Projection – Since the earth is round and not flat, simple XY coordinates become inaccurate over longer distances due to the earth’s curvature. Projections are mathematical algorithms to adjust coordinates such as latitude and longitude for the curvature. Typical projections include:

NAD27 (North American Datum of 1927) NAD27 was developed using the Clarke 1866 spheroid, referring to the geometric definition of the earth’s shape.

NAD83 (North American Datum of 1983) is a replacement for the long-established NAD27 is based on the GRS80 spheroid, derived from satellite imagery. A point assigned a latitude and longitude in NAD27 may be blocks away from another point assigned the same latitude and longitude using NAD83.

The significance to projection methods is that GIS users must be aware of the method(s) associated with geocoded datasets and geography files so that layers using different projections are not mixed on the same map. Most GIS software will automatically distinguish between layers using different methods and warn the user that the methods are inconsistent. Geocoded datasets, however, can be more problematic since they may not carry the projection method used to assign the coordinates, leaving the user to keep track of the method and ensure that it matches the projections of other datasets and geographic layers.