Merge pull request #5 from hybox/dplamap_pcdm

Dplamap / pcdm

notes/dpla_map_and_pcdm.md

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+
+## Relationship between DPLA MAP and PCDM
+
+This document notes the ways in which the DPLA Metadata Application Profile and PCDM can be used together.
+
+### Mapping from PCDM to DPLA MAP
+
+#### Collections
+
+One or more `pcdm:Collection`s map to a `dcmitype:Collection`. As `pcdm:Collection`s may be hierarchical, and DPLA's use of `dcmitype:Collection` is not hierarchical, information may be accumulated from multiple `pcdm:Collection`s into one.  
+
+For example, given:
+
+```
+_:maps a pcdm:Collection ;
+  rdfs:label "Maps Collection" ;
+  dc:description " ... " ;
+  pcdm:hasMember _:africa .
+
+_:africa a pcdm:Collection ;
+  rdfs:label "Maps of Africa" ;
+  pcdm:hasMember _:map1, _:map2, ... _:mapn .
+```
+
+The resulting `dcmitype:Collection` may take the description from the top level Maps collection, the label from the Maps of Africa collection, and apply it to all of the Map objects.
+
+The resulting collection will be associated with a `dpla:SourceResource` constructed from a `pcdm:Object`.  This is the inverse of the PCDM hierarchy, where Collections have members.
+
+
+#### Objects
+
+PCDM Objects can be used to describe objects (both conceptual and physical), and parts of those objects.  DPLA is mostly concerned with identifiable cultural heritage objects (`dpla:SourceResource`) and the digital views of that object (`dpla:WebResource`).
+
+When mapping from PCDM to DPLA, a `pcdm:Object` that is not `pcdm:hasMember` another `pcdm:Object` will be treated as a `dpla:SourceResource`.  Other resources may also be treated in this way.  These Objects may or may not be part of a `pcdm:Collection`.
+
+Such objects MUST have a stable web URL at which the object is rendered in a human readable way.  This is the object of `edm:isShownAt`.  Such objects SHOULD have a `dpla:WebResource` which is a `edm:preview` of the object, such as a thumbnail. 
+
+As per Collections, the resulting DPLA resource may conflate information from an entire hierarchy of objects into a single resource. 
+
+#### Files
+
+Some `pcdm:File`s are `dpla:WebResource`s, but not all.  It would be true in at least the following situations:
+
+* The `pcdm:File` is a view of the `dpla:SourceResource` (`edm:isShownAt`)
+* The `pcdm:File` is a preview / thumbnail of the `dpla:SourceResource` (`edm:preview`)
+
+### Further Notes
+
+* The reverse mapping is lossy and thus round-tripping is not possible. This is because DPLA MAP does not store the structure of the object or collection hierarchy, and thus information is lost in the above transformation.
+* Any enrichments should therefore happen at the PCDM layer, and then re-exported to DPLA.