That is a method I already use. The problem is that the population rectangle still has to calculate the non instances, so the larger the population area, the more non-instances it would have to spend working out. With a moving area, the instances will be less, although it would still have to calculate each frame, but then so would the other method.

It would be interesting to know when working out each instance, whether each x,y,z is worked out on the planet's coordinates defined by the population area. Or, the instance is worked out in the population area's x,y,z. Maybe a quick test could clarify....