Map Projections

There are a great many map projections. Some date back to ancient times and some are more modern. They fall into three broad groups. The cylindrical, conical and azimuthal groups. The best know of all is probably the Mercator projection which is a cylindrical projection. A few of the known projections are described and illustrated below. This is not therefore a definitive guide but simply a sample to help explain the nature and benefits of the three main types.

The Mercator Projection

Probably the most famous of the various map projections, the Mercator projection takes its name from Mercator who presented it in 1569. It is a cylindrical, conformal projection with no distortion along the equator. A major navigational feature of the projection is that a line of constant azimuth is straight. Such a line is known to seamen as a rhumb line. Thus, to sail from one point to another one only had to connect the points with a straight line, and keep this constant course for the entire voyage. This property is also known as comformality. The Mercator projection has been used extensively for world maps. You will notice that there is a marked distortion towards the polar regions and that countries such as Greenland appear larger then they are.

A transverse mercator projection is a mercator projection rotated through 90 degrees. This projection is widely used for land masses with a North/South expanse and is the basis for the UTM (Universal Transverse Mercator) co-ordinate system (among others).

Conical Projections

In the Conical Projection the graticule is projected onto a cone tangent to the globe along any small circle (usually a mid-latitude parallel). In the normal aspect (which is oblique for conic projections), latitude parallels are projected as concentric arcs of circles, and longitude meridians are projected as straight lines radiating at uniform angular intervals from the apex of the flattened cone. Conic projections are best suited for maps of temperate latitudes, especially those elongated in an east/west direction. The United States meets these qualifications and therefore is frequently mapped on conic projections.

Lambert Conformal Conic

Developed by J.H. Lambert in 1772 but further developed by others during the 19^th Century. The First World War gave this projection new life, making it the standard projection for intermediate - and large-scale maps of regions in middle latitudes for which the transverse Mercator was not then used.

Azimuthal Projections

The most noticeable feature of this azimuthal projection is the fact that distances and directions measured from the centre of the map are true. Therefore, a circle about the projection centre defines the locus of points that are equally far away from the plot origin. It is a useful projection for a global view of locations at various or identical distance from a given point and this should be located at the centre of the map.

Lambert Azimuthal equal-area projection

Polar Stereographic Projection

This is a conformal, azimuthal projection that dates back to the Greeks. It's main use is for mapping the polar regions. When a polar region is shown then all longitude meridians are straight lines and latitude parallels are arcs of (or complete) circles.

Which projection is right for your map?

If minimal distortion were the only issue then the choice would be relatively straightforward. Cylindrical projections are best for tropical areas, conical projections for temperate areas and azimuthal projections would work best for polar regions.

For navigational purposes conformality is essential and this might override any other choice factor. Distortion resulting from the projection used is unlikely to be evident on very large scale maps so one might well end up using a cylindrical projection in many more instances than the simple issue of overall distortion would imply.