CORS and OPUS for Engineers describes new global positioning system (GPS) technologies and procedures that are immediately relevant to civil engineering professionals engaged in high-accuracy positioning. This collection of 22 articles assembles the latest thinking on the use of two advanced services-CORS and OPUS-for obtaining accurate positional coordinates. Created and managed under the auspices of the National Geodetic Survey, the CORS (continuously operating reference stations) network contains more than 1,600 permanent, geodetic-quality receivers that collect GPS data around the clock at locations distributed throughout the United States, its territories, and a few foreign countries. These data are then made freely available to the public via the Internet. OPUS (online positioning user service) is a free, automated, Web-based utility that provides its users with accurate and reliable positional coordinates in a timely fashion by processing each user’s GPS data with corresponding data from the CORS network. Together, CORS and OPUS form the backbone of today’s high-accuracy, three-dimensional positioning activities.