How to handle centimeter from/to inches

Question

I have an international application that handles lengths and weights of people, and stores these in a database. I was wondering how to deal with this in case users can switch between using centimeters/inches in the application.

I was thinking to always use centimeters in the database, and convert to inches if the user chose to use inches. But of course, if the user enters a length in inches and it is converted to and stored as centimeters, the value may change slightly because of rounding errors.

How would you handle this scenario?

Thanks, L

Answer 1

There is much to consider in your question beyond the information that is available. Before deciding how to store and convert the information, you must know what your acceptable error is. For instance if you are calculating trajectory to intercept an incoming missile with another missile, extremely minute precision is necessary to be successful. If this is a medical application and being used to precisely control medication formulation it could be more important to be precise than if you are simply calculating BMI.

In short, pick a standard whether metric or other and stick with that for your storage type. depending on the precision required, choose the smallest unit of the measurement system that will provide you the accuracy you need. All display of units of a different measurement system would be converted from this base measurement.

And try not to over-engineer the solution. If it will not conceivably be important to measure out to 52 decimal places you are wasting effort and injecting unnecessary complication accounting for that scenario.

Answer 2

Personally I would use one of two methods.

1. Always store it in the same unit of measure
2. Store a unit of measure in a separate field so that you know if the units is CM or Inches.

I prefer the first method since it makes it easier to process.

Answer 3

So you would have rounding errors if you convert from inches to centimeters, and if you convert from centimeters to inches. The Problem would be the same, no matter what you store in the DB.

You could possibly store the values not in centimeters in the Database but in millimeters. So i think how smaller the unit is, so more exact it would be, even in case of conversion.

Answer 4

If the same user should be able to switch at front end, you should definitely store one field representing the value in one unit you decided, because the rounding errors will happen anyway.

If you have a group of users only dealing with inches and another only dealing with cm and each of these groups have their one database or at least "own values", then decide for two fields, value/unit (e.g. same software, different customer installation in different countries)

I'd store non float values representing for example micro meters (with an unsigned 32-bit you can represent everything from 4.2 km to 0.001 mm).

Answer 5

Not sure why you would need a database unless you were storing your conversion rates

There would be no way to detact metric or imperial because they are just numbers

Your rounding errors will happen in accordance with the degree of accuracy you wish to display....

Answer 6

Depending on what you're going to be doing with those values (whether you need to do much aggregation at the DB layer, etc), the best way to ensure there is no cumulative rounding error is to store in the original unit of measure with that unit of measure (id) in a separate column, and have a separate conversion table that you use for on-the-fly calculations when comparing, aggragating, etc.

This will not be super-efficient or convenient, however: you will always have to join to a conversion table before doing any work with the values stored.

Answer 7

We can do the task very simple. We have only coefficient for converting inches to cm and all calculations and result we did not save into DB. We can only multiply or divide the number on the ratio and we get result. So if you have cm need to multiply the ratio and get result. You can see how it work by the example I found it in 2 min: http://inchpro.com/metric-system/convert-inches-to-centimeters I think you know that store all the values in a database occupies a lot of space.