You can choose an existing spatial reference to use with an instance in the New Instance or the Instance Properties dialog box. You also have the option to choose a new spatial reference.
- Start ArcMap.
- If you do not have the New Instance dialog box open, right-click an existing instance from the Product Library tree view and choose Properties to open the Instance Properties dialog box.
-  Click the plus sign (+) next to Spatial Reference. 
 
- Verify the cell next to Use Existing is set to True.
- Click the cell next to Base Spatial Reference and click the ellipsis (...) that appears. 
The Choose Existing Spatial Reference dialog box appears.  
- Click the Spatial Reference drop-down arrow and choose the spatial reference that you want to use with all the areas of interest within the solution.
- Click OK. 
The New Instance or the Instance Properties dialog box appears. 
- Click OK to close the dialog box.