# Specify a datum transformation

A geographic datum transformation is a calculation that is used to convert between different geographic coordinate systems to ensure that data is properly aligned. Anytime the geographic coordinate systems used by layers in a map differ from the geographic coordinate system of the map, a transformation is required and is automatically applied. If all layers and the map use the same geographic coordinate system, a transformation is not needed.

The transformation that is applied is the optimal one based on its accuracy and the extent of the data, but you can specify a different transformation if necessary. You can also save additional transformations with the map for later use. This is helpful if you need to temporarily remove a layer but don't want to lose the transformation that was required for that layer.

## Apply transformations to a map

The Transformation tab on the Map Properties dialog box lists the geographic and vertical coordinate systems of the layers and the current geographic and vertical coordinate systems of the map. Between these coordinate systems are the transformations suggested to convert between them. Transformations are bidirectional. The order of the coordinate systems does not matter. You can specify a different transformation if necessary. Transformations are sorted by their accuracy and suitability for the location of your data. The coordinate systems cannot be changed here. Use the Coordinate System tab on the same dialog box to change the coordinate system.

To view the transformations applied in a map, or to choose different transformations, follow these steps:

1. Right-click the map or scene in the Contents pane and click Properties. On the Map Properties dialog box, click the Transformation tab.
2. In the Layer and map section, click the name of a transformation in the center column of the table to open the menu and choose a different transformation. Repeat this step as necessary for other coordinate systems used by layers in the map.

If the geographic coordinate systems of the layers and the map are identical, no transformation is necessary.

3. Click OK to apply the transformations and close the Map Properties dialog box.

## Store additional transformations in a map

In addition to the transformations currently used in a map, you can store transformations with the map that are not currently in use. These can include transformations that were previously used, for example, by a layer that was removed from the map. Additionally, you can add other transformations if you know they will be necessary when additional data is added to the map or if output circumstances require itâ€”for example, you plan to publish the map to a platform that uses a geographic coordinate system that differs from that of your map. In this storage section, you can specify the geographic and vertical coordinate systems and the transformation between them.

1. Right-click the map or scene in the Contents pane and click Properties. On the Map Properties dialog box, click the Transformation tab.
3. Click the Select Coordinate System buttons to choose geographic and vertical coordinate system pairs.

The transformation path is bidirectional so the order of the geographic and vertical coordinate system pairs doesn't matter.

4. Use the drop-down menu between the geographic coordinate systems to specify the transformation to convert between them.
5. Click Add to add another transformation, or click OK to save the transformation in the map.

## Show a warning if a transformation has been applied

It is useful to know when a transformation has been applied, especially if you will be creating or editing data in the layer. To ensure that you are always aware of this state, you can enable transformation warnings. With this option enabled, a warning appears anytime you add data to a map or scene that requires a geographic coordinate system transformation. This is an application setting so it applies to all the projects you work with.

To ensure that warning messages appear in the application when a transformation is applied, follow these steps:

1. Click the Project tab and click the Options tab to open the application and project settings.
2. On the Options dialog box, in the Application section, click the Map and Scene tab.
3. Expand the Spatial Reference heading and check the Warn if transformation between geographic coordinate system is required to align data sources correctly check box.
4. Click OK to close the Options dialog box.