[Tagging] Discussion about Multivalued Keys

Colin Smale colin.smale at xs4all.nl
Wed Jan 27 16:12:30 UTC 2016

Thanks Matthijs 

I am stating that the "list of values" syntax, whatever it turns out to
be, should *at this level* not make any assumptions about ordering. No
special status is conferred on the first or last value for example. This
level of semantics is going to vary by tag. In some cases it may be true
that a mulivalent shop has a dominant characteristic, but this forces
the mapper to make a choice, which in some cases is actually quite hard.
So I would like to defer that discussion to tag-level as much as
possible, although it may be reasonable to discuss a generic
construction like nominating one of the multiple values as "primary" for

Rendering a "printed" map is only one use to which the OSM data is put.
In that case I agree that mapnik et. al. have to make some tough
decisions. There are other use cases such as navigation (searching for
POIs) where you want the shop to be found under multiple categories,
where no one category takes precedence.

Of course any change of syntax (or indeed any change of tagging) imposes
a certain workload on data consumers. I believe that the hoops they have
to jump through to consume OSM data "accurately" are magnified
enormously by the myriad tagging conventions and exceptions. I hope that
the benefit of reducing this less productive side of their work will
outweigh the one-time investment that they will need to make to
accommodate a single, well-defined new syntax which can be re-used in
many, many situations. 


On 2016-01-27 16:52, Matthijs Melissen wrote:

> Hi Colin,
> Thanks for getting this discussion started. I agree it's fine to use
> the proposal template also for proposals that are not about proposing
> a concrete tag.
> One thing to take into account in this discussion is that multi-valued
> keys often move the problem to the data consumer. For that reason I'd
> recommend to avoid them in many cases.
> Take for example your example shop=supermarket, shop=bakery.
> Independent of the exact way of tagging, using a multivalued tagging
> scheme forces the renderer to make a decision between a supermarket
> and a bakery icon. Basically, there is no possible way for the
> renderer to support a multi-valued key here! The renderer might have a
> rule that considers supermarkers always more important than bakeries,
> or vice versa. But I think it's much more useful if the mapper decides
> what's the main function, supermarket or bakery, rather than forcing
> the renderer to make a choice.
> -- Matthijs
> On 27 January 2016 at 16:09, Colin Smale <colin.smale at xs4all.nl> wrote: 
>> Dear all,
>> I have created a proposal page as a channel for constructive debate about
>> the way forward. I hope you will all take a look and participate!
>> Although this subject is a bit more than just a proposal for a new tag, I
>> have used the same template. I will try and flesh it out a bit more in the
>> coming days, but everyone is of course welcome to add their stuff.
>> http://wiki.openstreetmap.org/wiki/Proposed_features/Multivalued_Keys
>> --colin
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