Map projections (and co ordinate systems) are a fundamental part of map making. They can be a bit of an enigma, even to cartographers. This guide is a basic introduction to the mechanics of projections.
Before a map can even begin to be a map, the data it's made with needs a particular set of display 'rules' to graphically represent the information properly - a projection.
A projection is the mathematical method of representing our three dimensional earth in a flat, two dimensional context i.e a map.
In order to be shown in the most suitable way for a given area, geographic data uses co-ordinate systems and projections specially crafted for that part of the world or the world as a whole.
In order to best flatten the spherical world, a developable surface, one that doesn't warp, rip or contract, is chosen to formulate a projection. The most common surfaces are cylinders, cones and planes.
These shapes are ideal because by cutting from top to bottom and unrolling, they can be turned into a flat sheet.
The shape is 'wrapped' around the globe. Imagine the globe is covered in ink. When the shape is unwrapped and peeled off, the places where the earth has touched the shape gives a rough representation of a two dimensional earth. But it's not that simple! Too much information would be lost, so in order to preserve shapes, angles and other important information, lots of math is involved to carefully modify the way the sphere is projected onto the shape.
The way the shape touches the globe is called the aspect. There are commonly used aspects, normal and transverse, though it's possible for the shape to touch along any line around the earth.
Central meridians and standard parallels are chosen to provide necessary reference positions which aid in the development of the rest of the projection. These points remain 'stationery' whilst the rest of the geographic information is modified to fit within and around them.
Every projection distorts the earth. No projection can show every characteristic accurately. It's the unfortunate reality of trying to transform a sphere into a different dimension.
Different types of projections exist to preserve particular qualities - areas, distances or angles.
Many projections take elements of each of these striking a compromise to make a projection with a more balanced appearance.
Over time cartographers, mathematicians and geographers have developed numerous map projections each with their own strengths. This variety enables map makers to create the most accurate map they can by choosing a projection that best suits the type of map being made. Luckily for us, not only are map projections necessary and useful, many are beautiful, too.
The next time you look at a map, take note of its projection. Projections are fundamental to how maps become maps - they quite literally shape how we view the world.