TBH I would simply have the entire thing in a bank
For example say your map is 12000 by 12000 thats 144,000,000 bytes of ram but it would be easier to access that than to have an array. For example 20,5 would be accessable via ( 5 * 12000 )+20 the 5*12000 as y is 5 and the map is 12000 wide, finally we add the X part that is 20 would give the location in the bank that we need to look at.
For example say your map is 12000 by 12000 thats 144,000,000 bytes of ram but it would be easier to access that than to have an array. For example 20,5 would be accessable via ( 5 * 12000 )+20 the 5*12000 as y is 5 and the map is 12000 wide, finally we add the X part that is 20 would give the location in the bank that we need to look at.