[OSM-talk] Tool for tag tracking

[Roland Olbricht]

Hi Pierre,

> This new query on the test server reports action=deleted when and object 
> is simply modified to remove all tags.
> See https://overpass-turbo.eu/s/uJ8

Thank you for pointing to this example. To get the information what has 
happened, please use [adiff:...] instead of [diff:...]. You then get 
back in the <new>-section an element that has "visible=true" or 
"visible=false", according to whether the object got irrelevant or got 
deleted. As a drawback, Overpass Turbo then does not show the results.

I should add a paragraph for this in the documentation.

The reason for this behaviour is the underlying metaphor: Filtering for 
a specfic set of data (here: waterways in the given bounding box) means 
that all other data is disregarded, i.e. it makes no difference whether 
it exists or not.

While this is pretty irrelevant as long as you work with the plain 
filtered data, it does make an implication for a diff: there is no 
difference whether an object is newly created or just got relevant. You 
do not even intend the database engine to spend extra time on figuring 
that out. Symmetrically this means that for the extract it is irrelevant 
whether an item got deleted or just irrelevant.

Hence, the database spends no time on that in [diff:..] mode but it does 
in [adiff:..] mode.

 > Should there be a an action subtype that reports either tag or geometry
 > only actions ?

I would like to avoid having a large number of different actions, I 
prefer giving the information in different attribute or so. Most things 
can happen in combination, further increasing the number of actions if 
actions telled all the details. There had been an older attempt for a 
diff format with more actions, and it was a pain to work with it.

Best regards,

Roland