A map set is a collection of map sheets that are considered part of the same intellectual work, but not necessarily published at the same time. An index map is an overview map of the geographic region covered by a map set, and shows the grid system with the corresponding grid cell codes to help users find which map sheet covers their area of interest.
When ingesting a scanned index map into a digital library along with scanned map sheets, the relationship between the index map and the map sheets is that of a related work, whereas the relationship between map sheets and the map set as a whole is that of a child and parent. This allows complex relationships such as when index maps cover more than one map set.
The Alexandria Digital Research Library (ADRL) digital library created by UC Santa Barbara Library uses this relationship in its data model to enable the display of index maps on web pages displaying the map set as well as each map sheet's web page.