Maps are one of the oldest collaborative perpetual works in human history, an evolutive process, a permanent product of improvement over the work of others that preceded us. Even though we may change many things, like place names and drawing of shapes, it's common that we maintain many of previous arbitrary assumptions, like orientation, geographic references, selected features to be represented, etc.

Maps are always at some level inaccurate, because they are always a selective and valorative representation of reality. Although when we first see some old maps, even if we realize how inaccurate they can be, we usually can intuitively recognize what they are representing.

Why are old maps usually more inaccurate when compared to contemporary ones, while some other even older human works of depiction, like artistic portraits from the Renaissance, can be almost perfect representation of their subjects?

One of the reasons is that maps, and mostly old maps, rely not exclusively on direct observations, but most times also on authoritative reports of explorers, as well as on indirect measurements.

Actually, both the “Mountains of the Moon” and “Mountains of Kong” never existed in reality, they were imagined and depicted based on reports of others.

What Cary introduced as a novelty in his 1805 map was the connection of both fictitious mountain ranges, creating a new fabulous single mountain belt diving Africa. As a result, the Niger River had eliminated any possibility to turn to the south, ending in the middle of the desert, as also imagined by previous mappers, waiting decades to be correctly surveyed by the middle of the 1800's, but even then kept being mistakenly represented up to the beginning of the 1900's.

As later explorers found, and we can see today, Niger River has its sources in the Guinea Highlands near the Sierra Leone border, flowing north-east to southern Mali. Then it turns to south, discharging in its vast delta in southern Nigeria, in the Gulf of Guinea.