[Tagging] waterway=derelict_canal

[Dave F.]

On 24/08/2015 21:49, Chris Hill wrote:
> On 24/08/15 18:56, François Lacombe wrote:
>> Hi Mateusz,
>>
>> It seems this tag is a combination of waterway=canal and disused=yes.
>>
>> I'm not so in favor of such value (derelict_canal). There are two 
>> different information in one value.
>
> I think that 'disused=yes' is a dangerous tag and should be avoided.
>
> Suppose someone uses foo=bar + disused=yes. Someone else searches for 
> foo=bar, he will find the objects with and without disused=yes. If he 
> doesn't notice he will treat the disused the same as the active objects.

Sub tags such as disused=yes have always been the way to describe 
additional attributes of an entity. It's even the syntax used by XML: 
you collect all 'waterway=canal' items then manipulate that selection 
set. If programmers "don't notice" then, quite simply, they're not very 
good at they're job.

An example of where the non use of sub tags lead to confusion is tree 
coverage:

natural=wood v. landuse=forest to distinguish between trees that are 
'managed' has led to misuse as can be seen in their wiki pages:

All tree coverage should be a single primary tag & any attributes 
described in sub tags
managed=yes.




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