Dallas /ˈdæləs/ is a major city in Texas and forms the center of a metropolitan area that ranks fourth in the United States. The city proper ranks ninth in the U.S. and third in Texas. The city's prominence arose from its historical importance as a center for the oil and cotton industries, and its position along numerous railroad lines. The bulk of the city is in Dallas County, of which it is the county seat. However, sections of the city are located in Collin, Denton, Kaufman, and Rockwall counties. According to the 2010 United States Census, the city had a population of 1,197,816. The United States Census Bureau's estimate for the city's population increased to 1,241,162 as of 2012.The city is the largest economic center of the 12-county Dallas–Fort Worth–Arlington metropolitan area, which had a population of 6,810,913 as of July 1, 2013. The metropolitan economy is the sixth largest in the United States, with a 2012 real GDP of $420.34 billion. In 2013 the metropolitan area led the nation with the largest year-over-year increase in employment, and advanced to become the fourth-largest employment center in the nation with more than three million non-farm jobs. In the latest rankings released in 2013, Dallas was rated as a "beta plus" world city by the Globalization and World Cities Study Group & Network. Dallas is also ranked 14th in world rankings of GDP by the Organisation for Economic Co-operation and Development.Dallas was founded in 1841 and formally incorporated as a city in February 1856. The city's economy is primarily based on banking, commerce, telecommunications, computer technology, energy, healthcare and medical research, transportation and logistics. The city is home to the third-largest concentration of Fortune 500 companies in the nation. Located in North Texas, Dallas is the main core of the largest inland metropolitan area in the United States that lacks any navigable link to the sea.It was developed because of construction of major railroad lines here; it became a hub in 1873. It was connected to Houston and other major cities that were railroad cities. It was booming by the late 19th and early 20th century, and was the world center of leather manufacture and harnessmaking. The interstate highway system in the 1950s and 1960s reinforced and consolidated Dallas' prominence. As with the railroads, east/west and north/south highways converged here. Four major interstate highways converge in the city, and a fifth interstate loops around it. Dallas developed a strong industrial and financial sector, and a major inland port, due largely to the presence of Dallas/Fort Worth International Airport, one of the largest and busiest airports in the world.