Specifying a coordinate system

To display your data correctly, each data frame uses a coordinate system. It determines the map projection for the map display in the data frame. The data frame's coordinate system need not be the same as the data you are using, although if ArcMap has to project your data on the fly, it can take longer to draw.

When ArcMap is started with a new, empty map, the coordinate system for the default data frame is not defined. The first layer added to an empty data frame sets the coordinate system for the data frame, but you can change it if necessary. As you add subsequent layers, they are automatically displayed using the data frame's coordinate system as long as the data source's coordinate system is defined.

If there isn't enough information, ArcMap will be unable to project the data in each layer and display it correctly. In this case, you'll have to supply the necessary coordinate system information yourself.

Generally, if you have a layer whose dataset does not have a coordinate system defined and you know which coordinate system it is using, you should use the Define Projection tool in ArcToolbox to assign projection information to your data. This is required for working with that data in ArcGIS.

Tip:

To see if your data source has a coordinate system defined, right-click the layer in the ArcMap table of contents and click Properties to open the Layer Properties dialog box. Click the Source tab, then look in the Data Source box. You can also use the Catalog window to see if your data has a coordinate system defined.

You can review and set the data frame's coordinate system from the Coordinate System tab on the Data Frame Properties dialog box.

Changing the data frame's coordinate system

Note:

To set the coordinate system of a data frame, you must not currently be editing. To end your editing session, click Stop Editing from the Editor menu.

1. Right-click the data frame name and choose Properties to bring up the Data Frame Properties dialog box.
2. Click the Coordinate System tab and navigate to the desired coordinate system for your map display.

   To set the data frame coordinate system to be the same as a particular layer in the data frame, open the Layers folder and select the layer.

   To set the data frame coordinate system to be the same as a layer that is not in the data frame, click the Import button and browse to a data source that is defined with the coordinate system you want to use.

   Note:

   You can modify parameters of the data frame coordinate system by clicking the Modify button.

Changing the coordinate system of a data frame doesn't alter the coordinate systems of the source data in the data frame.

For more information, see Coordinate systems for map display.

Working with Map Units

The map units are the units in which the layers in the data frame are displayed and used. The map units are determined by the data frame's coordinate system.

The map units will be shown as unknown until the coordinate system is specified for the data frame. If the data frame contains no data, add your first layer to the data frame, which will set the coordinate system automatically.

You can set additional appearance options for the x,y coordinate readout in the bottom right corner of the ArcMap window by changing the display map units in the Data Frame Properties dialog box.

1. Right-click the data frame name in the table of contents and click Properties.
2. Click the General tab. You can see the current Map units and also set the Display units.

You can also set additional coordinate display properties for the status bar by clicking Customize > ArcMap Options on the main menu, and clicking on the Data View tab.