Map projection and its properties
As the Earth’s shape is an ellipsoid and it has a curved surface, when crafting representations of the Earth upon flat paper or making a map, there are differences between both surfaces. If the Earth’s shape is cartographed into a flat map, it will cause distortion. In fact, when the Earth’s ellipsoid surface is crafted upon a piece of paper, no maps can accurately represent actual characteristics of the Earth’s surface. To ensure correctness, the Earth’s surface must be crafted upon a globe, which is in round shape similar to the Earth. However, there is a limitation of map scales since a large-scaled globe cannot be used. As a result, a great deal of details on the Earth’s surface cannot be placed on the globe. Otherwise, it may cause high expenses.
Consequently, it is necessary to portray the Earth’s surface upon flat paper. If every map has to maintain its properties involving direction, equal area or shape, the map will be distorted from the real Earth’s surface. As such, an idea to create a map projection was initiated to solve various problems in map-making process.
Map projection is created by projecting the shadows of shaped objects on a flat surface. This principle is applied for the creation of map projection by casting light through a transparent globe, so that the shadows of parallels and meridians on the globe will appear on a flat surface used for map-making. However, a map projection requires mathematic calculation combined with the creation of geometric images.
The surfaces used for shadow casting of parallels and meridians, which are developed into a map projection, include cone-shaped surface, cylinder-shaped surface and azimuthal surface.
Map projection is a system of lines that consist of parallels and meridians which are used to portray round shape of the Earth on a flat surface. The method is to create geometric images or make a mathematical analysis to maintain the distance, area, direction or shape in accordance with the real scale on Earth. The map projections that are used for mapping purpose have the following properties:
- Conformality property
- Areas on the map and on the Earth’s surface should be comparable
- The spacing between details on the map and on the Earth’s surface should be comparable
- The direction in the map should be the real direction.
- Details appearing on the map should be easy to understand and determine positions.
- The shortest distance between two positions on the curve of the great circle (meridian) appears on the map as a straight line, which is the shortest spacing between two positions on a flat surface.
However, since no map projections have all of these properties, the geographers have to choose only one type of map projections in accordance with the objective or the main goal of map making.