[OSM-talk] Mapping canals

[Richard Fairhurst]

Andy Allan wrote:

> 2) There are two different things that everyone is talking about, and
> keep getting them confused
>   * The distance, or speed, that you are recording (i.e. the physical
> property). Units are interchangable, can be converted etc to your
> heart's content.
>   * The manner in which the measurement is displayed in the real world
> (i.e. the evidence, signs etc)

And a third, which might sound obscure but is actually a big issue  
when we get back to canals:

     * The specification of the measurement

A UK narrow lock is 7ft wide. It isn't 2.14m wide.

It sounds like nit-picking, but I've just typed "2.14" in because I'd  
memorised that as narrowboat-width-in-metres and that's what some of  
the brokers put in their ads.

If you quote 7ft to 2 significant figures, it's 7ft. If you quote  
2.14m to 2 significant figures, it's 2.1m. There are narrowboats out  
there which will get through a 2.14m lock but not a 2.1m lock. There  
are locks (on the Chesterfield Canal) which have actually been  
built/restored too narrow because of measurement-cluelessness on the  
part of the contractors. It does happen.

(Actually, on checking with a calculator, 7*12*2.54=213.36. So I'm  
doing a bit of unconscious rounding already.)

Whether or not the maxwidth tag accepts measurements in feet is  
probably something that'll eventually be decided by renderer, routing  
app and editor authors, but there is certainly a need for the "7ft" to  
be recordable in the db.

cheers
Richard