Such wonders of modern technology as the Global Positioning System (GPS) have improved nautical navigation to an incredible extent, all owing mariners to navigate in real time (where am I?) as opposed to the time-late (where was I?) methods practiced by navigators for centuries. Today's nautical navigators must be familiar with certain aspects of chart making that once belonged to cartographers alone. The new terms, geodesy and datum, have entered the navigator's lexicon and can be ignored only at the greatest peril.