San Andreas is not an island, but an expansive fictional state based on sections of real life Nevada and California. It includes three cities: Las Venturas, San Fierro and Los Santos.
The three cities that make up the expansive San Andreas are based on three real-life cities. Los Santos is based on real-life Los Angeles, Las Venturas is based on Las Vegas, and San Fierro is based on San Francisco. Furthermore, the environmental settings are also set to mimic the southwestern region of the United States. Other important destinations in the game, such as Area 69, Mount Chiliad, Sherman Dam and Vinewood, are also based on real-world locations.