North American Overlay Mapper, all maps have an identical size for a 1° square. Obviously, the longitude distance (East to West) can not remain the same as the map moves North from the Equator towards the North Pole, thus to maintain an equal 1° square some level of distortion is introduced. The advantage of this projection, though, is that with an equal area throughout, it is easy to plot or position additional information, which explains why this is the most popular and frequently used in most mapping and GIS software.
A Background to Map Projections
The only true representation of an actual area is a globe. Directions, distances, shapes, and areas are all true, and the shortest distance between two points can be easily found. However, globes have their disadvantages - even the largest globe has a very small scale, and shows relatively little detail. Its costly to reproduce and update, difficult to carry around or store. Consequently, some means was needed to transfer a curved map to a flat piece of paper or computer screen, and a large number of different map projections were devised to do this. The first thing to say is that there is no 'best' projection. Each have their own advantages and disadvantages. All have some distortion.
Other Map Projections
The projection most familiar to Radio Amateurs is the Azimuthal Equidistant projection, commonly know as the Great Circle Map. Distances and direction to all places on the map are true only from the centre point, normally the operator's QTH. Distortion of other properties increases away from the center point. A straight line drawn from the centre point indicates the shortest route to a point, and is used to determine the correct direction to point antennas. With the North American Overlay Mapping System, the same can be achieved on any of the maps by simply holding the cursor over the destination and reading the antenna direction from the status bar.