Fundamentals of a shapefile's coordinate system
A shapefile often doesn't have any information that identifies which coordinate system was used to define its features. In this case, the Shape column's Spatial Reference property will be Unknown or Assumed Geographic. If the features' bounding coordinates are within the range of -180 to 180 in the x direction and -90 to 90 in the y direction, ArcGIS assumes the data to be geographic and its datum to be NAD27. If the bounding coordinates are not within this range, the software treats the spatial reference as unknown.
You can work with shapefiles even if their coordinate system hasn't been defined, but you may not be able to take advantage of all the available functionality. For example, your shapefile may not line up with other data, and its automatically created metadata will be incomplete.
You can define a shapefile's coordinate system in ArcCatalog in several ways:
- Select one of the predefined coordinate systems provided with ArcCatalog.
- Import the coordinate system parameters used by another data source.
- Define a new, custom coordinate system.
The coordinate system parameters must be saved in a .prj file in the same folder as the shapefile, and the .prj file must have the same file prefix as the shapefile. For example, if you're working with a shapefile named wells.shp, its coordinate system parameters must be stored in the same folder in a file named wells.prj.
Once a coordinate system has been defined, you can modify individual parameters. For example, you might want to modify one parameter in a coordinate system that was imported from another data source or customize one of the predefined coordinate systems. After creating a custom coordinate system, you can save it as a separate coordinate system file; you might want to share this coordinate system with others in your organization.