Ok I know it's impossible to map the GL, but I wanted to put that in the title to see the reaction I would get.
I have always thought that If anybody in One Piece has any info about Raftel and THE One Piece it would probably be the guys in the Gorosei. Think about it, these guys have inherited the most powerful single and united organization in the world. They are the ultimate source of power and information.
Several facts about the World Government has lead me to this theory:
- they are the longest standing single organization in the world and they have out lived or destroyed all other ancient organizations such as the acheologists in Ohara. Hence they probably have more documented information about the world than any single organization.
-they alledgedly know ways to travel across patches of the Calm Belt (ie the area between the gate of justice and the Marine Headquaters), which basically means they know something that every pirate wants to know. (however many pirates have been in and out of the Grandline, and we, the fans, don't really know how that works either)
there are bases all over the world and on Grandline so they probably have the most knowledge of how to get from one place to another. Perhaps they just have the resources to get Eternal posts made for them but that would mean they have largest data bank of knowledge about each known island on the Grandline, which makes gives them a HUGE advantage for survival in the GL.
they have captured/acquired Gol D. Roger. He is the only one who has seen all of the Grandline, and he and his crew probably knows more about it than any known figure of the manga. So if the government once had him in cuffs, it is likely that they either interogated him or already knew many of the things he knew. This ties into the mystery of how the Government ever captured Roger.
Aokiji had some info about where to find Strawhats and even more interesting is his knowledge of how to get there. He refered to a log of some sort, it told him that the SH would hit water 7 soon. That would mean that he knew the magnetic routes that would lead the SH to W7. What I'm trying to say here is that the world governments may not have mapped the Grandline, but they probably got close to the next best thing, they mapped some of the magnetic fields that controls traveling in the Grandline. All it comes down to is knowledge and experience, which they have more of than anyone else. So if anyone is the closest to the One Piece, I wouldn't say WhiteBeard, I would say the World Government.