Projection basics for GIS professionals
Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for communication about a particular place or area on the earth's surface. The most critical issue in dealing with coordinate systems is knowing what the projection is and having the correct coordinate system information associated with a dataset. There are two types of coordinate systems—geographic and projected.
A geographic coordinate system uses a three-dimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). In a geographic coordinate system, a point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads).
A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid.
In a projected coordinate system, locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position.
When the first map projections were devised, it was assumed, incorrectly, that the earth was flat. Later the assumption was revised, and the earth was assumed to be a perfect sphere. In the 18th century, people began to realize that the earth was not perfectly round. This was the beginning of the concept of the cartographic spheroid.
To more accurately represent locations on the earth's surface, mapmakers studied the shape of the earth (geodesy) and created the concept of the spheroid. A datum links a spheroid to a particular portion of the earth's surface. Recent datums are designed to fit the entire earth's surface well.
These are the most commonly used datums in North America:
- North American Datum (NAD) 1927 using the Clarke 1866 spheroid
- NAD 1983 using the Geodetic Reference System (GRS) 1980 spheroid
- World Geodetic System (WGS) 1984 using the WGS 1984 spheroid
Newer spheroids are developed from satellite measurements and are more accurate than those developed in the 19th and early 20th centuries.
You will find that the terms "geographic coordinate system" and "datum" are used interchangeably.
The coordinates for a location will change depending on the datum and spheroid on which those coordinates are based, even if using the same map projection and projection parameters. For example, the geographic coordinates below are for the city of Bellingham, Washington, using three different datums:
A principle of good data management is to obtain the coordinate system information from the data source providing the data. Do not guess about the coordinate system of data because this will result in an inaccurate GIS database. The necessary parameters are the following:
Geographic coordinate system (Datum) Unit of measure Zone (for UTM or State Plane) Projection Projection parameters
Projection parameters may be required, depending on the map projection. For example, the Albers and Lambert conic projections require the following parameters:
1st standard parallel 2nd standard parallel Central meridian Latitude of origin False easting False northing Unit of measure
You can define a coordinate system for data with the Define Projection tool in the Data Management toolbox.
If the data has a coordinate system definition, but it does not match the typical coordinate system used by an organization, you can reproject the data. You can reproject data in a geodatabase feature dataset, feature class, shapefile, or raster dataset using the Project tool or Project Raster tool in the Data Management toolbox.