The "S" in the product stands for "sensors". This unit incorporates a GPS receiver, barometric altimeter, and an electronic compass that deliver precise location, elevation and bearing information. This unit also contains a base map of North and South America, including major highways, thoroughfares, rivers, lakes and borders, with plenty of memory (24MB) for downloading MapSource data. Waterproof and floats in water. Larger screen, increased readability, bigger buttons and a rugged design.