A coordinate system is a reference framework that defines the position of features in two- or three-dimensional space. Maps and scenes use coordinate systems to locate and display data correctly on the earth's surface and relative to one another. Coordinate systems can be horizontal, defining where features are across the globe, or vertical, defining how high or deep features are relative to a surface. Coordinate systems can be defined in maps and scenes.

A map or scene should always have a horizontal coordinate system. Optionally, you can also define a vertical coordinate system. In a new, empty map or local scene, the default horizontal coordinate system is WGS84 Web Mercator. For global scenes, the default horizontal coordinate system is WGS84. There are numerous coordinate systems to choose from for maps and local scenes, but the horizontal coordinate system for a global is limited to WGS84 or China Geodetic Coordinate System 2000 (CGCS 2000). See the topic on scenes to understand the differences between local and global scenes.

Empty maps and scenes derive their coordinate systems from the first layer added to them. When you add more layers to a map or scene, they are automatically displayed using the current coordinate system of the map or scene. If the map or scene's geographic coordinate system is different than the geographic coordinate system of the layer, the data is projected in real time using a transformation.

##### Caution:

Projecting data in real time can take longer to draw and is not advisable if you are editing data or performing analysis. It is better to ensure that all the data is in the same coordinate system. Use the Project or Project Raster tool to project spatial data from one coordinate system to another.

Follow the steps below to choose or import a coordinate system, or you can create a coordinate system instead.

## Set the coordinate system

You can set the coordinate system of a map or scene directly, you can define it from a layer in the map or scene, or you can set it from another data source. In some specific workflows, you may need to use an unknown coordinate system.

#### Choose a coordinate system from a list

Follow these steps to set the coordinate system directly by choosing one from a list:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
The buttons below the Current XY and Current Z headings show the current horizontal and vertical coordinate systems of the map or scene, respectively. There may be no vertical coordinate system defined. Click Details for either coordinate system to see how they are defined.

- To change the horizontal or vertical coordinate system, click the button below the Current XY or Current Z heading, respectively. Choose an appropriate coordinate system from the corresponding Coordinate Systems Available list.
- To learn more about a coordinate system, right-click it in the list and click Details to see the coordinate system parameters. The valid area of use for each coordinate system is specified in the list of details, and also visually as a blue rectangle on the map at the bottom of the dialog box. If the area is very small, it may appear as only a circular point instead.
- Optionally, search for coordinate systems. Enter a search term in the Search box to help locate a specific coordinate system.
- Optionally, filter the list of coordinate systems. Click the Spatial Filter button and click Set spatial filter . On the Spatial Filter Index dialog box, choose Extent of data in all layers or Custom extent. Calculate a custom extent from a layer in the map or by defining an exact numeric extent.

- Click Apply to set the coordinate system, or click OK to apply the settings and close the Map Properties dialog box.

#### Set the coordinate system from a layer

Follow these steps to define a coordinate system for a map or scene from a layer within it. It is recommended that you do this to ensure that all layers in a map share the same coordinate system.

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- Under the Coordinate Systems Available list, expand Layers. Expand a coordinate system heading to see the layers that reference it.
- To set the coordinate system from another data source, click the Add Coordinate System button and click Import Coordinate System . Browse to a data source (or previously saved .prj file) that is defined with the coordinate system you want to use. You can then add the coordinate system to your Favorites list to make it available to all your projects.
- Click Apply to set the coordinate system, or click OK to apply the settings and close the Map Properties dialog box.

#### Set the coordinate system from a data source

Follow these steps to define a coordinate system from a data source:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- Click the Add Coordinate System button and click Import Coordinate System . Browse to a data source (or previously saved .prj file) that is defined with the coordinate system you want to use.
- Click Apply to set the coordinate system, or click OK to apply the settings and close the Map Properties dialog box.

