Multiple map references / chain of provenance

[davenquinn]

Source information for maps is currently stored in the maps.sources table, with a single set of reference information per source. This is convenient for direct data entry, but is a bit limiting in several cases:

• If we are entering a map compilation that has been synthesized from previous mapping
• If we are entering a map that has been updated or corrected

This is especially a problem for large compilation sources such as the USGS state geologic map compilation. Some mapping in the SGMC has up to three credible references. Only one is in broken out properly, while the rest are incorporated as comments.

We should update our schema to handle multiple references per source, possibly in tandem with better support for itemizing the individual maps that make up large compilation sources.

[davenquinn]

There will of course still be many compilation/synthesis maps (e.g., continental-scale ones such as the geologic map of North America) where the source information for each map feature is too far removed to be relevant to the final mapping. This is more relevant in the case where maps have been integrated into a generalized source with few changes.

[davenquinn]

Another example from SGMC: