Why we need map projections

Map projection is the method of transferring the graticule (the network of parallels and meridians) from the round earth onto a flat surface. On a globe, parallels are circles and meridians semicircles; on a flat sheet they become straight or curved lines that cross each other.

Why not just use a globe? A globe is the best model of the earth: it shows the shape and size of continents and oceans, and directions and distances, accurately. But:

• it is expensive;
• it cannot be carried everywhere easily;
• it cannot show small details, and it is hard to compare two regions on it.
• So for detailed study we need accurate, large-scale maps on flat paper.

The problem. A sheet of paper laid on a globe touches it only along a line or at a point; it cannot fit a large part of the surface without creasing. Light shone from the globe's centre gives a picture that is more distorted the farther it is from where the paper touches. The reason is that a sphere is not a developable surface: it cannot be flattened without stretching or tearing.

• So some distortion is unavoidable in every projection.
• Many projections have been devised, each keeping some properties true for a particular purpose. Map projection is also the study of these methods.

From the video: the teacher asks you to imagine cutting a ball with scissors and pressing it flat: the pieces split apart and stretch. That is the problem every projection tries to manage. He also notes that today you choose a projection in GIS software, and you can only choose well if you know each one's strengths and weaknesses.

Example (new): try pressing half an orange peel flat on a table: it tears or wrinkles at the edges. A map-maker has the same trouble with the earth, and must decide which kind of error to accept.