The purpose of the study is to substantiate the stages of the architectural method of designing a geoinformation platform as part of a digital one, for interdepartmental cooperation, in the view of National defense. Five approaches to designing complex digital management platforms are explained. The content of their architecture concept is introduced and considered. The essence of the functional, system and technical architectures of the design object, which together make its systemic form, is described. The principles of developing the architecture of a digital geoinformation platform are formulated. Based on them and the content of the functional, system and technical architectures of the object planning, the stages of the method of projecting a geoinformation platform as part of a digital one, for the declared goal, are described. It is concluded that the use of the architectural design method at the preliminary stage will allow, on the one hand, to perform reengineering of information on the mentioned functional processes, taking into account spatial data and cartographic visualization; and, on the other hand, to form operational and technical requirements for establishing the required platform