Specifying a coordinate system
Release 9.1 Last modified December 7, 2006	Print all topics in : "Creating maps"

About specifying a coordinate system

If all the data you want to display on your map is stored in the same coordinate system—for example, if you're using your organization's database—you can just add it to a map and not consider whether the layers will overlay properly; they will. If, however, you've collected data from a variety of sources, you'll need to know what coordinate system each dataset uses to ensure ArcMap can display them together.

For more information on coordinate systems, see the book, Understanding Map Projections, or the 'Map projections' section in this Help system.

Coordinate systems and data frames

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 transformed to the data frame's coordinate system as long as there's enough information associated with the layer's data source to determine its current coordinate system. If there isn't enough information, ArcMap will be unable to align the data and display it correctly. In this case, you'll have to supply the necessary coordinate system information yourself.
Learn more about defining projections

Map units

The map units are the units in which the spatial data in the data frame is drawn. The map units are determined by the coordinate system specified on the Coordinate System tab of the data frame's Properties dialog box. The map units dropdown list is disabled once a coordinate system has been specified; map units cannot be changed once a coordinate system has been specified for the map.

The map units will be shown as unknown if no coordinate system has been specified for the data frame, or if the coordinate system is unknown because the data in your map doesn't have spatial reference information. When the map units are unknown, you should either specify a coordinate system for the data frame or—if the data frame contains no data—add your first layer to the data frame, which will set the coordinate system automatically if the data has spatial reference information. In some situations, you can set the map units manually. For example, you can specify what the map units will be in for any graphics you draw or features that you edit.

How coordinate system information for a data source is stored

ArcMap expects coordinate system information to be stored with the data source. For a layer in a geodatabase, this information is part of the layer's metadata. For coverages, shapefiles, and rasters, it's stored on disk in a separate file named after the data source but with a .prj file extension, for example, streets.prj. These files are optional files; and you may still need to define the coordinate system for one of these data sources. You can create a .prj file using ArcCatalog.

Learn how to define a shapefile's coordinate system
Learn how to define a coverage's coordinate system

If no coordinate system information is associated with a data source, ArcMap will examine the coordinate values to see if they fall within the proper range: -180 to 180 for x-values; and -90 to 90 for y-values. If they do, ArcMap assumes that these are geographic coordinates of latitude and longitude. If they don't, ArcMap simply treats the values as planar x,y coordinates.

How to specify a coordinate system

Finding out what coordinate system your data is currently displayed with

1. Right-click the data frame for which you want to determine the coordinate system and click Properties.
2. Click the Coordinate System tab.
The details of the current data frame coordinate system display in the dialog box.

Tips

Changing the coordinate system of a data frame doesn't alter the coordinate system of the source data contained in it. To see if your data source has a coordinate system defined, right-click the layer in the ArcMap table of contents and click Properties. Click the Source tab, then look in the Data Source box. You can also use ArcCatalog to see if your data has a coordinate system defined.

Finding out what map units your data is currently displayed with

1. Right-click the data frame for which you want to determine the map units and click Properties.
2. Click the General tab.
The current map units are shown.

Tip

The data frame's display units, which were known as distance units in ArcView GIS 3.x and ArcInfo 8.x, can be set independently of the map units. The display units are the units in which distances and coordinates will be displayed in ArcMap.

Displaying data with a predefined coordinate system

1. Right-click the data frame for which you want to set the coordinate system and click Properties.
2. Click the Coordinate System tab.
3. Double-click Predefined.
4. Navigate through the folders until you find the coordinate system you want, then click it.
5. Click OK.
All layers in the data frame will now be displayed with that coordinate system.

Tip

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

Modifying the parameters of a coordinate system

1. Right-click the data frame for which you want to modify the coordinate system and click Properties.
2. Click the Coordinate System tab.
3. Click Modify.
4. Adjust the coordinate system properties as appropriate and click OK.
5. Click OK on the Data Frame Properties dialog box.

Tip

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

Setting the units for reporting distance and displaying coordinates

1. Right-click the data frame and click Properties.
2. Click the General tab.
3. Click the Display dropdown arrow and click the appropriate units.
If this dropdown arrow is unavailable, the map units of your data frame are currently set to Unknown because no coordinate system has been specified for it.

4. Click OK.

Tip

The data frame's display units, which were known as distance units in ArcView GIS 3.x and ArcInfo 8.x, can be set independently of the map units. The display units are used by the Measure tool, provide the default units used by scalebars, and are the units in which the map coordinate readout is shown in the status bar.

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