The Google Latitude API allows for websites and developers to integrate with Google Latitude, which lets users update and read their current location, their location history, and other geo-related information.
In Latitude, a location is a record of where a user was at a given time. A location always contains a latitude, a longitude and a timestamp. It might also contain more details about the position of the user at that time, such as altitude, or metadata about how the position itself was captured, such as accuracy.
A user typically has a current location, which represents the most recent known position of the user. In some cases, however, a user might decide to temporarily hide, making their position invisible to Latitude friends. A hidden user has no current location.
A user can opt to have their locations stored as a time series by Google. This collection of positions is called the user's location history.
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Theres another good mashup you can add
Google Latitude Mashup with Google Maps