[openstreetmap/openstreetmap-website] add optional coordinate epoch to nodes (#3203)

[Andrew Harvey]

> Right but I don't see how it helps unless data is normalised - so I download an area and I get a whole load of objects that were added at different times from different sources.
> Now even if all those objects have a date attached how does it practically help me edit?

Because the editor software can take those object coordinates, and transform them all into a common epoch. If using aerial imagery they could be transformed into the same epoch as the imagery so everything aligns, or without imagery then it doesn't really matter which epoch so long as it's some epoch.

> Not to mention that two different imagery sources might be different at the same date - in fact newer dated imagery from a different source might be less accurate.

Yes but all of this is only relavent for imagery sources that are accurate enough for it to make a difference and actually publish the epoch of the imagery. It's not the date the imagery was captured, it's the epoch of the imagery georeferencing. Because it's likely during the orthorectification process they transform to some fixed epoch.

> So without knowing the source as well the date tells me nothing useful.

Yeah, the epoch date needs to be published by the imagery provider (if not then simply not set the epoch value), or come from a higher accuracy GPS.

> You whole scheme is predicated on the fact that you happen to have very good imagery so you can assume it's perfectly aligned and shift data to normalise position based on date, but it seems to me that is very specific to your use case, yet you want to change the whole world on the assumption that is either true everywhere already or will be in due course.

Good imagery (which we do have in some regions already) and/or a GPS better than standard phone/consumer grade hardware.

It's not really changing the whole world, the `epoch` attribute would be optional and opt in, it's only set when either using a more accurate GPS or some compatible imagery layers, most of the uploaded nodes wouldn't use an epoch just yet.

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