The Kingdom of Thailand has an area of 514,000 sq. kilometres (198,455 sq mi), more than twice that of the United Kingdom. It lies between latitude 6° and 21° N and between longitude 97° and 106° E, with a length of 1700 km and a greatest width of 770 km. Thailand has a population of some 65 million.

The South-East Asian peninsula is traversed by three ranges of mountains running from north to south, a periclinal structure associated with the Himalayan folding movements known as the Minya Konka massif, extending from Assam in the west to Szechwan in the cast. To the west are the Arakan Mountains of western Burma; in the centre a system of cordilleras, the Tenasserim Hills (1800 m), separating Burma from Thailand and sloping down into the Malay peninsula; and to the east the mountains of Annam. Within these mountain ranges are wide fertile plains. The western plain is watered by the Irrawaddy and its tributaries; to the east is the larger basin of the Menam Chao Phya (the river Chao Phya or Phraya, known in the West as the Menam), the heart of Thailand.

The two main regions of Thailand are the Chao Phya plain and the Khorat plateau, which is bounded by the central cordilleras in the west and the mountains of Annam in the east. The valleys are like a continuation of the Gulf of Thailand; the Khorat plateau ranges between 100 and 250 m in altitude.

To the north the two ranges converge to form a series of higher mountains, the Yunnan massif. To the south the central cordilleras, here marking the western boundary of Thailand, run down to form the backbone of the Malay peninsula.

The country can be broadly divided into five regions:
(1) the Chao Phya or Menam plain, running from north to south, made up of the Bangkok basin and the upper Menam plain;
(2) the continental range, also running north-south, which bounds Thailand on the west;
(3) the Khorat plateau;
(4) the coastal regions of the south-east; and
(5) the Malay peninsula.

The Chao Phya or Menam plain, with the Menam Chao Phya river systems (Menam = “mother of the waters” or river, while Chao Phya or Chao Phraya is an aristocratic title: in other words, “noble lady, mother of the waters”), is the heart of Thailand. The soil is fertilised by the flooding of the river and bears generous harvests. This is Thailand’s main farming region, the most populous part of the country and the one with the best system of communications.

The plain is made up of three parts. The Bangkok basin begins at Nakhon Sawan, where the rivers Ping and Man join to form the Chao Phya. Near Ayutthaya, the Pasak, which flows down from the hills round the Khorat plateau, in turn mingles its waters with those of the Chao Phya. At Wat Sing the river divides into two mighty arms, the Nakhon Chaisi (or Suphan or Tachin) and the Chao Phya proper. The main stream is soon reinforced by the Noi and farther south the Lopburi, which itself has acquired the waters of the Pasak. To the west is the Meklong river system, which belongs to the Bangkok basin; to the east is the Prachin (or Ban Pakong).

As far north as Lopburi, 130 km from the sea, and between the mouths of the Meklong and the Prachin, a distance of some 96 km, the plain is subject to flooding. At present the sea is retreating fairly rapidly at the head of the Gulf of Thailand as a result of the deposit of alluvium by the river.

Altitudes are low, rising very gradually towards the north: 2 m at Bangkok, 4 m at Ayutthaya, 23 m at Nakhon Sawan (Paknam Pho). The river is still tidal at Ayutthaya, 95 km inland. During the floods which occur over most of the Bangkok region every year, mainly in October and November, silts of high fertility are deposited.

The plains with their dense covering of alluvial soil are broken up only by a few low hills. To the north the Bangkok basin is continued by the valleys of the rivers Ping, Yom and Nan, to the north-east by the long Pasak valley. All this region, like the piedmont of the central ranges and the upper Tachin valley, is covered with an even thicker mantle of alluvium. The landscape in this area is in general more diversified than in the delta, being broken up by ranges of low hills the foothills of the neighbouring mountain systems. and by isolated crags of limestone or quartzite (inselbergs).

The mountainous region to the north, north-west and west of the Chao Phya plain is known as the Continental Range. Two parts can be distinguished – the complex of valleys and mountains which make up northern Thailand, and the western ranges round Tak, Kanchanaburi and Ratchaburi.

Northern Thailand is traversed by a number of parallel ranges, between which are the valleys of the Ping, Wang, Yom and Nan, all flowing south to drain into the Chao Phya. The watershed is in the Phipanna (“spirit of the thousand waters”) massif, and beyond this other rivers, like the Fang, the Lao and the Ing, flow north towards the Mekong.

These deep and sometimes gorge-like valleys open out into basins of varying size inhabited by the Lao or other hill tribes. The Nan basin includes the large Nan valley (32 km long, alt. c.200 m) and other smaller valleys. North of Lampang is the Wang valley, which is too narrow to accommodate settlements of any size. The Ping valley runs into the Chiang Mai basin (alt.c. 300 m). Among the rivers flowing into the Mekong is the Ing, which drains the depressions of Pan, Payao (c. 450 m) and Thoeng (c. 350 m), as well as Muang Fang and Chiang Rai (350 m), which belong to the Mekok region.

