When we speak about dimensions, we talk about properties right?

So when I define the property "height of a person" as an entity, i would supply the SI unit (m) and the SI multiple (-2, cm) that it should be saved in (in the database).

When someone then inputs the height in meters (e.g. 1.86m) it would be converted to the matching SI multiple before being saved (i.e. 186 (cm)).

On the database side each SI multiple would get its own table so that indexes can easily be made. Depending on which multiple we choose in the property the datavalue would be saved to a different table. Did i get the idea correctly?


On Wed, Dec 19, 2012 at 4:47 PM, Gregor Hagedorn <g.m.hagedorn@gmail.com> wrote:
On 19 December 2012 15:11, Daniel Kinzler <daniel.kinzler@wikimedia.de> wrote:
> If they measure the same dimension, they should be saved using the same unit
> (probably the SI base unit for that dimension). Saving values using different
> units would make it impossible to run efficient queries against these values,
> thereby defying one of the major reasons for Wikidata's existance. I don't see a
> way around this.

Daniel confirms (in separate mail) that Wikidata indeed intends to
convert any derived SI units to a common formula of base units.

Example: a quantity like "1013 hektopascal", the common unit for
meterological barometric pressure (this used to be millibar), would be
stored and re-displayed as
1.013 10^5 kg⋅m−1⋅s−2

I see several problems with this approach:

1. Many base units are little known. "kg⋅m2⋅s−3⋅A−2" for Ohm... It
breaks communication with humans curating data on wikidata. It will
make it very difficult to compare data entered into wikidata for
correctness, because the data displayed after saving will have little
relation with the data entered. This makes Wikidata inherently
unsuitable for an effort like Wikipedia with many authors and the
reliance on fact checking.

2. Even for standard base units, there is often a 1:n relation. e,g,
both gray       and sievert have the same base unit. The base unit for lumen
is candela (because the steradians is not a unit, but part of the
derived unit applicability definition)

Gregor