Of course the old methods were'nt foolproof. I know of someone who set off to cut a 50 km path through a rain forest and dialed in the magnetic deviation on their survey gear on the "wrong" side". Only found out when they arrived, several weeks later, on a river 50+ kms away - several kms off plan...........
Asturias56, this reminds me of an almost similar when I was working in PNG. We had to mark out the alignment for a straight road through thick jungle with completely opaque canopy, though not as far as yours. I was all prepared to do a proper survey, but one of the locals was insistent that we didn't need to. I decided to give him his chance, and he walked and marked a straight line, and popped out on the river exactly where we were meant to.