A spatial reference is a set of parameters, typically defined by a WKID, that define the coordinate system and spatial properties for geographic data. Applications use a spatial reference to correctly display geographic data in a map or scene. Spatial references ensure that geographic data from different sources can be used together for accurate viewing or analysis. Each spatial reference is defined either by a well-known ID (WKID), also called a spatial reference ID (SRID), or by a well-known text (WKT). Common spatial references include WGS 84 (WKID: 4326), used by GPS systems, and Web Mercator (WKID: 102100), used in most online maps.