Every international land border in the world, measured and sorted: the ruler-straight lines, the rivers, the mountains and the rest. Then the straightest lines of all, and the treaties, years and powers behind them.
Look at a map of Africa, or Arabia, or the American West, and the lines that jump out are the straight ones: borders that run for hundreds of kilometres without a bend, across desert, savanna and lake, along a line of latitude or between two points someone chose in a distant capital. Africa's borders are often said to be the straightest in the world, the mark of Europeans dividing a continent with a ruler. I wanted to know how much of the world's border really is straight, where it is, and who drew the longest lines.
So I took every international land border in Natural Earth's detailed world map, 233,534 km of line between 170 countries and territories, and sorted each stretch into one of four kinds: ruler-straight (a parallel, a meridian or another straight line, for at least 50 km), river or lake, through mountains, and other. For the 40 longest straight lines I looked up the treaty that drew them. Click any border on the map.
Borders: Natural Earth 1:10 million admin-0 boundary lines (public domain). Classes are measured, not official: see How I did this. Numbered circles mark the 40 longest straight segments (more appear as you zoom in); tap one for its treaty. Disputed and indefinite lines are included as Natural Earth draws them, which is not a judgement on any claim.
Of 233,534 km of land border, 45,232 km (19.4%) runs in straight segments of at least 50 km. Rivers and lakes carry 20.9%, mountains 24.7%, and the remaining 35.0% wanders across lowland, following old estates, villages, ethnic lines, survey marks or compromise. Of the straight length, 21% follows a parallel of latitude, 16% a meridian, and 64% is a straight line between two chosen points.
Share of each continent's land border length, by kind
A border counts once, in the continent where it lies. Hover for kilometres.
The continents could hardly be more different. Europe's borders are almost never straight: 1.5% of their length, in five short pieces, the longest being the line across old East Prussia that has divided Poland and Russia's Kaliningrad since 1945. Asia's are dominated by mountains (42%), the Himalaya, Pamirs, Tian Shan and Altai. South America's follow rivers and the Andes. Oceania's only land border, across New Guinea, is 88% straight, the 141st meridian, but one border is not much of a continent.
Partly, and less than you might think. Africa has by far the most straight border of any continent: 24,332 km, 54% of the world's total. Of the five continents with more than one border, it also has the largest share of borders that are mostly straight: 28 of its 110 borders (26%) are at least half straight, against 21% in North America and 14% in Asia. But measured by share of length, Africa's borders are 30.2% straight, and North America's are 45.9%.
North America's figure rests on one huge, very straight border: Canada and the United States, 59% straight over 8,352 km, with the 49th parallel and the 141st meridian. It holds 72% of the continent's straight length. Take it out and North America drops to 28.9%, just below Africa. Most of Africa's borders, meanwhile, are not straight at all: 41% of their length is irregular, often following old local or colonial survey lines, and 20% follows rivers such as the Ubangi, Congo, Limpopo and Orange.
Share straight, by continent, under twelve different definitions
Tolerance (how far a vertex may stray from the line) and shortest segment counted. The ringed settings are the ones used throughout. Oceania (one border) left out. Hover for values.
The ranking does not depend on my settings. Under all 12 combinations of tolerance and minimum length I tried, North America comes first among the five continents with more than one border and Africa second. A figure often quoted, that 44% of Africa's borders follow parallels and meridians, is usually traced to a 1961 study of tropical Africa by K. M. Barbour; here, parallels and meridians make up 12.3% of Africa's border length. That study used different maps and categories, so the two numbers are not strictly comparable, but on today's borders the claim looks too high.
Africa has more straight border than anywhere else, 54% of the world's. But kilometre for kilometre, North America is straighter.
The longest straight segment on Earth's land borders is the 49th parallel between Canada and the United States: 2,020 km from Lake of the Woods to the Strait of Georgia, set by Britain and the United States in the Convention of 1818 and carried west to the Pacific by the Oregon Treaty of 1846. Second is the 141st meridian between Alaska and Canada's Yukon (1,042 km), agreed by Russia and Britain in 1825. Then come the Sahara's lines: Mali and Mauritania (945 km), Libya and Chad (812 km), and Egypt and Libya's 25th meridian (803 km). Egypt and Sudan's 22nd parallel, from the Anglo-Egyptian Condominium Agreement of 1899, adds up to 1,212 km in three pieces.
The 40 longest straight segments, with the year the line was drawn
Coloured by period; pale bars are lines whose origin I could not pin down. Click a bar to see it on the map.
25 of the 40 are in Africa. The United Kingdom was a party to 21 of the 40 lines, France to 11 and Germany to 6. But not all were drawn by European powers dividing someone else's land: the Arabian lines were agreed by Saudi Arabia, Oman, Yemen and the Emirates themselves between 1974 and 2000, and the Gadsden line of 1853 and the Tierra del Fuego meridian of 1881 by American states. Several of the Saharan lines were never treaties at all, but boundaries France drew between its own colonies, which became international borders at independence.
