In the lower 48, there are 5 map "series" or scales that you can view:
In the lower 48 states, the NGS file layout groups files into 1x1 degree blocks that I call "sections". In Alaska, the "sections" are now 3x1 degrees in size (3 degrees in longitude, and 1 degree in latitude). The Alaskan section directories hold the 250K, 63K, and where available the 24K maps.
There is one 250K map file covering a 3x1 degree section, with a name like Y69144A1.TPQ. It contains 100 maplets in a 10x10 array, so a maplet covers 0.30 by 0.10 degrees. (Note that I always list longitude, then latitude when describing map geometries.)
There are 20 63k map files covering a 3x1 degree section, with names like A69144A1.TPQ. Each file contains a 4x10 array of 40 maplets. Each maplet covering 0.15 by 0.025 degrees. Each file covers 0.6 by 0.25 degrees. In a 3x1 degree section there are 5 maps left to right and 4 top to bottom.
The 24k map files in Alaska are different from the 24K map files in the lower 48 states. In Alaska they have names like N59135A1.TPQ. They contain a 5x10 array of maplets, each maplet covering 0.375 by 0.0125 degrees. The entire map file covers 0.1875 by 0.125 degrees. In a 3x1 degree section, there are 16 maps left to right and 8 top to bottom. Note that the Alaskan 24K files have the initial letter "N" just like some of the 24K files for certain states in the lower 48 states. This is in some ways unfortunate given that their layout is so different.
With one exception, the northern limit of the lower 48 states is the line of 49 degrees north latitude. The exception is the "Lake of the Woods" area of Minnesota which extends somewhat north of the 49 degree line. In addition, the 125 west longitude line is west of any part of the lower 48 states. Alaska, on the other hand, is always well west of the 125 west longitude line (the easternmost part of Alaska reaches the 130 west longitude line). If we ignore the Aleutian Islands, Alaska is always north of 54 degrees north longitude, and the Aleutians extend somewhat south of the 53 degree north line, and are always north of the 52 line. The Aleutians extend to 178 west.
This makes it easy for software to discriminate between maps in Alaska versus maps in the lower 48 states. A latitude test against 50 degrees would do on one hand, or a longitude test against 126 on the other.
Gtopo / email@example.com