[OHM] Some mapping questions

[Hannes Röst]

Dear list
 
Hi, I have just started to map with OHM and it has been a fun ride. I do have a few questions:
 
1. Mapping pre-historic buildings, I am not sure how to encode times before 1582 since they require additional conventions according to ISO 8601 [1] and it would be great to clarify this, I have now used the conversion of 3384 AD = -3383, see my buildings here [2]. Can you comment on whether this is the right way to do it? It is counter intuitive to be "one year off" but I think its most consistent to use ISO 8601, however this should be clarified on the wiki.
2. I noticed on the mailing list some discussion about OSM imports and I cannot help but notice that major geographical and political features are missing in the areas I am interested in (eg lake Constance and Lake Ontario) and to me it seems like a reasonable idea to get things off the ground and tag imports with "licence=ODbL" for things like country boundaries and mountains/rivers/lakes. I am not sure whether spending a lot of time re-mapping geographical features simply so that they are under CC0 is a good use of (my) time. I am happy to have all content by default under CC0 but it seems like we would duplicate a lot of work simply for the purpose of having it under CC0.
3. Furthermore, I see rivers etc mapped in changeset 1 [3] and I wonder whether that data is truly CC0 or also from some OSM import / satellite (probably depends on how the import was done). I have used ESRI images to trace lake Constance since ESRI is free of restrictions and will produce CC0 content but I dont know about other imaging data and whether it has been used in OHM
4. I would like to ask how replaced structures should be handled such as the bridge [4] which was built in 1520 replaced 1828 with another wooden bridge and finally replaced in  1839 with the (current) stone structure. I saw some mention on the mailing list with reference to the date namespace [5] and I wonder whether this has consensus and how to handle a complete replacement -> would it be better to have 3 separate ways here that have independent start/end dates since these are completely different bridges. It seems the date namespace makes more sense if roads change names or importance but is not intended for a different physical object. Personally I would prefer the different ways solution since one could then refer to these entities using a unique identifier for each (eg in Wikidata).
5. How should data from a map best be tagged? I have used the "as_of" tag since I dont know the exact starting time for a road on a map, I only know it was present at a certain time. However, this leads to some rendering artefacts: the roads will be there from the beginning of time, however the bridge for which I know when it was built will suddenly appear in 1520 and before 1520 there is simply a hole where the bridge was [4]. Any suggestions on how to deal with this situation? Would it be better to use "after 1838" for a map produced in 1838, how would that be rendered? (see suggestion here [6])
6. Similarly, I have traced the data of a river from a map of 1838 and the course of the river has changed quite a bit. Now using the "as_of" tag now means that there are 2 rivers displayed in the map which is not really what I wanted. [7] Any thoughts on best to handle this? Of course I could choose an arbitrary start/end date to switch from one river to the other but without more information this is not optimal either. 
7. I have another bridge where there is clear evidence (dendrochronological) when it was built, around 150 BC, but unclear when it was destroyed. We only know that its defunct today, so it was destroyed somewhere between 150 BC and 2020 BC, how should that best be tagged?

Best regards

Hannes

1. https://en.wikipedia.org/wiki/ISO_8601#Years
2. https://www.openhistoricalmap.org/relation/2690490
3. https://www.openhistoricalmap.org/way/4515328
4. https://www.openhistoricalmap.org/way/198531945
5. https://wiki.openstreetmap.org/wiki/Proposed_features/Date_namespace
6. https://wiki.openstreetmap.org/wiki/Key:start_date#Approximations
7. https://www.openhistoricalmap.org/#map=16/47.5895/8.9490&layers=O
8. https://www.openhistoricalmap.org/way/198531929#map=18/47.55831/9.09103&layers=O