A map or scene always has a horizontal coordinate system, and optionally has a vertical coordinate system. The default horizontal coordinate system is WGS84 Web Mercator for maps and scenes. For global scenes, the default horizontal coordinate system is WGS84.

Empty maps and scenes derive their coordinate systems from the first layer added to them. When you add layers to a map or scene, they are automatically displayed using the same coordinate system of the map or scene. You can specify a coordinate system for the map or scene, or you can define a new one. You can create a coordinate system if you know the exact parameters required, or you can generate a suggested coordinate system based on the extent of your data and what aspects of it you intend to preserve.

## Create a coordinate system

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

- 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.

- 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 available 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. If you are creating a new 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.

### Work with custom datums and spheroids

When you create a geographic coordinate system, projected coordinate system, or ellipsoidal-based vertical coordinate system, you can specify a custom datum or spheroid by choosing <Custom> from the top of the list. Type a name for these in the Datum Name and Spheroid Name fields in the table, respectively. Enter values in the Semimajor Axis and Semiminor Axis fields for the custom spheroid.

When you create a gravity-related vertical coordinate system, you can specify a custom vertical datum by choosing <Custom> from the top of the list. Type a name for the datum in the Vertical Datum Name field in the table.

Custom datums and spheroids are not created independently from the new coordinate system, meaning you will not see them listed as choices in datum or spheroid menus when you create additional coordinate systems.

## Generate a new suggested projected coordinate system

You can generate a custom projected coordinate system by specifying an underlying geographic coordinate system, the intended map extent, and the projection property you want for the map. The list of available projection properties changes depending on the extent you have chosen, since certain properties are not valid for certain extents.

To generate a new suggested projected coordinate system, follow these steps:

- On the Map Properties dialog box, on the Coordinate Systems tab, highlight the Current XY button.
- Click the Add Coordinate System button and click New suggested projected Coordinate System to open the Create a new suggested projected coordinate system dialog box.
- In the Coordinate System Name field, type a name for the custom coordinate system you are creating.
- Click Geographic coordinate system to choose the underlying geographic coordinate system of the new projected coordinate system. It is recommended that you choose the geographic coordinate system used by the data in the map.
- Specify the extent to optimize the projection for, either manually, from the current extent, or from one or all layers. This does not change the extent of the map.
You cannot manually set an extent that intersects the 180th meridian for some geographic coordinate systems. Use the extent of a view, the extent of a layer, or specify a geographic coordinate system where the 180th meridian does not cross through the area of interest instead. See Allow panning across the international date line in maps.

- Choose the Projection property for the map. Depending on the data and options chosen, some of these options may not be available.
Equal-area

Optimize to preserve areas.

Conformal

Optimize to preserve local angles, which gives the appearance of preserving shape at the expense of area and distance.

Equidistant from one point

Optimize to preserve distances only along lines running through the center point of the projected coordinate system, (the point where the latitude of origin and the central meridian intersect). No projection can preserve distances in every direction everywhere on a map.

Equidistant along meridians

Optimize to preserve north-south distances only. Distances in any other direction are not preserved. No projection can preserve distances in every direction everywhere on a map.

Compromise

Optimize to preserve none of these properties specifically, but attempt to minimize the distortion for each property. Distortion can still become severe in certain places or circumstances.

- Review the resulting parameters of the custom projected coordinate system, and click Save. The projected coordinate system becomes the current XY coordinate system and is added to the list of Custom coordinate systems. Click OK to close the Map Properties dialog box to save this new projected coordinate system to your Favorites. If you click Cancel to close the Map Properties dialog box without adding the new projected coordinate system to Favorites, it will be deleted.

## Create a custom coordinate system from an existing one

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

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.