> Why do ships sometimes appear to move across land?
> In some cases this is because there are ships navigating via canals or rivers that aren’t visible on the map. Generally, though, this effect is an artefact of animating a ship between two recorded positions with missing data between, especially when the positions are separated by a narrow strip of land. We may develop the map to remove this effect in the future.
Found the answer in the sites FAQ:
> Why do ships sometimes appear to move across land?
> In some cases this is because there are ships navigating via canals or rivers that aren’t visible on the map. Generally, though, this effect is an artefact of animating a ship between two recorded positions with missing data between, especially when the positions are separated by a narrow strip of land. We may develop the map to remove this effect in the future.