I think I know understand what usage you want to do of that "waterway length" 
datum (/or at  least that's what I'm reading in your last message/): use it as 
a "control" for checking if the waterway's segments add up to the "official" 
(/whatever that can mean.../) waterway length.  Or at least in part: that datum 
will be close to useless to check waterways with the complexity of the "/river 
of a hundred waterways/" and many similar ones.

For that I guess  a better solution would be to use the fixme=* tag: "fixme: 
check that this river length is between 5499 and 7088 Km", for the Nile.

TBH I see A LOT of issues with this tag:

  * it is wrongly named (/distance instead of length/)
  * it is unverifiable "on the ground"
  * it can assume multiple different values according to different sources
  * it is IMNSHO useless (/just point to a Wikipedia article to get this 
information/)

Personally I'm leaning to propse to deprecate the usage of this key and subject 
that to a vote. What is the process for that?

Sergio


On 2019-02-17 14:07, Eugene Podshivalov wrote:
> вс, 17 февр. 2019 г. в 15:18, Sergio Manzi <[email protected] 
> <mailto:[email protected]>>: 
>
>       That's as old as data processing: "/garbage in, garbage out/". Let's 
> fix the data. 
>
> Fixing data is a good thing but from utilization in production point of view 
> the choice between unstable and stable data is not questioned.
> Competeness of data is even more important than its stability, and that 
> unfortunately cannot be achieved that quickly. One can create a waterway 
> relation with a length defined and then there may be a long run until all 
> waterway segments are drawn properly to finally be able to compare it to an 
> official length.
>
>     You'll probably can find many different estimations about its length. 
> Which one are you going to choose? 
>
>  I would take one from any encyclopedia (subject to its license) and that 
> figure will at least serve other mappers as a guidence when searching for 
> incomplete or broken rivers.
>
> Cheers,
> Eugene
>
>  
> вс, 17 февр. 2019 г. в 15:18, Sergio Manzi <[email protected] <mailto:[email protected]>>:
>
>     On 2019-02-17 12:55, Eugene Podshivalov wrote:
>>
>>     It will work but only if the entire river from its spring to mouth is 
>> drawn precisely enough, all relation roles are labeled properly and nobody 
>> breaks the labeling by intent or mistake some day.
>
>     That's as old as data processing: "/garbage in, garbage out/". Let's fix 
> the data.
>
>     And yes, the river you pointed at is particularly complex and probably 
> geographers are pulling each other's hairs about computing its length. You'll 
> probably can find many different estimations about its length. Which one are 
> you going to choose?
>
>     Sergio
>
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