A few years ago, I visited the Greenwich Observatory and waited in line for hours to get a photo on the prime meridian. To my surprise, its GPS coordinates were

A few years ago, I visited the Greenwich Observatory and waited in line for hours to get a photo on the prime meridian. To my surprise, its GPS coordinates were not at 0 degrees longitude. It turns out that while older datums such as NAD27 use the Greenwich meridian, GPS is based on WGS84 which uses the IERS Reference Meridian. It runs through the park about 100 meters away from the observatory (and you can stand on it without waiting in line at all!)