enables the user to plot the locations associated with study data on a global coordinate system overlaid with geophysical ("satellite") or geopolitical ("streets") maps. The street and satellite maps are, in essence, built-in Base Layers that supply geographic reference information onto which study data can be layered. User-defined Base Layers can be provided in the form of local shapefiles or remote data from KML/KMZ files or a map server on the Internet. Study data is applied in Data Layers, which can be divided into two groups. Both choropleth and dot density maps associate quantitative data with map regions, such as those defined in shapefiles or Keyhole data files. These map layers are useful for displaying large-scale public health information, where the number of cases, for example, is represented by color of, or dot density in, the jurisdictions drawn on the map. The other Data Layer group, consisting of case-cluster and spot maps, uses latitude and longitude to map the locations of subjects or other physical entities relevant to the study. The coordinates may be entered from GPS units in the field or translated from street addresses by the Check Code command GEOCODE. The user can also add markers, labels, and zones manually, by clicking on the map display.