If you have data that does not share the same geographic coordinate system in a single map or scene, you need a geographic transformation. Geographic transformations translate coordinates from one geographic coordinate system to another. When you add a layer to a map with a different geographic coordinate system, a transformation is applied automatically. You can specify a different transformation to the map as necessary.
Apply a coordinate system transformation
To apply a transformation, right-click the map or scene in the Contents pane and click Properties. On the Layer Properties dialog box, click the Transformation tab. The horizontal coordinate system of the map is shown on the dialog box, along with the vertical coordinate system if one is defined. Choose the geographic coordinate system to which you want to apply a transformation. Select the transformation method, or series of methods if no direct transformation is available. Methods are sorted by accuracy and location of your data.
Show a warning if a transformation has been applied
Spatial data is projected on the fly whenever the native coordinate system of a layer does not match that defined by the map or scene it is in. This happens with the automatic application of a transformation. It's 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 choose to enable transformation warnings.
Click the Project tab and click the Options tab. On the Options dialog box, click the Map and Scene tab. Expand the Spatial Reference heading and check Warn if transformation between geographic coordinate system is required to align data sources correctly.
With this option enabled, a warning appears anytime you add data to a map or scene that requires a geographic coordinate system transformation. You can click OK on the dialog box to accept the transformation, or choose a different one.