It's about as long as The Panama Canal.
It's about four-fifths as long as The English Channel.
In other words, 16 leagues is 0.8310 times the length of The English Channel, and the length of The English Channel is 1.20 times that amount.(average width between Isle of Ushant and The Walde Lighthouse) (La Manche, Ärmelkanal, Mor Breizh, Mor Bretannek) (a.k.a. The Channel)
The English Channel measures between 19.30 leagues, narrowing as it flows northeast. The world speed record for a Channel crossing by swimmer was set by Australian Trent Grimsey in 2012, with a time of 6 hours, 55 minutes.
It's about three-fourths as long as The 405.
In other words, 16 leagues is 0.762790 times the length of The 405, and the length of The 405 is 1.3110 times that amount.(a.k.a. I-405, Interstate 405, San DIego Freeway) (Southern California)
Running from Irvine to San Fernando, The 405 runs a total of 20.9760 leagues. The 405 is the most heavily-trafficed freeway in the United States, with an average speed as low as 8 kph during rush hours.
It's about three-fourths as long as Hadrian's Wall.
In other words, the length of Hadrian's Wall is 1.40 times 16 leagues.(a.k.a. Roman Wall, a.k.a. The Wall, a.k.a. Vallo di Adriano, a.k.a. Vallum Aelium, a.k.a. Severus' Wall) (from North Tyneside, Tyne and Wear, England, United Kingdom to Cumbria, North West England, United Kingdom)
Hadrian's Wall, which crossed England form the North Sea to the Irish Sea during the time of Roman rule, measures 22 leagues (about 79 Roman miles). Roman soldiers — at one time up to 10,000 of them — only occupied Great Britain for about 246 years after the wall was built, after which time the local Brits began to take residence the wall's garrisons.
It's about three-and-a-half times as tall as Olympus Mons.
In other words, the height of Olympus Mons is 0.30 times 16 leagues.(a.k.a. Mount Olympus) (Mars)
The tallest mountain in the solar system, Olympus Mons rises to approximately 4.90 leagues. The mountain has been known to astronomers since the nineteenth century because it is tall enough to rise above Mars' frequent dust storms.
It's about one-fifth as long as The Grand Canyon.
In other words, 16 leagues is 0.1990 times the length of The Grand Canyon, and the length of The Grand Canyon is 5.03 times that amount.(a.k.a. Gran Cañón) (Arizona) (Colorado River miles measure)
The Grand Canyon has a total length of 80.30 leagues along the course of the Colorado River at its bottom. Having opened in early 2007, the Grand Canyon Skywalk allows tourists to stand on a horseshoe-shaped glass walkway 0.1980 leagues above the floor of Granite Gorge.
It's about eight times as tall as Maxwell Montes.
In other words, 16 leagues is 8.10 times the height of Maxwell Montes, and the height of Maxwell Montes is 0.120 times that amount.(Ishtar Terra, Venus)
Maxwell Montes rises to a total height of 2 leagues. Since Venus does not seem to have the kind of tectonic activity which gives rise to mountains on Earth, the origin of the Venusian mountain remains the subject of some dispute.