International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B. ch. 2.5, p. 286

Symmetry analysis is necessarily tied to examination of patterns near relevant zone axes, since the most intense Nbeam interaction occurs amongst the zerolayer zoneaxis reflections, with in addition a limited degree of upperlayer (higherorder Laue zone) interaction. There will generally be several useful zone axes accessible for a given parallelsided single crystal, with the regions between axes being of little use for symmetry analysis. Only one such zone axis can be parallel to a crystal surface normal, and a microcrystal is usually chosen at least initially to have this as the principal symmetry axis. Other zone axes from that crystal may suffer mild symmetry degradation because the Nbeam lattice component (`excitation error' extension) will not have the symmetry of the structure (Goodman, 1974; Eades et al., 1983).
Upperlayer interactions, responsible for imparting threedimensional information to the zero layer, are of two types: the first arising from `overlap' of dynamic shape transforms and causing smoothly varying modulations of the zerolayer reflections, and the second, caused by direct interactions with the upperlayer, or higherorder Laue zone lines, leading to a sharply defined fineline structure. These latter interactions are especially useful in increasing the accuracy of spacegroup determination (Tanaka et al., 1983), and may be enhanced by the use of lowtemperature specimen stages. The presence of these defect lines in convergentbeam discs, occurring especially in lowsymmetry zoneaxis patterns, allows symmetry elements to be related to the threedimensional structure (Section 2.5.3.5; Fig. 2.5.3.4c).
To the extent that such threedimensional effects can be ignored or are absent in the zerolayer pattern the projection approximation (Chapter 5.2 ) can be applied. This situation most commonly occurs in zoneaxis patterns taken from relatively thin crystals and provides a useful starting point for many analyses, by identifying the projected symmetry.
References
Eades, J. A., Shannon, M. D. & Buxton, B. F. (1983). Crystal symmetry from electron diffraction. In Scanning electron microscopy, 1983/III, pp. 1051–1060. Chicago: SEM Inc.Google ScholarGoodman, P. (1974). The role of upper layer interactions in electron diffraction. Nature (London), 251, 698–701.Google Scholar
Tanaka, M., Sekii, H. & Nagasawa, T. (1983). Space group determination by dynamic extinction in convergent beam electron diffraction. Acta Cryst. A39, 825–837.Google Scholar