Geonode notes
The OGC CSW 2.0.2 standard provides a discovery mechanism of geospatial metadata. For the HTTP protocol binding, the baseline information model is the Dublin Core (DC) Metadata Element Set. Additional profiles (such as APISO 1.0) exist to support specific information models (i.e. ISO 19115:2006) and output schemas (e.g. ISO19139:2007, OGC APISO, ISO EBRIM, ISO OWL).
Both ISO and DC allow for the advertisement of HTTP links. ISO's CI_OnlineResource construct allows for a certain level of granularity to express links (linkage, protocol, applicationProfile, name, description, function). For discussion of usage of this ISO metadata element, see this page at lab.usgin.org. Dublin Core defines a 'dct:references' [0..*] with the comment "A related resource that is referenced, cited, or otherwise pointed to by the described resource." This elements is typically used to encode URL's to get described resources in CSW record implementations. The csw:record XML implementation of the Dublin Core elements and terms allows any element to have a 'scheme' attribute of type xs:anyURI (see http://schemas.opengis.net/csw/2.0.2/rec-dcmes.xsd). There is no further granularity within dct:references.
Implementations have used dc:URI, however dc:URI is not actually defined as a Dublin Core element (its not standard).
A finer model is required to communicate different types of links (opaque, binding parameters, etc.). For an in depth discussion of Machine actionable Links, look here.
Geonode, as a CSW client, requires the ability to identify various online linkages and be aware of their types/model to be able to process them accordingly (i.e. opaque, binding parameters, etc.).
This is possible given the ISO model, however it would be valuable to have an lightweight approach within DC that is inspired by ISO's CI_OnlineResource as well as fit within DC's dct:references element.
Express ISO's CI_OnlineResource model in JSON using in dct:references/@scheme.
Example:
<dct:references scheme="{'protocol': 'OGC:WMS-1.1.1-http-get-map', 'name': 'base%3Adistrits', 'description': 'distrits (JPEG Format)', 'linkage': 'http://localhost:8001/geoserver/wms'}">http://localhost:8001/geoserver/wms?layers=base%3Adistrits&width=620&bbox=-86.144%2C8.041%2C-82.555%2C11.221&service=WMS&format=image%2Fjpeg&srs=EPSG%3A4326&request=GetMap&height=550</dct:references>
Where:
- dct:references.text: the full URL to the resource
- dct:references/@scheme: a JSON representation of ISO's CI_OnlineResource:
- name: the name of the link. In the case of an OWS, this should be the resource (layer|featureType|coverage) name
- description: a human-readable description.
- protocol: the type assigned to the URL, from the GeoNetwork gmd:online/gmd:CI_OnlineResource/gmd:protocol enumeration *
- linkage: the URL of the resource. In the case of an OWS, this should be the service endpoint with query parameters
The scheme attribute documentation in the rec-dcmes.xsd schema states "The scheme attribute may be used as a qualifier to reference an encoding scheme that describes the value domain for a given property.", it would be more consistent to make the JSON object in the dct:references element contain all the necessary information to understand the link. There are a number of proposals out there for JSON encoding of machine-actionable links (hypermedia profiles): HAL, SIREN, Hydra. The 'scheme' URI would identify which of these schemes was being used for the JSON in the dct:references element content. Multiple dct:references could be provided using different schemes. See examples in a fuller development of the proposal
