Specifying a coordinate system
Release 9.3 Last modified December 3, 2008	Print all topics in : "Using ArcMap"

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 Map projections and coordinate systems.

Coordinate systems and data frames

To display your data correctly, a data frame uses a coordinate system. This can be any coordinate system ArcGIS supports. 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 does take longer to draw.

When ArcMap is started with a new, empty map, the coordinate system for the default data frame is not defined. When you add data with a defined coordinate system, ArcMap will automatically set the data frame's projection to be the same as that of the data. The first layer added to an empty data frame sets the coordinate system for the data frame, but you can change it if necessary. If the first layer does not have a coordinate system, ArcMap will set the coordinate system to Unknown.

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.

When the data frame's coordinate system is unknown, you may set the map units (on the General tab of the Data Frame Properties dialog box), provided you know the units of the coordinate system in which the data is stored. Generally, if you have data that 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 will make working with that data in ArcMap much easier.

If you want to use a specific coordinate system for your map, you can use the Data Frame Properties dialog box to set the coordinate system. Once you do this or if ArcMap has already automatically defined the coordinate system for a data frame, ArcMap will not override the coordinate system settings.

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 Properties dialog box. The Map Units drop-down 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 and is stored in the geodatabase. For shapefiles, it's stored on disk in a separate file named after the data source but with a .prj file extension, for example, streets.prj. Coverages and ESRI Grids store the coordinate information in a .prj.adf file located in the data source's folder. Other raster formats store the coordinate system information in the .aux file or the raster header (the beginning of the raster file). These are optional files; and you may still need to define the coordinate system for one of these data sources. You can define a coordinate system using ArcCatalog or geoprocessing tools.

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. The map and display units are set to Unknown.

How to specify a coordinate system

Setting a data frame's coordinate system

1. Right-click the data frame in the table of contents and click Properties.
   
2. Click the Coordinate System tab on the Data Frame Properties dialog box.
   
3. Choose a coordinate system from the tree or click the Import button and browse to a data source that is defined with the coordinate system you want to use.
   
Once you have chosen a coordinate system, its parameters are displayed in the Current coordinate system box.

4. Click OK.
   
All layers in the data frame will now be displayed with that coordinate system.

Tip

To set the coordinate system of a data frame, you must not currently be editing, since changing the coordinate system used by your map while you are editing could introduce alignment and accuracy problems. To end your editing session, click Stop Editing from the Editor menu.

Setting a data frame's coordinate system to a predefined coordinate system

1. Right-click the data frame in the table of contents and click Properties.
   
2. Click the Coordinate System tab on the Data Frame Properties dialog box.
   
3. Double-click the Predefined folder to expand its entries.
   
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.

Setting the data frame's coordinate system from a layer in the map

1. Right-click the data frame in the table of contents and click Properties.
   
2. Click the Coordinate System tab on the Data Frame Properties dialog box.
   
3. Double-click the Layers folder to expand its entries.
   
4. Double-click the name of the layer that will supply the coordinate system for your data frame.
   
5. Click the coordinate system.
   
6. Click OK.
   
All layers in the data frame will now be displayed with that coordinate system.

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.

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 on 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 and ArcGIS 8, can be set independently of the map units. The display units are the units in which distances and coordinates will be displayed in ArcMap.

Finding out the coordinate system of a layer's data source

1. Right-click a layer in the table of contents and click Properties.
   
2. Click the Source tab.
   
3. Scroll through the information in the Data Source box. The coordinate system details are at the bottom of the list.
   

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 drop-down arrow and click the appropriate units.
   
If this drop-down 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.
   

Tips

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 scale bars, and are the units in which the map coordinate readout and the area and dimensions of graphics you draw are shown in the status bar. You can set additional appearance options for the x,y coordinate readout in the bottom right corner of the ArcMap window by clicking the Tools menu, then Options, and the Data View tab. These settings are stored in the current map document (.mxd file) and apply to all the data frames in the map. You can change the number of decimal places shown (three is the default) and turn on thousand separators and zero padding. In addition, these coordinates normally are reported in display units, but you can override this by checking the box for Use these units instead of the data frame's display units and clicking a unit type. Note that this option does not affect the units used to report the dimensions of graphics that you draw with the tools on the Draw toolbar, which are always reported in display units. This option also does not affect the units used to report the dimensions of features you create or edit in an edit session using the Editor toolbar, which are reported in the map units of the data you are editing.

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