In this paper we introduce a formal specification of the problem of modelling geometric constrained objects adopting OBJSA nets, a high level Petri net. The geometric objects are defined imperatively while constructing them. The approach is innovative in that it solves constraints during manipulation, propagating messages among the objects involved in the geometric figure and it does not require numerical techniques. The formalization of the geometric constructions with OBJSA nets has been an important step to validate the system and in particular the constraint maintenance algorithm. Each class of the system is modelled by a OBJSA component and their compositions allows to describe a generic construction. The algorithm specified in OBJSA can be simulated in the ONE (OBJSA Net Environment) environment.