Though most people did not really need maps when traveling on foot or horseback, as they usually followed already well-known paths and roads, cartography was extremely useful in the field of sea travel, as there were often no orientation points for thousands of miles and one could get lost very easily if he did not know how to determine his position with enough accuracy. Same process as when mapping land areas applies when measuring the length and relative angle of a distance traveled marine vessel, however, it is much more difficult to do it accurately, therefore distortions happen easily.
Mapping land regions from within was exceptionally accurate even in ancient times mainly because one could determine walking distances with a very high degree of accuracy. It was also often possible to create a very detailed and accurate map of more than a hundred square miles of land in a very short time just by climbing on a mountain of a decent height and looking around. This map of Europe was drawn in 200 AD by Ptolemy, and similar accuracies were not achieved in medieval Europe until about 14th century.
However, on the open sea, there are no discernible landforms one could peg his location to, and therefore determining the location of a ship on a latitude and longitude grid was always the only way to accurely estimate the distances between different continents and archipelagos. And this is difficult. So difficult, in fact, that it wasn’t until 1762 when the longitude relative to the Prime Meridian could finally be measured for the first time within a margin of error of 1 arc minute (±1 nautical mile), using a state-of-art chronometer H4, manufactured by John Harrison, who received an because of it. However, these clocks remained quite expensive for a long time, and decreased below the sum of the annual wages of a skilled worker few years after Napoleon was exiled to St.Helena.