have a scenic river canyon, waterfalls, and caves. Doi Khun Tan National Park, which is located midway between the two provincial capitals of province Lampang and Lamphun, is best known for Thailand's longest railroad tunnel, which is long. Doi Phu Kha National Park in province Nan is northern Thailand's largest national park. Regional classification of northern Thailand The northern region, as defined by the National Geographical Committee in 1978, consists of nine provinces. Geographically the division, in conformance with the six-region system, includes most of the mountainous natural region of the Thai highlands. In the four-region classification system, northern Thailand gains the eight upper-central-region provinces: Kamphaeng Phet, Nakhon Sawan, Phetchabun, Phichit, Phitsanulok, Sukhothai, Uthai Thani and Tak, bringing the total to 17 provinces. In 2019 it is common to subdivide the northern region into: nine provinces of the upper northern region and eight provinces of the lower northern region. All websites of these eight provinces state: "located in the lower northern region". Economy For FY 2018, Northern Thailand Region had a combined economic output of 1,266 trillion baht (US$40.8 billion), or 7.7 percent of Thailand's GDP. Lamphun province had an economic output of 84.395 billion baht (US$2.7 billion). This equates to a GPP per capita of 211,489 baht (US$6,822), half more than for Chiang Mai province, next in the ranking and double than for Lampang province, which is third in the ranking. Kamphaeng Phet province had an economic output of 117.705 billion baht (US$3.8 billion). This amounts to a GPP per capita of 150,783 baht (US$4,864), half more than for Tak province, which is fifth in the ranking. Languages Central Thai is the sole official language in Thailand. However, in the six-region classification system, it is the second largest native language in Northern Thailand, amounting to roughly two hundred thousand people, found in the lower part of Uttaradit Province. Central Thai spoken in southern regions of Northern Thailand in the four-region classification system comprise a group of dialects classified as Ne (). Because there are no significant Thai Chinese communities in Northern Thailand, the Krung Thep accent (prestige dialect) is not commonly found in Northern Thailand. The main language is Northern Thai, which is a southwestern Tai language spoken in the 9 changwat of Northern Thailand. It is spoken by roughly six million people. There are also various hill tribe languages such as Lolo-Burmese, Karenic, Mienic, Mienic and Palaungic. Notes