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).