Continuum and HI surveys working together

SKA1 will offer an enormous improvement in observational capabilities for radio astronomy. The astronomical community is very eager to exploit these new possibilities, so much so that the ambitions for using SKA1 far exceed what is possible given the available observing time. An obvious strategy to alleviate this problem is to try to define commensal surveys, i.e. observing programs from which the same data serves a number of (even very) different science topics. The community is aware of the large potential benefits of this approach and many groups are actively discussing how to define the best commensal observing strategies. This, turns out, is not a trivial exercise unless one has some kind of quantitative measure that defines what an optimal combination of survey strategies is. In this paper, we discuss a possible approach, from the HI science point of view, to quantitatively investigate how one can optimally combine different surveys.