In this paper, we study the selection process of resources for teaching geometry, of a 5th Grade primary-school teacher in Colombia.For that, we use some concepts from the Documentational Approach to Didactics (DAD), such as "resource system" and "scheme"; and from the Mathematical Work Space (MWS) theory, the "geometrical paradigms" defined by Kuzniak and colleagues.We analyze the teacher's documentational work, and by using the reflective investigation methodology, we study the evolution of this teacher's resource system for teaching geometry through his production of representations or maps of his selection process.As a main research finding, we were able to infer the teacher's scheme underlying his selection of (digital or non-digital) resources for teaching geometric transformations, and identify some operational invariants or criteria involved in that scheme, derived from his professional knowledge.We conclude that the knowledge is diverse and involves several aspects related to: mathematical content, curriculum, resource affordances, cognitive and ergonomic aspects.