San Diego County in the context of "Indian reservation"

Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. Its densely populated coastal region includes Greater Los Angeles (the second-most populous urban agglomeration in the United States) and San Diego County (the second-most populous county in California). The region generally contains ten of California's 58 counties: Los Angeles, San Diego, Orange, Riverside, San Bernardino, Kern, Ventura, Santa Barbara, San Luis Obispo, and Imperial counties.

Although geographically smaller than Northern California in land area, Southern California has a higher population, with 23.76 million residents as of the 2020 census. The sparsely populated desert region of California occupies a significant portion of the area: the Colorado Desert, along with the Colorado River, is located on Southern California's eastern border with Arizona, and the Mojave Desert shares a border with Nevada to the northeast. Southern California's southern border with Baja California is part of the Mexico–United States border.

San Diego (/ˌsæn diˈeɪɡoʊ/ SAN dee-AY-goh; Spanish: [san ˈdjeɣo]) is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. and second-most populous city in California with a population of over 1.4 million, while the San Diego metropolitan area with over 3.3 million residents is the 18th-largest metropolitan area in the country. San Diego is the county seat of San Diego County. It is known for its mild Mediterranean climate, extensive beaches and parks, long association with the United States Navy, and recent emergence as a wireless, electronics, healthcare, and biotechnology development center.

Historically home to the Kumeyaay people, San Diego has been referred to as the Birthplace of California, as it was the first site visited and settled by Europeans on what is now the West Coast of the United States. In 1542, Juan Rodríguez Cabrillo claimed the area for Spain, forming the basis for the settlement of Alta California, 200 years later. The Presidio and Mission San Diego de Alcalá, founded in 1769, formed the first European settlement in what is now California. In 1821, San Diego became part of the newly declared Mexican Empire. California was ceded to the U.S. in 1848 following the Mexican–American War and was admitted as the 31st state in 1850.

Imperial County is a county located on the southeast border of the U.S. state of California. As of the 2020 census, the population was 179,702, ranking as the least populous county in Southern California. The county seat and largest city is El Centro. Imperial is the most recent California county to be established, as it was created in 1907 out of the eastern half of San Diego County.

Imperial County is located in the far southeast of California, in the Imperial Valley. It borders San Diego County to the west, Riverside County to the north, the U.S. state of Arizona to the east and the Mexican state of Baja California to the south. It includes the El Centro metropolitan statistical area and is part of the Southern California border region, the smallest but most economically diverse region in the state.

The Southern Border Region is one of nine geopolitical designations in California used for economic analysis. The region includes San Diego County and Imperial County, each of which borders Mexico. Its largest city is San Diego.

The Southern Border Region is adjacent to the Southern California Region, which consists of five counties (Orange, Los Angeles, San Bernardino, Riverside, and Ventura). These regions were created in about 1998, when the California Regional Economies Project identified 9 regional economies. The California Economic Strategy Panel used employment and wage information reported by employers for public policy-making, planning, and program administration. As an example of information reported by the panel, according to the 2009 report, the gross domestic product of the region had grown 68.5% in five years. Other agencies such as the California Public Utilities Commission also provide reports for the region.

The Palomar Testbed Interferometer (PTI) was a near infrared, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County, California, United States. It was built by Caltech and the Jet Propulsion Laboratory and was intended to serve as a testbed for developing interferometric techniques to be used at the Keck Interferometer. It began operations in 1995 and achieved routine operations in 1998, producing more than 50 refereed papers in a variety of scientific journals covering topics from high precision astrometry to stellar masses, stellar diameters and shapes. PTI concluded operations in 2008 and has since been dismantled.

PTI was notable for being equipped with a "dual-star" system, making it possible to simultaneously observe pairs of stars; this cancels some of the atmospheric effects of astronomical seeing and makes very high precision measurements possible.