High ranges of hills with peaks rising to between 1500 and 1800 m separate the valleys of the Fang, Lao, Ing and Mekok. West of the Fang, near the Burmese frontier, one peak even exceeds 2300 m. Throughout this whole region the rivers turn into torrents during the rainy season, while in the dry season there is hardly any flow at all. To the south the valleys of the Ping, Wang, Yom, Nan and their tributaries are separated by ridges of hills with the same general north-south orientation. North-east of Chiang Mai the range which runs between the Wang and Ping valleys reaches altitudes of 2000 m. The highest peak in Thailand, Doi Inthanon (2575 m), lies between the Ping and its tributary the Chom. The hills between the Wang and the Yorn are much lower, barely exceeding 1000 m, but their steeply scarped slopes give them an impressive appearance. In north-eastern Thailand the hills along the Laotian frontier reach heights of around 2300 m. This region is difficult of access, since the valleys are always narrow, and sometimes close in even farther to form deep gorges. The frontier with Burma is also marked by isolated steep-sided valleys rising towards the Salween watershed. The Merai, Yuan and Thaungwin, indeed, are tributaries of the Salween. These western ranges of hills along the Burmese frontier, with their high peaks, canyons and deep valleys, are thinly populated: here there are none of the open valleys suitable for human settlement so typical of northern Thailand, apart from the Massot plain, a wide extension of the Moei valley.

The Thanon-Thongchai massif is divided into three distinct ranges. The largest of these, the Tanasoi or Tenasserim range, runs from the Three Pagodas Pass (1400 m) along the Burmese frontier into the Malay peninsula. Between the Khwae Noi and the Khwae Yai is the Meklong range; the third range lies east of the Khwae Yai, on the edge of the Menam plain. Near Kanchanaburi, which is already in the Menam plain, the Khwae Noi and Khwae Yai join to form the river Meklong, beyond which there are no more hills. This western group forms a barrier between the Salween and Menam valleys.

We now come to the Khorat plateau, in fact a series of low hills sloping gently to the north and east and bounded by the Mekong and the mountains of Laos. The centre is depressed (Ubon area, under 100 m). On the west and south it is bounded by the steep rock faces of the Phetchabun and Dong Phrayayan hills, exceeding 1000 m in the south-west. At the south-west corner of the plateau the border ranges meet the Sankamphaeng and Dang Rek hills (Khao Laem, 1300 m; Khao Khampheng, 1000 m). To the south, towards the Cambodian plain, are hills of something over 700 m. To the east, beyond the Mekong in Laotian territory, is a range of hills near the mouth of the Mun rising to almost 1400 m. The river Mun drains almost the whole of this region, though to the north a few rivers flow directly into the Mekong. The Khorat plateau is studded with low hills. Drainage is poor, as is shown by the numerous lakes and ponds which grow larger during the rainy season and contract again during the dry season.

The valleys are wide and flat, with ridges of alluvial deposits through which the rivers have carved their way. In the rainy season the rivers overflow. The water drains away slowly on account of the low altitude and gentle gradients, favouring rice-growing in the lowest areas. In summer white patches are formed on the ground by incrustations of salt, deposited by the flood-water of the rivers, leached out and crystallised in the sun. It is harvested and purified by the local people.

The south-eastern coastal regions include Trad, Chanthaburi, Rayong and, to the south, the province of Chonburi. To the north are the hills on the south side of the Prachin valley. To the south and west is the Gulf of Thailand. The flattened summits of the Banthat Hills, outliers of the Cardamom range, form the frontier with Cambodia. Inland is a hilly region from which numbers of little streams make their way to the sea through the intricate coastal relief. Among those that qualify to be called rivers are the Chanthaburi, Pra Sac, Wen and Trad. At the muddy tide-washed mouths of the Pra Sac and Wen are areas of mangrove swamp. Between Rayong and Chonburi extends a wide plain, studded with isolated hills, the highest of which, Khao Kui (700 m), lies near the sea and forms a useful landmark for shipping. A string of beaches of magnificent sand bears witness to the progress made in regularising the east coast.

A series of rocky wooded islands lie off the coast. At the island of Si Chang, opposite Sri Racha, is an anchorage used by steamers drawing too much water to be able to negotiate the sandbanks of the Menam estuary. Chang is the largest of the islands, but Kut, to the south-east, is also of some size.

The southern part of Thailand consists of the northern and central portions of the Malay peninsula, the backbone of which is formed by the end of the Tenasserim range. The plains, valleys and coastal regions are highly fertile. The ranges of hills are very far from being continuous. The Tenasserim hills fall sharply to a mere 100 m near Chumphon, and then branch into two near the river Pak Chan the western branch runs into Burma, while the eastern branch forms the Mergui archipelago in Burma and the Phuket hills, and reaches the Indian Ocean in the Isthmus of Kra and the island of Phuket. Here and there are hills rising to 1000 m. Its rich resources of tin make this the country’s principal mining region.

Near the island of Tao begins the eastern range of hills, the largest in the peninsula, known as the Si Thammarat Hills – a range which has been partly covered by the sea but emerges in the islands of Phagan and Samui off the east coast. The highest peak, Khao Luang, falls just short of 1800 m. Farther south the summit ridge is lower. Between the ranges of hills is a plain studded with isolated hills.

An old-established trade route runs across the Isthmus of Kra from sea to sea, a distance of some 100 km. Various plans have been put forward in the past for constructing a canal across the isthmus which would considerably shorten the sea route between Europe and the Far East, and the idea is now again being canvassed.

The low east coast is made up of a series of huge bays with magnificent beaches. Here too the process of regularisation is leading to an extension of the coastal area by alluvial deposits from the rivers. Just inland from the coast are lagoons abundantly stocked with fish, the largest being the Tale Luang (Tale Sap) lagoon north of Songkhla. There are numerous valleys and depressions running far into the interior. The west coast is quite different. Here the hills mostly fall directly into the sea, and the coast is frequently and deeply indented. The coastal plain, where there is one, is fringed by rocky islets. Mangrove swamps are numerous.