[OSM-talk] Good practice, and should we rely on defaults?

[Greg Troxel]

Yves via talk <talk at openstreetmap.org> writes:

> I wonder if using defaults from an external database (let say we have
> it) and analyse it with our boundaries is really more efficient than
> tagging everything?

I don't understand how you can think that.   Simplifying, there are two
paths forward:

  A) Don't produce a file that maps regions to defaults and promulgate its
  use.  If a property is not tagged, apply some global default from the
  wiki.  Encourage everything to tag every property always.

  B) Produce the file and use it.

To talk about efficiency, we need to estimate both the total amount of
effort, and the result in terms of accuracy of rendered information.  It
seems obvious that B leads to much better results with less total
effort.  Producing the file probably only takes 1000 hours and using it
per rendered another 100 or so.  Tagging every road, done honorably,
would take a very long time.


> Also, asphalt is certainly granted in Germany on major roads, but this
> is not the case where the majority of the world population lives. Why
> would a data consumer have to post-process our data differently
> depending on the place in scope on a worldwide dataset ?

Because the point of OSM is to represent the world, and if having a file
that maps regions to defaults, combined with explicit data, is a better
representation of reality, that's something to consider seriously.

> Last, assuming a default where no knowledgeable mapper have put is foot (or mouse) is certainly a mistake. 

So if there is a road shown on the map, drawn from imagery, and it is
not tagged oneway=yes or oneway=no, and you are a router, do you allow
routing over it?  In one direction or both?  I don't see how you can say
"assuming a default is a mistake" as a general statement.  One has to
make assumptions to operate in the world, and the question is what
assumptions lead to better outcomes.
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