To translate the curved surface of the Earth to a flat (planar) computer screen, it must undergo a projection. Types of projections include the well-known conformal Mercator projection, which preserves shape, and the equal-area Albers projection, which preserves relative size. Choosing an appropriate projection for your map reduces distortion as much as possible for your region of interest.
A coordinate reference system (CRS) connects projections to real locations on Earth. There are two types of CRS: a geographic coordinate system (GCS) and a projected coordinate system (PCS). A GCS is a 3-D surface model that accurately defines where data is located, whereas a PCS provides information for how to draw data on a planar projection. Each GCS is based on a unique datum. Since the Earth is not a perfect sphere, datums offer estimates for the size and shape of the Earth; they also decide where to center ellipsoidal GCS surface models in order to best fit a region.
Geographic coordinate systems measure data in angular units such as degrees and utilize longitude and latitude. Projected coordinate systems are also known as Cartesian coordinate reference systems; they use linear units such as meters or feet. A PCS is a GCS that has been flattened according to a specific map projection.
Maps of the United States displaying differences between the Mercator (left) and Albers (right) projections.
In the Albers projection, the top of the United States is slightly curved. In comparison, the Mercator version is straighter and stretched out; Alaska is also oversized, and Mexico appears smaller than in reality.
Resources and Tutorials
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Getting Started
An overview of coordinate reference systems, including PCS, GCS, and when to use different types of CRS
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ArcGIS Lab
A tutorial for changing CRS in ArcGIS
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QGIS Lab
A tutorial for changing CRS in QGIS
Click here to download the dataset for the above ArcGIS and QGIS labs: titanosaurs.csv
Questions?
Reach out to the Yale Geospatial Consulting Team: link here
See this Esri article
Please reach out to us at geospatial@yale.edu