The EMDB map distribution format is based on CCP4 map format. See http://emdatabank.org/mapformat.html for further information; click here to download the complete EMDB map distribution format description.
Conventions for exchange of cryoEM data have been addressed by J.B.Heymann, M.Chagoyen and D.M.Belnap in a 2005 publication entitled 'Common Conventions for Interchange and Archiving of Three-Dimensional Electron Microscopy Information in Structural Biology', with corrigendum. The 3DEM conventions website summarizes conventions and provides information about checking for adherence to the proposed conventions. A web resource has also been created to collect and compare conventions of individual 3DEM software packages: http://conventions.cnb.uam.es