Data items in the EM_MAP category record details
about the type of the 3d-em map. The map is represented
logically as a three-dimensional array of data-values
of the same data-type. To interpret the contents of of a 3d-map file
it is necessary to know the data-type of the array and the size of the
array in three dimensions (i.e.the number of columns, rows and sections).
In a 1d-array representation columns are the fastest changing, followed by
rows and sections. The first element of the array will have index 0.
The 3d-em map is in a defined orientation/position in Universal 3D Space.
This space is described by a right-handed cartesian coordinate system
(and is the same coordinate system as that used for structures deposited
in the PDB).