recorder with arbitrary CRS

[tim-salabim]

This is to continue the discussion from da29ff0#diff-2500a1ff7485db873721e70572309874R26.

In case people are digitising in non-wemercator CRS, should playback still show the playback in webmercator or replay in native CRS without a map background? I think it has merit to replay in native CRS but would appreciate thoughts from @mdsumner for example

[mdsumner]

At first thought I think it's fair enough to expect that any new feature like this might be limited to the context-CRS in which it was created. Until there's general and robust support for moving between CRS in different contexts by the underlying tools then it would be overloading a specialist tool to provide that.

(I carried on a little, off-topic but if you're interested, been thinking about this a lot https://gist.github.com/mdsumner/0398479afc03c55722e6760ff406d1c7)

[tim-salabim]

Thanks for the input @mdsumner (also the sigh :-) ). Two reasons for supporting editing with arbitrary CRS (including no CRS) are:

• digitizing on top of raster/aerial reference images in corners of the world where web-mercator is not a good choice
• digitizing on top of non-geospatial imagery (such as e.g. floor plans) without coordinate reference systems (e.g. [this example)[https://stackoverflow.com/questions/38607045/place-non-geographic-mapview-object-into-shiny-app/38608437])

Using L.CRS.Simple seems like a good choice for these things, however, there is still no support for displaying images in L.CRS.Simple (see rstudio/leaflet#371). Nonetheless, I'd prefer to have this possiblity in mapedit, as there may also be instances where people wann draw over vector features of arbitrary projection.

[tim-salabim]

This would also mean that we would need to provide a map = argument in playback so that users can supply their background maps to review changes on top of them. This would make playback also more coherent with the other functions in mapedit.

[mdsumner]

To be sure I consider it absolutely essential and assumed that we must be able to edit in arbitrary coordinate systems (not just CRS - the "obvious" next step is editing any graphics produced in R).