# What is a Map Projection?

A map projection is a mathematical expression that is used to represent the round, 3D surface of the earth on a flat, 2D map.

1. 3D Earth
2. Mercator Projection

This process always results in distortion to one or more map properties, such as area, scale, shape, or direction. Because of this, hundreds of projections have been developed in order to accurately represent a particular map element or to best suit a particular type of map.

Data sources for maps come in various projections depending upon which characteristic the cartographer chooses to represent more accurately (at the expense of other characteristics). In the example above, the Mercator projection preserves the right angles of the latitude and longitudinal lines at the expense of area, which is distorted at the poles, showing the land masses there to be larger than they actually are.

The following are some common map projections:

Albers Equal Area Conic

Typically used for small regions or countries extending in an east-to-west direction, but not continents. Preserves angles between meridians and parallels. Attempts to minimize distortion for both shape and linear scale, but neither is truly correct. The example here shows how this projection looks over the entire earth.

Oblique Mercator (Hotine)

A cylindrical projection like the Mercator projections, but where the cylinder is shifted to align with a region that is oblique and follows neither a north-south nor an east-west axis. The region to be mapped typically is a small portion along the length of the meridian and close to it laterally. For example, this projection was developed originally for mapping the Malaysian peninsula.

Chamberlin Trimetric

Used by the National Geographic Society for mapping most continents. This projection is a three-point equidistant one, intended to preserve the distance between three reference points relative to any other point.

Lambert Conformal Conic

A projection ideal for middle latitudes and/or where the territory to be mapped has an east-west orientation. This projection is often found in USGS maps created after 1957. Scale is most accurate at the expense of area.

When building whole-earth databases, a single global projection is the most convenient to use. Google Earth uses Simple Cylindrical projection for its imagery base. This is a simple map projection where the meridians and parallels are equidistant, straight lines, with the two sets crossing at right angles. This projection is also known as Lat/Lon WGS84.

1. Simple Cylindrical (Plate Carree) Projection