The fourth continuum divides Web sites on the basis of how frequently the mapped dat are updated. Airport Monitor is one Web site that presents updated data showing near-real-time displays of incoming and outgo- ing flights from selected airports around the world. On the other hand, the Perry-Castaneda Map Library from the University of Texas at Austin contains an online catalog of thousands of non-updated maps In other words, once the map is posted for downloading, it is never updated.