Defining a shapefile's coordinate system
Release 9.3 Last modified November 19, 2008	Print all topics in : "Shapefiles"

About defining 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 considers the spatial reference Unknown.

Learn more about coordinate systems.

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. You can

• 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.

To add or modify the coordinate system of a shapefile, you can follow the steps below, or you can use the Project or Create Spatial Reference geoprocessing tools.

How to define a shapefile's coordinate system

Selecting an existing coordinate system

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   
4. Click Select.
   
5. Navigate to the coordinate system you want to use.
   

For example, you can use one of the predefined coordinate systems in the Coordinate Systems folder that was provided with ArcCatalog. Or you can use a projection file that was created with ArcInfo Workstation.

6. Click the coordinate system.
   
7. Click Add.
   
The coordinate system's parameters are listed in the Details box.

8. Click OK on the Shapefile Properties dialog box.
   

Importing a coordinate system

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   
4. Click Import.
   
5. Navigate to the data source whose coordinate system parameters you want to copy.
   

For example, you can get coordinate system information from coverages, rasters, or feature datasets and feature classes in a geodatabase.

6. Click the data source.
   
7. Click Add.
   

The coordinate system's parameters are listed in the Details box.

8. Click OK on the Shapefile Properties dialog box.
   

Defining a new geographic coordinate system

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   


4. Click New and click Geographic.
   
5. Type a name for the new geographic coordinate system.
   
6. Type the appropriate semimajor and semiminor or inverse flattening values, then type a name for your custom spheroid and datum.
   

Alternatively, click the Datum or Spheroid drop-down arrow and click a predefined datum or spheroid.

7. Type the appropriate radians per unit, then type a name for your custom units.
   

Alternatively, click the Angular Unit drop-down arrow and click a predefined unit of measure.

8. Type the appropriate degrees, minutes, and seconds defining the prime meridian, then type a name for this line of longitude.
   

Alternatively, click the Prime Meridian drop-down arrow and click a predefined line of longitude.

9. Click OK.
   
10. Click OK on the Shapefile Properties dialog box.
    

Tip

When creating a custom coordinate system, if you choose an object, such as a datum or spheroid, from one of the drop-down lists, those parameter values will be read-only. To modify them, click the drop-down arrow again and click <custom>. Change the default values appropriately and type a name for your custom set of parameters.

Defining a new projected coordinate system

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   


4. Click New and click Projected.
   
5. Type a name for the custom coordinate system.
   
6. Click the Projection Name drop-down arrow and click one of the supported projections.
   


7. Type the appropriate parameter values for the projection.
   
8. Click the Linear Unit drop-down arrow and click a predefined unit of measure.
   

Alternatively, click <custom> in the drop-down list, type the appropriate meters per unit, then type a name for your custom units.

9. Specify the projection's datum by selecting a predefined geographic coordinate system or defining a new geographic coordinate system. Afterward, you can modify the geographic coordinate system's parameters if you want.
   
10. Click OK.
    
11. Click OK on the Shapefile Properties dialog box.
    

Modifying a coordinate system's parameters

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   
4. Click Modify.
   

The Geographic or Projected Coordinate System Properties dialog box appears.

5. Follow the steps for defining either a new geographic or a new projected coordinate system to change the appropriate parameters.
   
6. Click OK.
   
7. Click OK on the Shapefile Properties dialog box.
   

Saving a coordinate system to a file

1. In ArcCatalog, click the shapefile whose coordinate system you want to define.
   
2. Click the File menu and click Properties.
   
3. Click the XY Coordinate System tab.
   
4. Click Save As on the Shapefile Properties dialog box.
   
5. Navigate to the location where you want to place the coordinate system file. For example, place it in a shared folder on the network.
   
6. Type a name for the coordinate system file and click Save.
   
7. Click OK.
   

The coordinate system file appears in the folder's Contents list.

Please visit the Feedback page to comment or give suggestions on ArcGIS Desktop Help.
Copyright © Environmental Systems Research Institute, Inc.