The Yellow God by H. Rider Haggard

Reflecting that time evidently had made little change in Jeekie, Alan studied this route map with care, and found that it started from Old Calabar, in the Bight of Biafra, on the west coast of Africa, whence it ran up to the Great Qua River, which it followed for a long way. Then it struck across country marked "dense forest," northwards, and came to a river called Katsena, along the banks of which the route went eastwards. Thence it turned northward again through swamps, and ended in mountains called Shaku. In the middle of these mountains was written "Asiki People live here on Raaba River."

The map was roughly drawn to scale, and Alan, who was an engineer accustomed to such things, easily calculated that the distance of this Raaba River from Old Calabar was about 350 miles as the crow flies, though probably the actual route to be travelled was nearer five hundred miles.