Complex engineering systems development comprises many technological elements that have to be integrated together to function as one system. Traditionally, a project based approach will create a new product. However after many years of product engineering, the systems engineer can now be faced with engineering of technologies that need to integrate with legacy systems which may continue their deployment over a long period of time. Legacy integration poses problems for traditional systems engineering methods, such that the success of a complex engineering product cannot be measured simply in terms of the successful commissioning of the system, but requires a measure of product performance within the larger scale system and pre-existing development system over the course of the product's deployment. This paper uses the development of a hydrological smart phone system to illustrate the concept of a “Heuristic” approach to Systems Engineering. The authors propose that the traditional systems engineering method needs flexibility to perform a Heuristic translation to provide error correction shortcuts in the legacy engineering methodology.