Map projections are one of the fundamental concepts of geography and cartography. Selecting the right map projection is one of the important first considerations for accurate GIS analysis.
The Robinson, Transverse Mercator, Lambert Conformal Conic, and Space Oblique Mercator projections are discussed in this article.
Map Projections Distort Reality
The problem with projections (and the reason why there are so many types) is that it is very difficult to represent the curved 3D surface of the Earth on a flat 2D surface of a map; some distortion is bound to occur (read about what is a map projection).
Many geographers through the ages have tried to solve the map distortion problem through various map projections. An example of a pseudocylindrical projection is the Robinson map projection which views the entire world at once and one that compromises both area and angles.
The longitudinal lines are curved while the latitude lines remain horizontally straight in the Robinson map projection. The lines of longitude are depicted as nonparallel lines that get increasingly curved as you move away from the map’s central meridian.
The Robinson is a compromised view of the Earth’s surface with greater amounts of distortion occurring at the poles.
With area, the Robinson map projection distortion increases with latitude but not with longitude. The Robinson map projection is considered a compromise projection. This means that while shapes, areas, distances, directions, and angles are distorted, the amount of distortion is fairly low over most of the map.
The Robinson Projection was developed by Arthur H. Robinson in 1961 and was indeed to make world maps “look right” rather than measure precisely. This now common projection and has been used in many popular maps such as the Rand McNally series (from the 1960s) and the National Geographic Society (since 1988).
In the words of Arthur Robinson: “…I decided to go about it backwards. I started with a kind of artistic approach. I visualized the best-looking shapes and sizes. I worked with the variables until it got to the point where, if I changed one of them, it didn’t get any better. Then I figured out the mathematical formula to produce that effect. Most mapmakers start with the mathematics.”
A number of map projections have been used throughout history and deciding which projection to use is largely based on what is being mapped. Each projection has its tradeoffs and some are better at depicting the Polar Regions while others are better at depicting mid latitude areas. The scale of the map is also an important consideration as some projections are useful for small areas such as cities and counties while other projects are better for large areas such as continents or world maps.
Further considerations regarding choosing which map projection to use are the complexity of the mathematical functions that transform the coordinates from the curved surface of the earth to a flat plane. With the popularity of GIS software and robust computer hardware, these calculations are now primarily done by computer but without this convenience most mapmakers choose suitable projections with simpler mathematical equations.
Transverse Mercator Projection
The Transverse Mercator projection is widely used around the world and works especially well for mapping areas smaller than a few degrees longitudinally, such as a state or county.
The Transverse Mercator is a revision to the standard Mercator projection in which the cylinder is longitudinally along a meridian instead of the equator.
This conformal projection does not maintain true direction (especially evident at large scales) but this distortion can be minimized by placing the central meridian at the region of interest; this is why the Universal Transverse Mercator (UTM) coordinate system uses “zones” that each have their own central meridian. This projection is commonly used on topographic maps, geological maps, and U.S. Geological Survey maps.
Lambert Conformal Conic
Another common projection currently in use is the Lambert Conformal Conic (LCC). This projection is one of the best to use for middle latitudes and is often used for aeronautical charts, aviators, and maps with wide east-west extents.
It is a conformal conical projection with two reference parallels secant lines which help to minimizes distortion; in fact, there is no distortion along the standard parallels but distortion increases further from the chosen parallel.
Space Oblique Mercator
A unique and specialized projection is the Space Oblique Mercator (SOM) projection. This map projection was developed fairly recently, in 1976, for the specific purpose of mapping of imagery from an orbiting satellite around the ellipsoidal Earth and is completely free of distortion along the path of the satellite.
Originally Space Oblique Mercator was intended for the Landsat satellites but this projection can be used for any satellite in a circular or elliptical orbit around the Earth. This projection has been referred to as, “one of the most complex projections ever devised” by Library of Congress cartographic historian, John W. Hessler.
Many other map projections are currently in use around the globe. The U.S. Geological Survey, charged with mapping the United States, uses more than 18 different map projections with no one particular projection that is used for all applications.
Understanding map projections is critical to ensuring accurate and precise mapping.