The navigation devices in the One Piece universe are:
- The compass for the Eastblue, Southblue, Westblue, Northblue
- The Log-Pose for the Grand Line
- The New-World Log-Pose for the New World
Other navigation instruments (in exceptional cases) are:
- The Eternal-Pose for Grandline (always points at the same island)
- The South Birds on Jaya whose bills always point south.
- The Kame-Pose who always points to Mecha
All of these navigation instruments have limitations and disadvantages. The compass, for example, does not work for the Grandline and New World. The Log-Pose can follow only one magnetic wave and takes time to record the next magnetic wave. And the Eternal-Pose works for only one island.
As you can see, navigation in the One Piece universe is very complicated. And this is the point where my question is. Besides the Eleven Supernovas, all other pirates, marines and other ships are free to travel through the whole Grandline and Blues, and to go on any island they want.
In episode 392, Shakuyak explains how hard it is to travel through the Grand Line for the Eleven Supernovas as they tried to get to the Sabaody Archipel. However, the Marines and other pirates commute easily between every island. Do they have an extra expensive Eternal-Pose for every island?
So why is navigation so easy for the Marines and other pirates whereas it is so hard for the Straw Hat crew (as example)?