Actually, it's okay The main reason for inaccurate longitude positioning is that the earth is a sphere with slightly flat poles, which leads to the shorter meridian length of 1 degree with higher latitude. At that time, every meridian on the chart was the same, which led to positioning error.

But in fact, it has little impact. 1. Because the navigation in the era of geographical discovery is mainly concentrated in the middle and low latitudes, where the error is very small.

2. It is because in 1687, Newton published the first law of the universe in human history: the law of nature. Put forward gravity and Newton's law. At the end of the paper, Newton made two predictions through calculation: A. The earth is a sphere with slightly flat poles. B. you can use the acceleration g of gravity to calculate the weight of the earth! (The experimental method is given). The French Academy of Sciences quickly verified a prediction (although their real purpose was to try to prove that Newton's paper was nonsense). That is, from then on, the chart was recalibrated, and the problem of longitude positioning was basically solved. A hundred years later, B's prediction was confirmed by an American scientist. In the process of verifying B prediction, human beings invented contour lines.