Actually personally I'd be happier with your original tank example.

That's the sort of overlapping usage that can easily arise if you merge documents from different fields and it might be easier to just handle the overlapping usage of the element name rather than rename the element in  all the documents.

But the new example shows a purpose designed schema with altitude intentionally designed to take a grandchild element of the same name but a different type, which seems unnecessarily confusing.

David