Choosing a coordinate system

When you create a new standalone feature class or feature dataset, you need to specify its coordinate system. You can select a predefined coordinate system, import a coordinate system from a supported spatial data format, or create a new coordinate system.

You have three options for specifying the coordinate system for new feature classes and feature datasets.

· Select

When you select an existing coordinate system, you can choose either a geographic or projected coordinate system. Because all projected coordinate systems are based on a geographic coordinate system, you set one or the other, never both.

ArcGIS supports over 600 geographic coordinate systems, each optimized for a specific area on the earth, as well as over 2,700 projected coordinate systems.

· Import

You can import a coordinate system from an existing coverage, shapefile, feature class, or feature dataset provided that the coordinate system has been documented for the source data (for coverages, coordinate system information is stored in a file called prj.adf; for shapefiles, this information is found in the .prj file). When you import a coordinate system, you have the option of modifying its parameters.

· New

You can create a custom coordinate system by choosing the desired parameters in a dialog. Your new coordinate system can be saved as a .prj file.

The coordinate system that you specify when setting the spatial reference simply documents your data—your data is not reprojected. Therefore, if you accidentally set the wrong projected coordinate system when creating a feature dataset, you can change it by choosing a different one in ArcCatalog.