It just means "subjectivity" shapes the "creativity of map making". "Defining" here means, in effect, the creation of the limits for the decisions, and the boundaries where the results will have to work within. In other words, this sentence can be understood as:" subjectivity (of working with the scant information provided by surveyors) will shape the creativity of map making (by testing the skill of the cartographer in creating a usable map based on that scant information.)" Actually, I'm not sure if I made the problem worse, but hope the long explanation helps.