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?

>>13
Is there a lossless algorithm that is so efficient you can compress something to even one-sixth of its original size (at which point you can store the world onto a dual-layer DVD)?