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

[Andrew Harvey]

Nodes contain lat/lon in WGS84, however as pointed out https://twitter.com/nyalldawson/status/1393050234041757698 and https://geosupportsystem.se/2021/05/19/det-ror-pa-sig/ without the coordinate epoch those coordinates have an implict inaccuracy.

This is especially a problem for parts of the world where plate movement is significant, combined with higher accuracy GPS and/or availability of high resolution and high positional accuracy aerial imagery we have reached a point where OSM is and can be accurate enough to start seeing the problems of no coordinate epoch.

Each node has a timestamp, but this is the edit date, and is not necessarily the coordinate epoch.

I'm proposing we add a new `epoch` attribute along side existing `lat`, `lon`, `timestamp`, where the value shall be a date `YYYY-MM-DD`. The field shall be optional, and only set automatically by editors when using an imagery layer which has a known epoch, or when using a GPS trace or live GPS coordinates.

There is no need for the API to transform data into a common epoch, that can be left to downstream clients who choose to do so (optional, but improves precision).

In my opinion OSM absolutely needs to eventually support a coordinate epoch field, I understand this is a significant undertaking to modify the data format and software to support this.


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