purformance vs resources

I'm making myself a game and it's going to have a huge map 50000x50000 tiles -> 20bytes per tile atm.


Now i already have split the map up into sections of about 500x500 and i can store these as their own Objects @ a cost of ++500^2*20 bytes per object or i could sacrifice some speed and store the map to a image with a color->byte conversion to determine what's located @ the specified x,y coord.

wat do?

50000x50000 tiles -> 20bytes per tile
Have you even done the math on this?
50 000 000 000b
48 828 125kb
47 684Mb
46.57Gb
Your total world map is just over 46 gigbytes.  How the hell do you plan to make something like this available?  Blu-ray!?
The only way you could do something like this is downloading and uploading map sections as the player navigates (4.77Mb per map section per your calculations).