For what it's worth, Cosma Shalizi's notebook page on emergence has a very reasonable discussion of emergence, and he actually mentions macro-level properties of gas as a form of "weak" emergence:

The weakest sense [i]s also the most obvious. An emergent property is one which arises from the interaction of "lower-level" entities, none of which show it. No reductionism worth bothering with would be upset by this. The volume of a gas, or its pressure or temperature, even the number of molecules in the gas, are not properties of any individual molecule, though they depend on the properties of those individuals, and are entirely explicable from them; indeed, predictable well in advance.

To define emergence as it is normally used, he adds the criterion that "the new property could not be predicted from a knowledge of the lower-level properties," which looks to be exactly the definition you've chosen here (sans map/territory terminology).