Before jumping in the heat of projections and its repercussion on a GIS analysis, here are some important distinctions and definitions to understand the basis.
A datum is a mathematical model of the Earth surface that can be used as a reference system to plot locations across the globe (Shelito, B.A., 2012). There are many different datums used depending of the year of the collection of the data and the location. The most common datum are the North American Datum of 1927 and 1983 (respectively NAD27 and NAD83) and the World Geodetic System 1984 (WGS84). One thing to keep in mind is that datums do not necessarily line up with one another. If you need to transform data from one datum to another, you need to effectuate a datum transformation which are available in most GIS software.
A Geographic Coordinate System is a global reference system for determining the exact position of a point on earth (Shelito, B.A., 2012). In a geographical coordinate system, a location is given by a latitude and a longitude which can be expressed in decimal degree or in degree minute second. A given coordinate is only exact if used in conjunction with the right datum.
Projections are an essentials part of every datasets. Basically, a projection is the mathematical operation needed to go from the planet actual shape to a flat map according to the Geographic Coordinate System. Just think about a orange peel. In order to go from the actual orange to a flat peel on your desk.
The process of systematically transforming positions on the Earth’s spherical surface to a flat map while maintaining spatial relationships, is called map projection. This projection process is accomplished by the use of geometry and, more commonly, by mathematical formulas. In geometric terms, the Earth as a spheroid (that is, a slightly flattened sphere), is considered an undevelopable shape, because, no matter how the Earth is divided up, it cannot be unrolled or unfolded to lie flat. Some of the simplest projections are made onto geometric shapes that can be flattened without stretching their surfaces. These shapes or forms are considered to be developable. Examples of shapes that reflect these properties are cones, cylinders, and planes.
The CONE, CYLINDER and PLANE are developable geometric shapes. The curved surface of the Earth can be projected on to these forms which can be unrolled to make a flat map. (The PLANE is already a flat surface!) —The Atlas of Canada
Intrigued? Visit one of these website for more information and projections gallery.
- Nevron Map projections gallery
- International Institute for Geo-Information Science and Earth Observation (ITC) – Map projections
When working in a GIS software, projections issues can be the cause of many difficulties For example, layers with different projections might not align at the same place. Also, projections need to be defined in order to calculate area or length of a feature.
Here are some resources that can be helpful when facing problems associated with projection in GIS.
- Map projections : cartographic information systems
- Offers a review of map projections (sphere, ellipsoidal, rotational surfaces) and of the geodetic datum transformations, in the context of Geographical Information Systems (GIS). This book also includes the reviews of computer vision and remote sensing space projective mappings.