We take our units of analysis for granted by/via @Randy_Au

In a perfect world, you choose the unit of measurement that is optimal for the question you’re trying to answer in some analysis. But practically, more often than not, this isn’t the case. Take Randy Au’s advice and think twice next time you’re deciding on a unit of measurement.

This is really important. To give a global view of anything inevitably means you will have to do some kind of data conversion between date/times, units of measurements, etc. Let's take a quick example - there are three gallon types - are you talking about imperial gallons, US gallons or even US dry gallons. How do they relate to litres?  Clearly, the chances are that you will need master tables to enable translations between the different values.