Just how good were the deep ocean navigators-of-old in maintaining a good knowledge of their position at sea? They did not have an absolute reference, such as a GPS receiver, to check on their workings and sightings as we do, so it is difficult to know just how well 19th century deep sea mariners knew their actual position.

A novel way of answering this question suggests itself in a paper "Spatial and Seasonal Distribution of American Whaling in the Age of Sail" by Tim Smith et al. They used data culled from whaling logbooks to determine where in the oceans the various whale species could be found. However, they also noted that they found 32 examples where the logs of two ships each recorded a sighting of other, and gave the position. The mean absolute difference  between pairs of reported positions was 0.22 degrees of latitude (with an SD of 0.315 degrees) and 0.54 degrees in longitude (with an SD of 0.667 degrees)

Naively, we could suppose that on average, the actual position was midway between the recorded positions in each pair and so the average errors in longitude and latitude were in fact half the figures given above - that is about 6.6 minutes in latitude and about 16.2 minutes in longitude.