GOM - the Global Overlay Mapping System

Map Projections

A variety of different map projections are needed in any large mapping project. The ones used in the Global Overlay Mapping System are:

• Main maps - drawn in an Equidistant Cylindrical projection.
• Antarctic map - drawn in a Polar Stereographic projection.
• Some Toolbar maps - drawn in Albers Conic projection.
• The opening image of the world is drawn in Orthographic projection.

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.

Specific Map Projections

An Equidistant Cylindrical 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.

Stereographic projections are used mainly for Arctic and Antarctic maps. Directions are true only from the point of projection. Scale increases away from the centre point, unlike the usual 'Great Circle' map (see below), where it is constant. Any straight line through the centre point is a great circle line. Distortion of large areas increase away from the centre point.

Conic projections distort scale and distance except along standard parallels. Areas are proportional and directions are true in limited areas. Used in the United States, Russia, and other large countries with a larger east-west than north-south extent and that require equal-area representation.

An Orthographic projection is best visualised as a view of the earth from deep space. It is used to obtain a perspective view of hemispheres. Area and shape are distorted. Distances are true along the equator and other parallels.

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 Global 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.