Coordinate Systems and Projections

Coordinate systems and projections are two important mapping concepts about which you should have a basic understanding. Projection refers to how a map is displayed on a flat surface such as a paper map or computer screen, while a coordinate system describes how map features are spatially arranged. Both are important considerations when developing applications, especially those where spatial precision and accuracy are important.

A projection is a method of reducing the distortion that occurs when objects from a spherical surface are displayed on a flat surface. There are two main trade-offs to be considered: the preservation of equal area, and the preservation of the true shape of a feature. There are many different types of projections, each designed to reduce the amount of distortion for a given area. Some projections preserve shape; others preserve accuracy of area, distance, or direction.

A coordinate system is a set of parameters that tells you how to interpret the locational coordinates for objects. One of those parameters is projection. Coordinates can be of two types: Spherical or Cartesian. Spherical relates to locations on the curved surface of the Earth, while Cartesian describes flat surface locations in two dimensions. Both are represented by x and y coordinates. The difference comes when calculating distance or area of features that represent real Earth locations such as streets or rivers (Spherical), or relative locations, such as a map of brain anatomy or a chess board (Cartesian).

Knowing which coordinate system your map uses is an important consideration when developing applications. Analytical operations involving distance and area calculations, such as buffering, routing, and querying, use the coordinate system and projection to yield the correct results.