From TrekBuddy Wiki
An atlas is a set of maps wich can have diferent detail-levels or sizes (layers). Trekbuddy automaticaly selects the right map out of the atlas (depending on your actual position) and switches them while you move.
You can switch layers with 7, maps with 9 key.
Details to Maps and Atlas: http://www.trekbuddy.net/forum/viewtopic.php?t=21
Atlas format (unpacked)
An atlas is a set of different maps organized in a directory structure as shown below.
The structure shows an atlas an atlas named example.tba. It has two layers: roadmap and hikingmap. Each layer contains two maps of different name.
example-atlas/ example-atlas/example.tba example-atlas/roadmap/ example-atlas/roadmap/24/ example-atlas/roadmap/24/P24_ozf.map example-atlas/roadmap/24/P24_ozf.set example-atlas/roadmap/24/set/ example-atlas/roadmap/24/set/24_0_0.png example-atlas/roadmap/24/set/24_0_320.png example-atlas/roadmap/24/set/... example-atlas/roadmap/25/ example-atlas/roadmap/25/P25_ozf.map example-atlas/roadmap/25/P25_ozf.set example-atlas/roadmap/25/set/ example-atlas/roadmap/25/set/25_0_0.png example-atlas/roadmap/25/set/25_0_320.png example-atlas/roadmap/25/set/... example-atlas/hikingmap/ example-atlas/hikingmap/18_Prague_north/ example-atlas/hikingmap/18_Prague_north/18_Prague_north.map example-atlas/hikingmap/18_Prague_north/18_Prague_north.set example-atlas/hikingmap/18_Prague_north/set/ example-atlas/hikingmap/18_Prague_north/set/18_0_0.png example-atlas/hikingmap/18_Prague_north/set/... example-atlas/hikingmap/19_Prague_south/ example-atlas/hikingmap/19_Prague_south/19_Prague_south.map example-atlas/hikingmap/19_Prague_south/19_Prague_south.set example-atlas/hikingmap/19_Prague_south/set/ example-atlas/hikingmap/19_Prague_south/set/19_0_0.png example-atlas/hikingmap/19_Prague_south/set/...
The file example.tba is the root file of the atlas. When loading an atlas using TrekBuddy select this file. Please note that the name of the tba file is not static! Any other name is also valid like xyz.tba or myatlas.tba.
The file itself is only a marker file with a static content: a text file with single line:
Atlas format (packed)
A packed atlas is a set of packed maps and an additional special atlas root file (archive).