Sources: Brownlie (1979) African Boundaries; Biger (1995) The Encyclopedia of International Boundaries; the US State Department's International Boundary Studies; the International Court of Justice (1994); the International Boundary Commission and the International Boundary and Water Commission; Wikipedia's border articles. "High": a well-known treaty set this line. "Medium": the treaty is known, but which instrument drew this exact stretch, or when, is less certain. "Low": an administrative line or an origin I could not pin down. Corrections welcome. Full table in data/straight_segments.csv.
The years cluster. Of the 40 longest straight lines with a date, 21 were drawn between 1880 and 1914, the years of the Scramble for Africa, and they make up 47% of the length of the 40. 6 are older, all in North America, from 1783 on, and 4 are recent: three in Arabia and the provisional line between Ethiopia and Somalia. The median year is 1899.
Year each long straight line was drawn, against its length
Hollow circles: origin uncertain. Hover for the treaty.
Straight lines were the natural tool for powers that claimed land they had not surveyed. A parallel or a meridian can be agreed in a European capital from a coarse map, and marked on the ground later, if ever. Many of these lines cross desert where few people lived, or were thought to. They were not empty: the Saharan lines cut through the grazing lands of Tuareg, Moors and Tubu, and the Kenya and Tanzania line through Maasai country.
Water is the commonest natural border: 20.9% of the world's length follows a river or lake. China and Russia's border is 81% river (the Amur and the Ussuri), and Argentina and Paraguay's is all river. Mountains carry 24.7%: Argentina and Chile share 3,881 km through the Andes, and China and India's whole border runs through the Himalaya and Karakoram.
The borders with the most river and the most mountain
Dark: length of that kind; pale: the rest of the border. Hover for values.
Two cautions. Natural Earth's world map of rivers misses many small ones, so some borders that follow a stream show up here as other; the river share is a lower bound. And the mountain class says only that the border runs through mountainous terrain: most such borders follow crests or watersheds, but a world elevation model with 9 km cells cannot show which. As a check, 59% of the length I class as mountains lies inside one of Natural Earth's named mountain ranges, against 13% of the other class.
Any measure of straightness depends on the map. Natural Earth draws the world at three scales. On the most detailed, 1:10 million, used here, 19.4% of border length counts as straight. On the 1:50 million map it is 25.4%, and on the 1:110 million map 83.6%: simplify a winding border enough and it becomes a chain of straight lines. Even at 1:10 million some small wiggles are smoothed away, so the true straight share on the ground is, if anything, a little lower than mine.
Share of world border length counted as straight, at three map scales
Same method and settings (1 km tolerance, 50 km minimum) on each.
Among countries with at least 1,000 km of land border, the straightest is Somalia (93%), then Egypt (81%), Somaliland (80%; Natural Earth draws it apart from Somalia) and Saudi Arabia (79%). The United States' land borders are 54% straight. 78 of 170 countries and territories have no straight border at all, among them France, Germany and the United Kingdom. Search any country on the map for its own figures.
Borders. Natural Earth's 1:10 million admin-0 land boundary lines (version 5.2.0-pre), keeping international boundaries and the disputed, indefinite and line-of-control lines Natural Earth draws, and leaving out UN buffer zones, demilitarised zones and leased bases: 233,534 km in 332 borders. Disputed, indefinite and ceasefire lines are 4.6% of the length; without them the straight share is 19.1%.
Straight. Walking along each border, a stretch counts as straight if every point on it lies within 1 km of a single great circle or a single line of constant compass bearing (which includes every parallel), and it runs for at least 50 km without doubling back. I tried twelve combinations of tolerance (0.25 to 2 km) and minimum length (25 to 100 km): with a 50 km minimum the world figure ranges from 18.0% to 24.0%, and it only balloons, to 51.4%, when short, loose stretches of 25 km within 2 km are allowed, which takes in plenty of ordinary wiggly border. Lines Natural Earth marks as running through water count as water even where drawn straight; the straight courses through the Great Lakes are not marked so, and count as straight.
Rivers and lakes. A stretch follows water if it lies within 3 km of a Natural Earth river centre line or lake shore, or inside a lake, for at least 15 km. Varying the distance (1.5 to 5 km) and minimum length (10 to 30 km) gives 19.4% to 23.6%.
Mountains. From the ETOPO5 world elevation grid (5 arc-minutes, about 9 km): above 2,500 m, or above 300 m with at least 300 m of relief within about 18 km, a simplified version of the UNEP-WCMC mountain definition; at least 20 km at a time. I first tried to detect crests directly (border higher than both sides), but at this resolution it found a crest on only a small part of even the Andes border, so I use terrain instead. Other thresholds give 11.0% to 32.4%. The order of priority is straight, then water, then mountains.
Who drew them. The treaty table is hand-compiled from standard references, not a database, and dates are the year a line was first set in roughly its present form. I mark 1 of the 40 as low confidence (the provisional line between Ethiopia and Somalia, whose date is uncertain) and 18 as medium; the colonial administrative and Soviet lines were checked against the English Wikipedia articles on each border, cited in the data. Continents follow Natural Earth, with borders between continents placed where they lie (Russia's Asian borders in Asia, Sinai in Asia, New Guinea in Oceania).