Map projections and coordinate systems
Features on a map refer to the actual locations of the objects they represent in the real world. A coordinate system is used to translate locations on the surface of the earth to an image on a flat surface, like your computer screen. A map in Explorer Online has a coordinate system that translates the locations of data into locations in the map.
The layers in the map also have a coordinate system. You can find out the coordinate system used by a layer based on a service by clicking Service Details in the Layer Details view of the Layers panel.
You can create new maps whose source are the provided basemaps, or you can create new maps based on other sources. All ArcGIS Online services and ArcGIS Explorer Online basemaps are stored with a continuous tiling system to support the seamless display of map data for large scale subareas (for example, the imagery and streets within a city). This requires a single map projection for the world. The spherical Mercator projection is used. This is often referred to as the WGS 1984 Web Mercator [Auxiliary Sphere] projection.
Any tiled map services you add to ArcGIS Explorer Online must match the coordinate system of the ArcGIS Explorer Online basemap source. If you have access to a dynamic map service, it will be projected on the fly to match the basemap. WMS services advertise a number of coordinate systems supported by the service; if there is no coordinate system matching that of the basemap source, the layer cannot be displayed.
About coordinate systems
The positions of objects on the earth's spherical surface are measured in degrees of latitude and longitude, also known as geographic coordinates. Coordinates are transformed using mathematical formula so that features can be displayed on a flat computer screen. Though latitude and longitude can locate exact positions on the surface of the earth, they are not uniform units of measure; only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude.
To overcome measurement difficulties, data is often transformed from the three-dimensional geographic coordinate system to the two-dimensional planar surface in a projected coordinate system. Projected coordinate systems describe the distance from an origin (0,0) along two separate axes: a horizontal x-axis representing east–west and a vertical y-axis representing north–south.
Because the earth is round and maps are flat, getting information from the curved surface to a flat one involves a mathematical formula called a map projection. A map projection transforms latitude and longitude to x and y coordinates in a projected coordinate system.
The term coordinate system, which includes both geographic and projected coordinate systems, is used to describe the information about the projection, as well as other specifics, such as datum, units, and meridians. The term spatial reference is used by Esri systems to refer to a coordinate system, and also information such as the accuracy and resolution of coordinates stored.