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

[Fabrizio Carrai]

Dermot,
I thinked to your comment and I still found several reason to tag in local
units but also in km/h as well.
It really depends from the scope of the application.
But I have to admit that finally in the doubt of the most efficent method
and in rule with the OSM philosofy to not map for an application but for the
reality, we should have to remain with the SI (km/h).

Ciao!
	Fabrizio



> -----Messaggio originale-----
> Da: newbies-bounces at openstreetmap.org
> [mailto:newbies-bounces at openstreetmap.org]Per conto di Dermot McNally
> Inviato: sabato 2 maggio 2009 15.09
> A: newbies at openstreetmap.org
> Oggetto: Re: [OSM-newbies] R: Road Maximum speed in UK units
>
>
> 2009/5/2 Fabrizio Carrai <fabrizio.carrai at gmail.com>:
> > In  my opinion, the speed limit should be recorded in the local
> speed unit.
> > During the driving I would be able to check my speed vs the
> limit reported
> > on the traffic signals, whatever their units are.
> > No conversions have to be done.
>
> This is an old topic and a divisive one, but regardless of my personal
> opinion of how this should be tagged, you're taking too narrow a view
> of the purpose of the tag. If I interpret your point correctly, you're
> assuming that the purpose of the tag is to somehow make you aware of
> the limit in force to allow you to compare it manually against your
> actual speed. To achieve this, clearly a simple number for human
> interpretation will suffice.
>
> However, the real gain from tagging speed limits is that a GPS device
> (which knows your current speed) can know whether you are currently
> within the limit or not - and to do this requires clarity of units for
> the tagged speed. That is, an application using the tag data has to be
> able to make one of the following assumptions:
>
> 1. All values are in a predetermined unit (earliest usage and wiki
> documentation assume this and specified km/h. Disadvantage is that mph
> values need to be approximated in km/h, representing a loss of
> accuracy and a separation from what is "on the ground")
>
> 2. All values are in the unit legally employed in the country in which
> they are recorded (so applications need accurate border information,
> something we don't yet have. In addition, any changes in legal units
> necessitate immediate retagging. Limits that are dual signed [height
> limits, say] create a tagging dilemma).
>
> 3. All values stored as number only are assumed to be in a default
> unit (km/h for maxspeed) with the option to add a unit suffix where
> you wish to represent a different unit. Snags: all processing requires
> pre-parsing of values, pure numeric analysis no longer works without a
> normalisation stage. Mappers will vary widely in style of suffix
> recording (space or no space, 'mph', 'm.p.h.', 'MPH', 'mp/h'....)
>
> None of the approaches are perfect, but any usable approach must allow
> a machine to "understand" the limit, not just present it to a human.
>
> Dermot
>
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