The concept of overlay in geographic information systems (GIS) refers to the Question 30 options: A) incorporation of multiple types of digital spatial data (maps) in answering research questions. B) constant, automatic updating of digital spatial data (maps) of the same type with newer data. C) use of digital spatial data (maps) to identify what lies deep beneath the Earth's surface. D) inclusion of a grid (such as latitude-longitude) on a digital spatial dataset (map).