The Log Pose is special compass that's needed for all pirates to travel the Grand Line. It's the one thing that is to be trusted on the unpredictable sea.
The Log Pose is unlike any known form of compass, and is unique to the One Piece universe. There are no letters on the compass and the needle is always shown in suspended within a transparent sphere. Having a Log Pose is required for navigating through the treacherous seas of the Grand Line. The seas are so wild that without one it's impossible to know the true direction you are traveling. It's the only instrument you must trust. Whether the needle points into the sky or the sea.
A normal compass will only spin and sway wildly. This is due to the abondance of minerals in the Grand Line. Due to these geographical make up of the islands each land mass has a different magnetic pull. This is how the Log Pose works. The red needle always points to the next island on the chain. Once on the directed island the wearer of the Log Pose must remain for a certain amount of time for it to acclimate the islands signature, then it will point the way to the next island.
The time for the Log to change varies from island to island. It can be as short as a few hours. To as long as an entire year, as in the case with the island known as Little Garden.
Grand Line Paths
From Reverse Mountain, the starting point of the Grand Line, there are several different paths to take. No matter the path you take. They all eventually lead to the Red Line and Merman Island. After Merman Island the path branches out again and will eventually meet at the island known as Raftel. The only man known to reach this island is the Pirate King, Gold D. Roger.
The only other way to find an island in the Grand Line is either by luck or the use of an Eternal Pose.
Why the Log Pose is Round
The Log Pose is set in a round shell, because of two rather different islands that littler the Grand Line. The first of which is the Merman Island that is set under the Red Line, deep at the bottom of the sea. The second type is the Sky Islands. These are islands of dense clouds where people actually inhabit.
Luffy and his crew arrived at the end of Reverse Mountain without a Log Pose. Their navigator, Nami, had never even heard of them until she tried to chart their course and her compass no longer worked. Crocus, the local doctor who ran the lighthouse on Reverse Mountain, was the one who explained the use of the Log Pose.
It was actually seen in the manga a chapter before it was explained. Two Baroque Works members, Miss Wednesday and Mr. 9, accidentally dropped their Log Pose and was picked up by Luffy. This one was eventually broken due to Sanji and Luffy's goofing around. It was replaced by Crocus with an older model.
Log Pose Bearers
While all crews need a Log Pose to navigate the Grand Line. Not every crew is seen to have a person wearing one on the wrist.
Nami - Straw Hat Pirates
The first Log Pose Nami had possession of was broken. Crocus gave the crew and older model to get them started, and it's been the one the Straw Hat crew has had ever since. It's was later revealed that Crocus had been the doctor of Gold Rogers crew. Though he never told them. it's possible this Log is the same one that was used on the original Pirate King's trip to Raftel.
Ace - Whitebeard Pirates
Ace, Luffy's brother, has also been seen to wear a wrist mounted Log Pose. He is traveling alone in his search for the pirate Blackbeard and following the same path on the Grand Line. Knowing the path Blackbeard must take. Ace only has to follow him until he catches up.
Eddy - Bellamy Pirates
Eddy is a member of the Bellamy Pirates. He is the navigator and wears his Log Pose on his right wrist.
The Eternal Pose is a type of Log Pose. The biggest difference is that an Eternal Pose will only hold a single magnetic frequency of an island. It's key to having quick travel along the Grand Line.
An Eternal Pose is normally displayed in a wooden base with three pillars and the name of the destination carved into the top platform.
The Marine forces often have an abundance of Eternal Poses that afford them quick travel to where ever they need to be. This gives them an advantage over pirates, who are normally dependent on the Log Pose.