Numerical simulation: unit system

Unit is always one of the most important factors in science. Most of the time, a number without unit is useless. For numerical simulation, a good design of unit system can be critical. Recently, I have written a short script/program to prepare climate data for my groundwater/surface water hydrology simulation. I downloaded some data from the National Climatic Data Center(NCDC) separately, and I did NOT notice the changes in unit system. In the end, I have to do everything over again.

So the question is: what kind of unit system should we use to improve our efficiency, and how do we actually implement it?

First of all, we need to identify the unit system used in the data obtained from third party. This is usually done through reading the meta data in the documentation. Never directly use the data without reading the documentation! A lot of time we don’t usually have choices of the unit system provided. In this case, the best practice might be stick with the original unit system.

However, we should use the SI unit system whenever possible. They are apparently a few benefits from it. For example, NCDC climate data online usually provide standard unit and metric unit system. In this scenario, I would prefer to use metric. And you can always convert them into other units without much efforts.

When we implement related numerical simulation program, using SI unit system for data I/O is also very straightforward. For some type of data, a scale factor can easily preserve the precision without losing efficiency. In modern programming language, the double floating data type can handle almost all type of data without memory issues.

Even when you are dealing with old algorithms which use standard unit system, you can always convert the units before actual calculations, and convert them back to SI afterwards.