[OSM-newbies] R: Road Maximum speed in UK units

[Fabrizio Carrai]

> -----Messaggio originale-----
> Da: newbies-bounces at openstreetmap.org
> [mailto:newbies-bounces at openstreetmap.org]Per conto di Thomas König
> Inviato: giovedi 7 maggio 2009 11.12
> A: newbies at openstreetmap.org
> Oggetto: Re: [OSM-newbies] Road Maximum speed in UK units
>
>
> 1) Only for clarification: hour is NOT an SI-unit. So when measuring in
> km/h we can't complain about the ridiculous unit system in the US/UK.

You  are right, but just to give a rationale to what we are doing: [1] says:

"The SI base unit for time is the SI second. From the second, larger units
such as the minute, hour and day are defined, though they are "non-SI" units
because they do not use the decimal system, and also because of the
occasional need for a leap second. They are, however, officially accepted
for use with the International System"

More info in [2]. I think that this should support the continuation of the
use of "hours" as time unit.

>
> 2) I think it shouldn't be too hard for a computer to convert units!
> (Regardless of the fact if someone uses kph, kmh, k.p.h. or whatever
> abreviation.) Isn't that what computers are made for - calculations?
> Regarding the loss of accuracy: As speed limits are always integers (and
> I guess all over the world they should be divisible by 5) the computer
> (renderer) should be able to perform the conversion without loss of
> accuracy!
>

Computer are built to compute, but we cannot abuse of it! Computing power is
a limited resource and we shoudn't waste it doing extra calculations.

Things change day-by-day, but mobile devices are still less powerful than
the desktop PCs.
I think that all of you agree that we (as OSM mappers) often act as "Data
designer". Although the "Data Designer" objectives are different than the
"Application Designers" we cannot avoid to think to the use of our product.
That means that our "product" must be "usable" in the best efficent way.

The reason of the discussion is to find the best way to tag the speed limits
avoiding extra effort during their use.

So , my personal propose would be:

a) Let the mappers to use their own units
b) Set the requirements for the application to convert all such data
according to well know function to a SI (or an accepted unit). e.g. meters
for distances, km/hours for speeds

Point a) is "justified" by the fact we want to map the reality and the
reality is that in saxon countries the signals are in Mph. As Mike said
""When in Rome, do as the Romans" !
Of course that strongly simplifies the job for the UK mappers, with the
request to add the info that explicit the unit. [4] indicates that there is
still a certain standardization. I don't know if this is the best solution
for a data processing application.
We have also to say that this is still in compliant with the wiki [3] "UK
mappers will often include units in the maxspeed tag, ie: maxspeed=20 mph
(miles per hour)".
A more literally aderence to the standard, i.e. "value"-space-[unit], could
support the string tokenizer function (many of them require a separating
character).

In my opinion that's not a detail. Think to a routing application that load
the speed limit indications and has to check if a unit has been specified
and do the proper conversion all the time. This operation should have to be
do done on each speed value that is in a tag!.

Point b) limits the action in point a). Only standardized units should be
specified, since the  application shall rely on the list of the standard.
Still free to add "furlongs per fortnight" as unit, just specify the
converting function to SI in the wiki.


Ciao!
	Fabrizio


[1] http://en.wikipedia.org/wiki/Time
[2] http://physics.nist.gov/cuu/Units/outside.html
[3] http://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Maxspeed
[4] http://tagwatch.stoecker.eu/Great_britain/En/tags.html