"Multi-volume" JNX maps
JNX format only allows having up to 50000 tiles per map level, but there is a way to cross this limit, so called "multi-volume" JNX files.
In this approach, the levels with bigger number of tiles are saved in separate files with the less than 50000 tiles in each level. The coordinates in the headers of these separate JNX files correspond to the coordinates of tiles they contain.
An example. We need to make a 3-level JNX map with 10000 tiles in the first level, 40000 in the second, and 160000 in the third. In this case, we make 4 files: 3-level JNX with 10000 tiles in the 1st level, 40000 - in the 2nd, and 50000 in the 3rd, and three single-level JNX files - two files with 50000 tile in their only level, and one file with 10000 tiles.
At the time of writing, the only program with the multi-volume JNX maps support is SAS.Planet, which uses JNXLib to produce the JNX maps.