Not using the same reference points is the odd part. If they were building a tunnel, it would make sense, but for a bridge they could see from one side to the o

Not using the same reference points is the odd part. If they were building a tunnel, it would make sense, but for a bridge they could see from one side to the other. They should have been able to set a reference point on one side and translated it across to the other side. That's a direct measurement, and it doesn't matter at all where that reference point is relative to sea level or anything else.