Standard Altitude

While some Sky Islands lie high or low in the skies, most have roughly the same altitude. The default level has been labelled the Standard Altitude (SA), and measurement stones on strings are sold which take exactly that height. This lets Skyfarer quickly check their current altitude.
Most Sky Ships are designed for their railing to lie at most five metres above SA at empty load, and around or slightly below SA at full load. Their hull at full load can easily be at -5 SA. This allows them to easily dock at most Sky Islands, without having to overcome large height differences.
Sky Pirates often use light ships, for easy raiding. Meanwhile, islands that are significantly below SA are often supplied by heavily ballasted ships. The biggest risk of a heavy ship is it ripping apart, so only the sturdiest of ships are weighed down that heavily.