Compass and nautical chart
A compass (jap, Rashin-gi) is a navigation instrument (note: needs link) that orients itself to the earth’s magnetic field. As a result, the metallic compass needle always forms a north-south line, which can be used to determine a course. In the four blues, compasses and charts serve as the main means of navigating a ship on the seas. However, as one approaches the Grand Line, the compass begins to go haywire – the needle spins uncontrollably in circles, rendering it useless. This is due to the fact that there are tons of mineral-rich islands on the Grand Line, all of which have their own magnetic field. In order to progress anyway, you need either a log port or an eternal port.
Since islands in the New World change spontaneously or even hide their magnetic field completely, a triple log port is used by default so that the crew does not run the risk of being permanently stuck on an island. Thus, there are always three islands to choose from and you have to choose your own route.
Various ports
Log Port

The log-port (jap. Recordkeeping Guidelines, Kiroku Shishin) is a special type of compass used only on the Grand Line. It consists of a glass dome in which a special compass needle hangs, and is usually worn on the wrist by the navigator of each pirate gang, such as Nami or Eddy. In the trade, a Log Port costs around 100,000 Berry.
The compass needle reacts to the individual magnetic fields that run between the islands. If you have chosen one of the seven magnetic currents at the beginning of the Grand Line, the needle will only log onto the next island within this current. How long the log port needs to recharge varies from island to island and ranges from a few hours to several days. In extreme cases, it can even take a whole year, such as on the prehistoric island of Little Garden.

Because of the many weather anomalies on the Grand Line, Vivi says the log port is also the only instrument you should rely on in the open sea.
The kanji mean recording compass needle.
New World Version

Because the seas of the New World are much rougher and more unpredictable than those of the first half, you also need an extended form of the log port. This is a full three log port needles arranged in a triangle on a round wooden base. One points diagonally to the left, one straight ahead and the other diagonally to the right.
Thus, three possible islands are always targeted on the direct course, because it happens that an island spontaneously changes and hides its magnetic field. As a rule of thumb, the more unusual and violent the deflection of the needle, the more dangerous the indicated island. In the Straw Hat Pirates’s case, the choices were Risky Red, Raijin, and Mystoria, of which Raijin was the most dangerous due to its lightning rain, according to Vice Admiral Smoker. However, Luffy and his crew ignored the islands and set course for the nearby island of Punk Hazard, which had no magnetic field and was sealed off by the world government.
Eternal Port
Huh? Are you guys really not familiar with this?
It is a permanently latched version of the log port.
Instead of recalibrating on each island like the log port does…
In other words, the Eternal port will point in the direction of the island forever.
And this one is pointed at Arabasta.
Igaram about the Eternal Port

A special type of log port is the eternal port (jap. permanently refers to the needle, Eikyū Shishin). This has the coordinates of an island permanently stored. Strictly speaking, it is a fixed log port in a wooden socket. In this way, one can easily commute between two magnetic currents or travel long distances without having to take large detours via other islands on the way back.
Eternal ports are very expensive, however, and are not suitable for making real progress on the Grand Line. Every now and then, however, they are a good alternative to avoid the long recharging times of the log port. On the primeval island Little Garden, for example, it takes a whole year for a log port to reorient itself. With a direct Eternal port to Drum or Arabasta, this time limit can thus be avoided.
The kanji mean eternal compass needle.
Eternal ports that have occurred so far:
Arabasta, Elbaf Enies Lobby, Jaya | Long Ring Long Land, Nada, Impel Down, Navarone (filler) | Partia (Movie 4), Omatsuri (Movie 6), Foolshout, Laugh Tale (Movie 13) | Samua (TV Special 2), Nebulandia (TV Special 16) |
Cameo Port (Movie 7)

The Kame Port (Japanese: 亀針儀, Ki Shishin) only appeared in the seventh feature film. Due to a log port and Eternal port respectively, the path to Roba’s home island of Mecha-Island is not shown, so she and the Straw Hat Pirates used a cameo port to get there.
It is a small casket. If you open it, a small Turtle appears, which always swims in the direction of this island. This function is similar to that of the Southbirds on Jaya. For orientation, a compass cross with the four cardinal directions is drawn on the floor.
The kanji mean turtle compass needle.
Miscellaneous

- In the original they are called Log and Eternal Pose.
- Aboard the Rocketman are Eternal ports to every station the sea train calls at. So are San Faldo, Pucci, St. Poplar, Water 7 and Enies Lobby.
- A kind of natural compass are the Southbirds, which can be found on Jaya, among other places. As their name suggests, they are a species of bird that always points south with its beak. They were used by Mont Blanc Cricket and the Straw Hat Pirates to locate the knock-up stream south of Jaya. Southbirds are also thieving and very intelligent. As it turns out later, there are also North-, East- and Westbirds pointing to the other three cardinal directions.
- Vivre Cards also work on a similar principle to a compass. Made from a piece of fingernail in a certain New World store, they always point in the direction of the person from whose DNA they were made. When the owner passes away, the card burns completely.