#### Use an unknown coordinate system

In some circumstances, it may be necessary to clear the coordinate system entirely for a map by setting the coordinate system to Unknown. When the coordinate system is unknown, the application does not perform automatic transformations, scaling, or projection to align data. For this reason, you might use an unknown coordinate system to visually compare or otherwise troubleshoot the spatial references of two or more layers.

A map with an unknown coordinate system cannot have a vertical coordinate system. Global and local scenes must use a valid coordinate system. The coordinate system cannot be set to unknown in scenes.

When you import an ArcMap document (.mxd) with an unknown coordinate system into ArcGIS AllSource, the resulting map has an unknown coordinate system.

Follow these steps to define an unknown coordinate system for a map:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- From the Coordinate Systems Available list, choose <Unknown>.
- Click Apply to set the coordinate system, or click OK to apply the settings and close the Map Properties dialog box.
If a vertical coordinate system is set on the map, it is removed when you apply the change.

##### Dive-in:

In a map with an unknown coordinate system, all values are unitless, meaning coordinate values of 10 degrees, 10 meters, and 10 feet are all interpreted as just 10 (equal to each other.) Layers draw using these unitless coordinates, positioned relative to the origin of their spatial reference and the 0,0 origin in this unitless system. In other words, layers of the same geographic area likely will not align in an unknown coordinate system if they have different spatial references.

Editing is disabled in a map with an unknown coordinate system. Most analysis operations in ArcGIS AllSource assume that a map has a properly defined coordinate system, so unexpected or unreliable results are likely to occur.

## Save coordinate systems

If there are coordinate systems you use often in your projects, you can add them to your list of Favorites to make them easier to find. Favorites are available across all your projects. You can also save any coordinate system as a projection file (.prj file extension). Projection files can be used to define a coordinate system in geoprocessing.

If you are creating a geographic coordinate system that you want to reuse in a new projected coordinate system, you must save the geographic coordinate system as a favorite for it to appear in the list of geographic coordinate system choices in the projected coordinate system.

##### Tip:

If you have a lot of favorite coordinate systems, consider organizing them in a folder structure. Create folders to organize your favorite coordinate systems (.prj files) in your operating system. Find your coordinate system that's set as a favorite at [install drive]:\Users\[YourName]\AppData\Local\ESRI\ArcGISPro\Favorites. This structure is then reflected in the Coordinate Systems Available list.

#### Save a coordinate system as a favorite

Follow these steps to save a coordinate system as a favorite:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- Right-click a coordinate system in the Coordinate Systems Available list and click Add to Favorites to have the coordinate system appear under the Favorites heading in this list for easy reference.

#### Save a coordinate system as a projection file

Follow these steps to save a coordinate system as a projection file:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- Right-click a coordinate system in the Coordinate Systems Available list.
- Click Save as Projection File .
- On the Save Coordinate System as PRJ File dialog box, browse to a location on your file system, and type a file name.
The default location to save the projection file is the Favorites folder, located at [install drive]:\Users\[yourname]\AppData\Local\ESRI\ArcGISPro\Favorites. If you save to this folder, the coordinate system you saved to a projection file will appear in the Favorites section in the Coordinate Systems Available list on the Map Properties dialog box.

## Work with vertical coordinate systems

Maps and scenes must have a horizontal coordinate system defined, but a vertical coordinate system is optional. To set an ellipsoidal vertical coordinate system, it must share the same datum as the horizontal coordinate system. The datum name, the spheroid name, and all the spheroid properties of the two coordinate systems must exactly match.

If the current horizontal coordinate system is incompatible with the current vertical coordinate system, follow these steps to choose compatible horizontal and vertical coordinate system pairs:

- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- Click the button below the Current Z heading and choose None from the list.
- Click the button below the Current XY heading and choose a coordinate system from the list with a datum that matches that of the ellipsoidal vertical coordinate system you need.
To see a coordinate system's datum, right-click it in the list and click Details .

- Click the button below the Current Z heading and choose an ellipsoidal vertical coordinate system that shares the same datum.