Coordinates were measured in grades on official French terrestrial ordnance charts from the French revolution well into the 20th century. 1 grade (or in modern symbology, 1 gon) = 0.9° or 0.01 right angle. One advantage of this measure is that the distance between latitude lines 0.01 gon apart at the equator is almost exactly 1 kilometre. It would be exactly 1 km if the original definition of 1 metre = 1 ⁄ 10,000 quarter-meridian had been adhered to. One disadvantage is that common angles like 30° and 60° are expressed by fractional values (33 + 1 ⁄ 3 and 66 + 2 ⁄ 3 respectively) so this "decimal" unit failed to displace the "sexagesimal" units equilateral-vertex – degree – minute – second invented by Babylonian astronomers.