CORS is the acronym for a GPS "Continuously Operating Reference Station". Originally established in the United States by the U.S. Coast Guard to provide a precise positional reference for shipboard navigators, the system has become popular with Land Surveyors, Engineers, and with Geologists that research the movements of the Earth’s continental plates.
Congress asked for a solution to improve the way reliable elevations and heights were being determined in the United States, and the National Geodetic Survey responded with a proposal for a Height Modernization Program to utilize GPS CORS sites as the basic elevation reference tool for the 21st century.
Around the same time, the U.S.S.R. fell with the Iron Curtain, and the U.S. Department of Defense began to de-classify the secret gravity model of the Earth that had been used for targeting Inter Continental Ballistic Missiles. That secret gravity model is now available to the general public, and it is one of the key components of the National Height Modernization Program. Since GPS does not provide elevations, the gravity model termed the Geoid is the connection between GPS heights and elevations. Useful for most areas of the world, the Geoid has been specifically refined and its accuracy enhanced for the United States. In fact, it is especially precise for the State of Louisiana because it represents the only way we can obtain current, up-to-date elevations! Using the CORS with the latest version of the Geoid is how Land Surveyors and Engineers are now obtaining reliable, current elevations.
More information can be found at the NGS CORS Website.
The old way of doing things was based on "spirit leveling", which had its roots in the old surveys of the Babylonians, dating back some 3,000+ years. When the government establishes the elevation of a benchmark, that elevation is good as long as the benchmark does not move up or down. Louisiana is subsiding, and benchmarks become useless because they do not keep their elevations for more than a few years at best, some only for a year when located near the coast!
The CORS sites have GPS antennae bolted to buildings and structures in Louisiana. As the ground and the buildings subside, so does the antenna such that the reference elevation is computed and re-determined every few days - the value is always up-to-date!