In the northeastern part of the country the Khorat Plateau, a region of gently rolling low hills and shallow lakes, drains into the Mekong River via the Mun River. The Mekong system empties into the South China Sea and includes a series of canals and dams. Together, the Chao Phraya and Mekong systems sustain Thailand's agricultural economy by supporting wet-rice cultivation and providing waterways for the transport of goods and people. In contrast, the distinguishing natural features of peninsular Thailand are long coastlines, offshore islands, and mangrove swamps. A recent global remote sensing analysis suggested that there were of tidal flats in Thailand, making it the 45th ranked country in terms of tidal flat extent. Area Total: Land: Water: Thailand uses a unit of land area called the rai, which is . Extreme points Extreme points of the mainland Northernmost point: Myanmar border, Mae Sai District, Chiang Rai Province, at Southernmost point: Malaysian border, Betong District, Yala Province, at Easternmost point: Laos border, Khong Chiam District, Ubon Ratchathani Province, at Westernmost point: Myanmar border, Mae Sariang District, Mae Hong Son Province, at Elevation extremes Highest point: Doi Inthanon, , at Lowest point: Gulf of Thailand, 0 metres - sea level Regions The National Research Council divides Thailand into six geographical regions, based on natural features including landforms and drainage, as well as human cultural patterns. They are, namely: the North Region, the Northeast Region, the Central Region, the East Region, the West Region and the South Region of Thailand. Although Bangkok geographically is part of the central plain, as the capital and largest city this metropolitan area may be considered in other respects a separate region. Each of the six geographical regions differs from the others in population, basic resources, natural features, and level of social and economic development. The diversity of the regions is in fact the most pronounced attribute of Thailand's physical setting. Northern Thailand Northern Thailand is a mountainous area. Parallel mountain ranges extend from the Daen Lao Range (ทิวเขาแดนลาว), in the southern region of the Shan Hills, in a north-south direction, the Dawna Range (ทิวเขาดอยมอนกุจู) forming the western border of Thailand between Mae Hong Son and the Salween River, the Thanon Thong Chai Range (เทือกเขาถนนธงชัย), the Khun Tan Range (ดอยขุนตาน), the Phi Pan Nam Range (ทิวเขาผีปันน้ำ), as well as the western part of the Luang Prabang Range (ทิวเขาหลวงพระบาง). These high mountains are incised by steep river