Abstract: The author analyzed the present state of research on the field digital data collection technique applied in geological mapping and existing problems during the past 30 years and studied the segmentation techniques of the semantic, descriptional, spatial and storage granularity and their relationships with scale-independent mapping. The author proposes the theoretical framework of PRB granularity for digital geological mapping according to these studies and based on establishing the expression of spatial data in geological mapping and complying with the characteristics of traditional mapping. Through studies of the structure mode and relationships of the interpretative data and absolute data of the field traverse observation, the author designs the “grammatical” structure of the field geological mapping process with scale-independent mapping, determines the structure mode of mutual connection of the collected data item and data item and their relationships in the description of geological mapping and defines and elucidates the PRB “granularity” and process in digital mapping. Through a study of the content required for an integrated geological analysis and geological map linking, the basic requirements for structural description are studied, the structure granularity is determined based on the semantic granularity and descriptional granularity and the PRB data model is defined and discussed.