Maps and scenes use coordinate systems to locate and display data correctly on the earth's surface and relative to one another. A coordinate system is a reference framework that defines the position of features in either two- or three-dimensional space. 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 both maps and scenes.

In a new, empty map or scene, the default horizontal coordinate system is WGS 1984 Web Mercator. The horizontal coordinate system cannot be changed for a global scene, it must always be WGS 1984 Web Mercator, but you can choose a different horizontal coordinate system for maps and local scenes. A map or scene always has a horizontal coordinate system. You can, optionally, define a vertical coordinate system for a map or a scene.

Empty maps and scenes take their coordinate systems from the first layer added to them. When you add additional layers to a map or scene, they are automatically displayed using the same coordinate system of the map or scene. If the map or scene's coordinate system is not the same as the native coordinate system of the layer's source data, the data is projected on the fly using a transformation. Be aware, however, that projecting on the fly can take longer to draw and is not advisable if you are editing data or performing analysis. It is far preferable 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.

##### Tip:

To see the coordinate system defined for your data source, or if your data source has a coordinate system defined, right-click the layer in the Contents pane and click Properties. On the Layer Properties dialog box, click the Source tab and expand the Spatial Reference group.

## Horizontal coordinate systems

Horizontal coordinate systems are either geographic or projected. A geographic coordinate system is based on a three-dimensional ellipsoidal or spherical surface, and locations are defined using angular measurements, usually degrees of longitude and latitude. A projected coordinate system is a planar system that uses two-dimensional coordinates and uses linear distance measurements as units. A projected coordinate system is based on a geographic coordinate system and a map projection. The map projection contains the mathematical calculations that convert the geodetic locations to the planar system.

## Vertical coordinate systems

Vertical coordinate systems provide a reference for z-coordinates, which are measurements of the height or depth of features. Vertical coordinate systems are always in linear units such as meters or feet. Using a vertical coordinate system improves locational accuracy in analysis and editing. Vertical coordinate systems are not applied by default to new maps and scenes, you must explicitly choose one.

There are two types of vertical coordinate systems. Gravity-based vertical coordinate systems are more commonly used. With this type, the reference surface is defined by a mean sea level calculation (or, in some cases, it is derived from the level of a single point.) Ellipsoidal coordinate systems are based on a reference to a mathematically derived spheroidal or ellipsoidal volumetric surface. Since they are calculated on a mathematical model, they are simpler than gravity-based vertical coordinate systems, but they may lack significant accuracy, especially in large-scale applications. For example, a stream in a large-scale map may appear to flow in an uphill direction using an ellipsoidal vertical coordinate system. When you use an ellipsoidal vertical coordinate system, you must ensure that it matches the geographic coordinate system. For example, if z-value height is defined in NAD 1983, the geographic coordinate system must be defined in NAD 1983 also, not WGS 1984.

Vertical coordinate systems in a global scene must be ellipsoidal. Or, they can be gravity-based only if they cover a full world extent. EGM2008 Geoid and EGM96 Geoid are examples of global gravity-based vertical coordinate systems.

##### Caution:

Be aware that the ellipsoidal coordinate system is not taken into account during drawing. This may be noticeable if you extrude features.

## Specify the coordinate systems of maps and scenes

- In the Contents pane, right-click a 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 either 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. You can enter a search term in the Search box to help locate a specific coordinate system.
When the vertical coordinate system is ellipsoidal, it must share the same datum as the horizontal coordinate system. The datum name, the spheroid name, and all of the spheroid properties of the two coordinate systems must exactly match.

##### Tip:

The Map Properties dialog box is expandable. Drag the bottom of the box down to enlarge the space for the list and make it easier to see the choices.

### Set the coordinate system from a layer

You can define a coordinate system from an existing layer, even if it is not present in the map or scene.

- To set the coordinate system to be the same as that of a layer on the map, under the Coordinate Systems Available list, expand the Layers folder. Expand a coordinate system heading to see the layers that reference it. This is a good way to ensure all layers in your map sharing the same coordinate system.
- To set the coordinate system to be the same as that of a layer that is not in the map or scene, click Import , and 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.

### Filter available coordinate systems

Set a spatial filter to limit the Coordinate Systems Available list.

- Click the Spatial Filter button and click Set spatial filter .
- On the Spatial Filter Index dialog box, choose either 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.
- If necessary, click the Spatial Filter button and click Clear spatial filter to clear the spatial extent.

### Remove the vertical coordinate system

While maps and scenes always have a horizontal coordinate system defined, a vertical coordinate system is optional.

- To remove a vertical coordinate system definition from a map or scene, click Current Z and choose <None> from the Z Coordinate Systems Available list.

## Import a coordinate system

You can import a coordinate system from a spatial dataset, such as a layer, or from a projection file.

- Click the Add Coordinate System button and click Import Coordinate System .
- On the Import Coordinate System dialog box, browse to a spatial dataset or a projection file (.prj).
- Click OK to apply the coordinate system.

## Create a custom coordinate system

You can create a custom coordinate system by copying an existing coordinate system and modifying its parameters or from first principles.

To copy and modify an existing coordinate system, complete the following steps:

- Right-click a coordinate system in the Coordinate Systems Available list and click Copy and Modify .
- On the Modify Coordinate System dialog box, update parameters as necessary.
- Click Save to save the new coordinate system.
It is placed under the Custom heading in the Coordinate Systems Available list. Apply it as the current coordinate system or save it as a favorite coordinate system to persist it; otherwise, it will be lost as soon as you close the Map Properties dialog box.

## Define a new coordinate system

To define a new coordinate system, complete the following steps:

- Click the Add Coordinate System button and click New Geographic Coordinate System , New Projected Coordinate System , or New Vertical Coordinate System . New Geographic Coordinate System and New Projected Coordinate System are enabled only when the Current XY button is highlighted. New Vertical Coordinate System is available only when the Current Z button is enabled.
- On the New Coordinate System dialog box, update parameters as necessary.
- Click Save to save the new coordinate system.
It is placed under the Custom heading in the Coordinate Systems Available list. Apply it as the current coordinate system, or save it as a favorite coordinate system to persist it; otherwise, it will be lost as soon as you close the Map Properties dialog box.

## Save a coordinate system as a favorite

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.

- 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

You can save any coordinate system as a projection file. Projection files have a .prj file extension. Projection files can be used to define a spatial reference in geoprocessing. For example, you can enter a path to a .prj file in the Create Feature Dataset tool to define the spatial reference of the dataset.

- 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.

## Allow panning across the international date line in maps

By default, maps are expressed as a single, finite earth, split at the international date line. If you need to visualize data across or near this line, you'll want to be able to draw and pan seamlessly over this line.

- To allow continuous panning across the international date line on your map, check Enable wrapping around the date line. This option is only supported for geographic coordinate systems and a select set of projected coordinate systems.

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