Over the centuries, many different ways of representing the round Earth on flat paper have been developed. Each of these methods is referred to as a map projection. What does it mean to project something? Have you ever been to the movie theatre? How does the movie get on the movie screen? The image you watch on the screen is projected using a high powered light from the back of the theater. Now, imagine placing a projector inside of a globe and projecting the different continents, islands, and other features onto a flat screen. What would the projection look like? This depends on where you place the screen. Map projections are attempts to portray the surface of the earth or a portion of the earth on a flat surface. Some distortions of conformality, distance, direction, scale, and area always result from this process. Some projections minimize distortions in some of these properties at the expense of maximizing errors in others. Some projection are attempts to only moderately distort all of these properties.
Conformality : When the scale of a map at any point on the map is the same in any direction, the projection is conformal. Meridians (lines of longitude) and parallels (lines of latitude) intersect at right angles. Shape is preserved locally on conformal maps.
Distance : A map is equidistant when it portrays distances from the center of the projection to any other place on the map.
Direction : A map preserves direction when azimuths (angles from a point on a line to another point) are portrayed correctly in all directions.
Scale : Scale is the relationship between a distance portrayed on a map and the same distance on the Earth.
Area : When a map portrays areas over the entire map so that all mapped areas have the same proportional relationship to the areas on the Earth that they represent, the map is an equal-area map.
Different map projections result in different spatial relationships between regions.
A cylindrical projection map is the most common type of map that we see. Imagine placing the movie screen around the globe in a cylinder shape. The projection that results is depicted in this image. Notice that areas close to the equator have very little distortion. However, the closer to the poles that one travels, the more distorted the map becomes. In this example, Greenland appears to be many times larger than it really is.
A conic projection map is created by placing a cone shaped screen on a globe. The resulting projection is more accurate than the cylindrical projection map discussed above. However, the further we travel down the map, the more distorted and less accurate the map becomes.
A plane projection is created by placing an imaginary screen directly above or below a globe. The image that would result is called a plane projection. This type of map projection is not commonly used.
There are many different types of interrupted projection maps. These types of maps try to depict the continents as accurately as possible by leaving blank space in the less important areas of the map, such as in the oceans.
Each of the map projections previously discussed as well as any other type of map projection must consider two important factors. What is more important, depicting the accurate sizes of objects on the map, or depicting accurate shapes of these objects? The challenge is that you cannot have both. The more accurately you depict shape, the less accurate will be your depiction of size, and vice versa.
A map which portrays shape accurately is called a conformal map. Conformal maps are useful in that they help us understand the true shape of the items on the map. However, these maps have many drawbacks. A conformal map tends to get quite distorted, especially towards the top and bottom of the map. This creates problems with scale. The scale may be accurate near the equator, but the further one travels form the equator, the less accurate the scale becomes.
Many maps are neither entirely conformal nor entirely equivalent. By blending both conformality and equivalency, we can create a map that balances the distortion of both size and shape.
Thus, some map projections are entirely conformal, while others are entirely equivalent. It is impossible for a map to be both conformal and equivalent. However, many maps are a hybrid between conformal and equivalent.
Map projection types all have their pros and cons, but they are incredibly versatile. Even though it is nearly impossible to create an entirely accurate map projection there are uses for even the most imperfect depictions of the Earth. Map projections are created for certain purposes and should be used for those purposes. In the end each and every map projection has a place, and there is no limit to the amount of projections that can be created.
Ref: Progonos.com, GISLounge, Kids GeoGraphy