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Select the coordinate reference system by typing any part of it in the System field, such as the name of the state ("Utah") or an EPSG number ("4326") and then choose from the CRSs that appear as you type. You may alternatively select one of the CRSs you use most often by choosing it from the Recently Used Systems drop-down menu.
If a particular EPSG code is not available in the System field, select the Use Custom WKT check box and select an option from the Select WKT drop-down list. If you’ve already created and saved a custom well-known text string (WKT), you can choose it from this list. Otherwise, select Add a new WKT to create a new custom WKT that describes the CRS. You can paste a WKT string in the WKT field, or you can use a text file or Esri CRS file (*.prj) as input by clicking Browse.
NOTE: For information on how to create a WKT, see the Open Geospatial Consortium (OGC) spec # 01-009 at http://www.geoapi.org/3.0/javadoc/org/opengis/referencing/doc-files/WKT.html. For a listing of common WKT strings, see http://www.spatialreference.org/.