[RFC] Historic main tag for defensive works

Hello friends,

I wrote a short proposal to add historic=defensive_works as main (top-level) tag to defensive_works=*. Details can be found at Proposal:Historic main tag for defensive works - OpenStreetMap Wiki. All feedback is appreciated. I am not active on the tagging mailing list, so I would appreciate it if someone could share my proposal there.

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I would appreciate it if someone could share my proposal there.


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Seems sensible but there might be some cross-over with archaeological sites. For example, archaeological_site=fortification.

When should one use historic=defensive_works + defensive_works=motte versus historic=archaeological_site + archaeological_site=fortification + fortification_type=motte for example?

Defensive works are often not archaeological sites. In many cases they simply exist and archaeology is irrelevant there.

archaeological_site=fortification is mainly used for full fortifications, whereas defensive_works is specifically for sections of it. In a motte-and-bailey castle for example, the motte can be mapped as an OSM object with defensive_works=motte.

defensive_works=* is already approved. This proposal only adds the historic=* tag to it. Changes to the meanings of existing tags are outside the scope of this proposal.

Sure, but you’re proposing to bring it, and all of its values, under the same top-level tag as the archaeological_site tagging.

So you will now have the same value (motte) under the same top-level key (historic) but with a different meaning. I think it’s definitely worth mentioning and discussing the difference.

I just think it has the potential to be a little confusing without some clarification. For example, the image used as an example for defensive=crownwork is of the exact same place as the image used in archaeological_site=fortification (Fort Bourtange). So which scheme should I use? Cross-overs like this have the potential to make it unclear which historic tag to use.


A big problem with this suggestion is that the new value is orthogonal to many of the existing historic values in use.

It does indeed make sense to say “… this is defensive” (as various tags already do), but not as a “historic” tag except in unusual circumstances.

Good point. I’ll see what I can do with that.

There is no archaeology at Bourtange. I did some mapping there myself, so I know a bit about it. There’s the fortress and its defensive works, and of course the village.
The image for archaeological_site is poorly chosen and can easily be replaced by a picture of a Roman castrum or an Irish ringfort. Unfortunately I cannot find a good a good image of just the crownwork of Bourtange or any other crownwork, which is why I used the zoomed-out image of Bourtange that shows the crownwork well.

Why would they not be historic? Defensive works are often important parts of heritage sites.

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I didn’t say that they weren’t historic. I just suggested picking a more appropriate historic tag for the feature (which may vary depending on what it is) and then adding other keys for “defensive”.

Having separate historic=* tags for each defensive_works feature is impractical, if that’s what you mean. There are simply too many and they’re all part of fortification engineering terminology, so a more generic main key seems appropriate to me.

The whole defensive_works thing is a bad idea (it dilutes and confuses fortfication_type, which has 17 times the usage), and requiring “historic=defensive_works” as a historic tag is a spectacularly bad idea too, because many of these objects have some other historic tag, like this city_gate or this battery. Even the ones that don’t have a historic tag (here, or here) have other main tags.***

As I’ve said before, the best way to improve OSM’s historical coverage would be to add more detail** (using existing, established keys) to the objects already mapped in OSM, not by diluting what is already mapped or tagfiddling from established values to new ones.

Neither the new page nor the previous one address the two key issues here:

  1. That one historical thing may have several components. Is “defensive_works” to be applied to the whole, or just the component, or both? If both, how do data consumers tell which is which?
  2. The historical / archaeological split, which forcing a “new” historical" value would cause more problems with.

** actual mapping, not some weird kind of architecture astronautery

*** I actually try and consume this data, and every time I see a combination of main tags I have to ask myself “is this mainly an X or is it a Y?”

Of the common values only motte and earthworks conflict with defensive_works, which is nowhere near 17 times the usage.

I would actually argue that fortification_type is a bad idea since it requires objects to be tagged as archaeological sites, which is usually incorrect.

  • A barbican is a type of city gate with a defensive purpose. I’ll think about that one.
  • Batteries by themselves are usually not forts but sections of fortifications, so (without a closer look) this might be a case of mistagging.
  • Caponiers are usually buildings, I don’t see how that conflicts.
  • Wall tagging is currently broken with way too many overlapping tagging schemes.

Thanks for the feedback, I’ll have a closer look at that.

I almost considered inventing tags for murder holes and indentations in walls for cannons, but I suppose that would be too deeply in the realm of architecture astronautery.

Many objects are multifunctional to a degree, but I can certainly try to mitigate the double tagging and confusion a bit with the input you just gave.