13 Making Earth Flat: Map Projections
Introduction
Maps are flat representations of Earth. As we’ve learned, Earth is large and complicated, but we can simplify it so the information we want to display can be preserved, represented, and analyzed. An important step in the simplification is taking the surface of Earth and presenting it on a flat (planar) surface. When Earth’s surface is converted (projected) from 3 dimensions to 2 dimensions, there are trade-offs. In addition to going from round (and 3-D) to flat, the representation must be reduced in size. This is the map’s scale (see and Scale chapter for a definition of cartographic scale). All maps are projections, but only on small scale maps (remember representational fraction) does the distortion of the projection become readily apparent to anyone but a keen observer (like a student of geomatics). Maps of countries, continents, and the world, show their map projection distortions readily.
Organization of Chapter and Key Aspects of Map Projections
When projecting Earth’s surface onto a planar (flat) surface something is lost, or given up. A map project can preserve the accuracy of some aspects of the arrangement and characteristics of features, but at the cost of something else. That which is preserved is one way to classifying map projection.
What Can be Preserved, and What is Lost
Shape: A map that preserve shape are called Conformal. The trade-off is that area is exaggerated. On a conformal map the shapes (of countries, islands, continents, etc.) are the same as they are on Earth’s surface or a globe. That area is distorted means that we can’t visually compare the size of different places on the map. On the map below compare (approximately) Alaska and Brazil. They might look similar in size, but Brazil is 5 times the area of Alaska. This is an example of a conformal map that minimizes distortion at the equator and preserves shape, necessarily exaggerating area as the distance from the equator increases.
Area: A map projection that preserves area is called an Equal Area projection. Like the conformal, there is a trade-off. That trade off is that shapes are distorted. Compare Alaska, or any place far from the equator, on this and the previous map.
In the two maps above (and the “unprojected” perspective image of a globe, on the left in the top image) the arrangement of the lines of Latitude and Longitude are an indicator of the nature of the distortion resulting from the projection. Lines (parallels) of Latitude and (meridians of) Longitude are lines that intersect one another at 90 degrees on Earth. Lines of longitude meet at the poles, lines of latitude are a constant distance apart and, other than the equator, are shorter than lines of longitude. A great circle is a line that bisects a sphere; all lines of longitude are great circles, the only line of latitude that is a great circle is the equator. Both lines of latitude and longitude follow a direction consistent with a compass bearing,