After a 7 month break, I have finally returned to ROM hacking. Today, I have finished learning about all the bytes in the map header. I am hoping this information will be helpful to you all working on your ROM hacks.
You can see the map header by pressing CTRL+H while you are in the header tab on your map in Advance Map. It is a long strand of bytes that the game uses to load everything for the map.
For example, the map header for Mt. Moon is this..
A43D7608 DCA17308 00000000 00000000 2001 72 00 7F 00 00 04 01 06 00 00
Now, I will go through all the bytes and explain what they mean. Experiment or use different maps as examples to get what you want.
Bytes 1-4: Pointer to map footer offset
Bytes 5-8: Pointer to event offset
Bytes 9-12: Pointer to map script offset
Bytes 13-16: Pointer to connection offset
Bytes 17-18: Music in reverse hex form.
Byte 19: Map footer #. Used for setmapfooter command, which changes the map to another. You should probably leave it be unless you're adding a new map.
Byte 20: ??
Byte 21: Defines what map name the map belongs to. (Thanks to Jambo51 for clearing this up)
Byte 22: Cave type
Byte 23: Weather type
Byte 24: Similar to byte 28. The trainer battle background may or may not change depending on the value chosen.
Byte 25: Set to 01 to be able to ride a bicycle on the map.
Byte 26: Changes the "Show name on entering" value that is visible in simple header mode. 06, 07 allow you to run on the map, 05, 08 do not.
Byte 27: Species floor number. Above-ground floors count up from 01, where 01 is "F1." Below-ground (basement) floor count down from FF, where FF is "BF1."
Byte 28: Species a battle background that will override the trainer battle background AND wild battle background. Leave at 00 if you do not want this. You can choose from 00-09. 0A and on seem to be the same background. This may be because I have JPAN's engine installed, but I'm not quite sure.
Perhaps someone knows what the twentieth byte is. If so, please say so and I will put it in this post.