| protocol | description |
|---|---|
| ESRI:AIMS--http--configuration | ArcIMS Map Service Configuration File (*.AXL) |
| ESRI:AIMS--http-get-feature | ArcIMS Internet Feature Map Service |
| ESRI:AIMS--http-get-image | ArcIMS Internet Image Map Service |
| GLG:KML-2.0-http-get-map | Google Earth KML service (ver 2.0) |
| OGC:CSW | OGC-CSW Catalogue Service for the Web |
| OGC:KML | OGC-KML Keyhole Markup Language |
| OGC:GML | OGC-GML Geography Markup Language |
| OGC:ODS | OGC-ODS OpenLS Directory Service |
| OGC:OGS | OGC-ODS OpenLS Gateway Service |
| OGC:OUS | OGC-ODS OpenLS Utility Service |
| OGC:OPS | OGC-ODS OpenLS Presentation Service |
| OGC:ORS | OGC-ODS OpenLS Route Service |
| OGC:SOS | OGC-SOS Sensor Observation Service |
| OGC:SPS | OGC-SPS Sensor Planning Service |
| OGC:SAS | OGC-SAS Sensor Alert Service |
| OGC:WCS | OGC-WCS Web Coverage Service |
| OGC:WCS-1.1.0-http-get-capabilities | OGC-WCS Web Coverage Service (ver 1.1.0) |
| OGC:WCTS | OGC-WCTS Web Coordinate Transformation Service |
| OGC:WFS | OGC-WFS Web Feature Service |
| OGC:WFS-1.0.0-http-get-capabilities | OGC-WFS Web Feature Service (ver 1.0.0) |
| OGC:WFS-G | OGC-WFS-G Gazzetteer Service |
| OGC:WMC-1.1.0-http-get-capabilities | OGC-WMC Web Map Context (ver 1.1) |
| OGC:WMS | OGC-WMS Web Map Service |
| OGC:WMS-1.1.1-http-get-capabilities | OGC-WMS Capabilities service (ver 1.1.1) |
| OGC:WMS-1.3.0-http-get-capabilities | OGC-WMS Capabilities service (ver 1.3.0) |
| OGC:WMS-1.1.1-http-get-map | OGC Web Map Service (ver 1.1.1) |
| OGC:WMS-1.3.0-http-get-map | OGC Web Map Service (ver 1.3.0) |
| OGC:SOS-1.0.0-http-get-observation | OGC-SOS Get Observation (ver 1.0.0) |
| OGC:SOS-1.0.0-http-post-observation | OGC-SOS Get Observation (POST) (ver 1.0.0) |
| OGC:WNS | OGC-WNS Web Notification Service |
| OGC:WPS | OGC-WPS Web Processing Service |
| WWW:DOWNLOAD-1.0-ftp--download | File for download through FTP |
| WWW:DOWNLOAD-1.0-http--download | File for download |
| FILE:GEO | GIS file |
| FILE:RASTER | GIS RASTER file |
| WWW:LINK-1.0-http--ical | iCalendar (URL) |
| WWW:LINK-1.0-http--link | Web address (URL) |
| WWW:LINK-1.0-http--partners | Partner web address (URL) |
| WWW:LINK-1.0-http--related | Related link (URL) |
| WWW:LINK-1.0-http--rss | RSS News feed (URL) |
| WWW:LINK-1.0-http--samples | Showcase product (URL) |
| DB:POSTGIS | PostGIS database table |
| DB:ORACLE | ORACLE database table |
| WWW:LINK-1.0-http--opendap | OPeNDAP URL |
| RBNB:DATATURBINE | Data Turbine |
| UKST | Unknown Service Type |
Extensions to protocol enumerations and descriptions
| protocol | description |
|---|---|
| WWW:LINK-1.0-http--image-thumbnail | Web image thumbnail (URL) |
WMS Layer with only binding information
<dct:references scheme="{'protocol': 'OGC:WMS-1.1.1-http-get-map', 'name': 'base%3Adistrits', 'description': 'distrits (JPEG Format)', 'linkage': 'http://localhost:8001/geoserver/wms'}">http://localhost:8001/geoserver/wms?layers=base%3Adistrits&width=620&bbox=-86.144%2C8.041%2C-82.555%2C11.221&service=WMS&format=image%2Fjpeg&srs=EPSG%3A4326&request=GetMap&height=550</dct:references>
Thumbnail
<dct:references scheme="{'protocol': 'WWW:LINK-1.0-http--image-thumbnail', 'name': 'base%3Adistrits', 'description': 'Web image thumbnail (URL)', 'linkage': 'http://localhost:8001/geoserver/wms'}">http://localhost:8001/geoserver/wms?layers=base%3Adistrits&width=20&bbox=-86.144%2C8.041%2C-82.555%2C11.221&service=WMS&format=image%2Fjpeg&srs=EPSG%3A4326&request=GetMap&height=20</dct:references>
