Projections and Coordinate Systems

To identify any location on the Earth, we need to know the longitude and latitude of that location, which is one spatial coordinate system. Usually we rely on a map to find the exact location of a place. How can we show the spherical surface of the Earth on a piece of paper? Projection is the method of transferring data from locations on the curved surface of the Earth to corresponding locations on the flat surface of the map. Unfortunately it is not possible to flatten curved surfaces without stretching, tearing, or otherwise distorting them. There are different projection processes that are suitable for different application and with different distortions. For a detailed explanation, please see information from ESRI’s Resource Center

ArcGIS allows you to project data “on-the-fly”, meaning data in two different projections will be lined up properly by the software as long as the data files include projection information. ArcTools can be used to specify the correct projection a dataset is in.

Projecting “on-the-fly” is fine for displaying data, but if you are going to be doing analyses with different sets of data, I recommend converting them all to the same projection using ArcTools. ArcGIS will allow you to view data in geographic coordinates (e.g. latitude and longitude in decimal degrees), but if you want to measure distances, it is better to use projected data.

Don’t know what projection to use? Find out the projection used by an appropriate government agency and adopt that one. For example, in Massachusetts, MassGIS in the Executive Office of Environmental Affairs uses North American Datum 1983, State Plane Coordinate System, Mainland Zone, units in meters, for all of their mainland data.

All spatial datasets must be accompanied by documentation (metadata) that specifies the projection the dataset is in.

Last modified 1/10/2013.