A basemap is used for locational reference and provides a framework on which users overlay or mash up their operational layers, perform tasks, and visualize geographic information. The basemap serves as a foundation for all subsequent operations and mapping. Basemaps provide the context and a framework for working with information geographically.
The ArcGIS Event Editor provides the functionality to add and change basemap layers.
- Open Event Editor and, if prompted, sign in to Portal for ArcGIS or ArcGIS Online.
 - Click the Map tab.
 - In the Contents group, click the Basemap button 
.The basemap selector appears.

 - Select any one of the nine basemap layers to add to your map.