Earlier quoted context omitted.
What do they say?
Long, straight lines for borders usually imply that the territory through which they are drawn was neither well explored nor terribly important to the people drawing them. Sometimes that's because the territory is mostly uninhabited and nobody really has a strong claim to want their particular patch of land to be on one side or the other, but frequently its because the mapmakers either don't know or don't care about…
Speaking of West Africa, you don't really have to think hard to figure out why Gambia's borders were drawn the way they were.