: Darwin Core Maintenance Group. 2023. Darwin Core XML guide. Biodiversity Information Standards (TDWG). <http://rs.tdwg.org/dwc/terms/guides/xml/2023-09-13>
This document provides guidelines for implementing application schemas based on [Darwin Core terms](../../terms/) using [XML](http://www.w3.org/XML/). The underlying metadata model is described (in a syntax neutral way), followed by some specific guidelines for XML implementations. Some guidance on the use of non-Darwin Core terms is also provided.
This document does not provide guidelines for encoding Darwin Core in RDF/XML. Nor does it take a position on the relative merits of encoding metadata in "plain" XML rather than RDF/XML. This document provides guidelines in those cases where RDF/XML is not considered appropriate.
This document is targeted toward those who wish to use or construct application schemas using Darwin Core terms in XML. It includes explanations of existing schemas such as [Simple Darwin Core](../simple/) and how to build new schemas to meet specific models of information.
Implementors SHOULD base their XML applications on [XML Schemas](http://www.w3.org/XML/Schema) rather than _XML DTDs_. Approaches based on _XML Schemas_ are more flexible and are more easily re-used within other XML applications.
Implementors MUST use [XML Namespaces](http://www.w3.org/TR/1999/REC-xml-names-19990114/) to uniquely identify elements. Darwin Core namespaces are defined in the [Darwin Core Namespace Policy](../../namespace/), while Dublin Core namespaces are defined in the [DCMI Namespace Recommendation](http://dublincore.org/documents/dcmi-namespace/).
The Darwin Core follows the [Dublin Core Metadata Initiative Abstract Model](http://dublincore.org/documents/abstract-model/) except that the Darwin Core _record_ is roughly equivalent to the Dublin Core _resource_.
- A Darwin Core term MUST be either a `class` or a `property` where `class` is defined as http://www.w3.org/2000/01/rdf-schema#Class and `property` is defined as http://www.w3.org/1999/02/22-rdf-syntax-ns#Property.
Darwin Core follows the guidelines for expressing [Dublin Core metadata using XML](http://dublincore.org/documents/dc-xml/) except in that Darwin Core implementors MUST encode `properties` as XML elements and `values` as the content of those elements instead of having each property contain a value representation and its associated value. The name of the XML element MUST be an XML qualified name (QName), which associates the value given in the `Term name` attribute in the [Darwin Core Terms](../../terms/) recommendation with the appropriate namespace name. For example, use:
[Simple Darwin Core](tdwg_dwc_simple.xsd) most closely models the "flat" nature of many data sets. It is a ready-made schema for sharing information with no structure beyond properties of a _record_ (equivalent to fields in a table, or columns in a spreadsheet). It is meant to accommodate all properties except those that require further structure to be meaningful (auxilliary terms in the classes [ResourceRelationship](http://rs.tdwg.org/dwc/terms/ResourceRelationship), [MeasurementOrFact](http://rs.tdwg.org/dwc/terms/MeasurementOrFact), and [ChronometricAge](http://rs.tdwg.org/chrono/terms/ChronometricAge). The schema has no required terms and no term is repeated within a given _record_. Refer to [Simple Darwin Core](../simple/) for the rationale behind this schema.
The term [`dcterms:type`](http://rs.tdwg.org/dwc/terms/dcterms:type) (which is controlled by the [Dublin Core Type Vocabulary](http://dublincore.org/documents/dcmi-type-vocabulary/)), gives the basic category of object (`PhysicalObject`, `StillImage`, `MovingImage`, `Sound`, `Text`) the record is about. The term [`basisOfRecord`](http://rs.tdwg.org/dwc/terms/basisOfRecord), which has a controlled vocabulary distinct from that of `dcterms:type`, shows the name of the Darwin Core class (e.g., [`Event`](http://rs.tdwg.org/dwc/terms/Event), [`LivingSpecimen`](http://rs.tdwg.org/dwc/terms/LivingSpecimen), [`PreservedSpecimen`](http://rs.tdwg.org/dwc/terms/PreservedSpecimen), [`FossilSpecimen`](http://rs.tdwg.org/dwc/terms/FossilSpecimen), [`MaterialEntity`](http://rs.tdwg.org/dwc/terms/MaterialEntity), [`MaterialCitation`](http://rs.tdwg.org/dwc/terms/MaterialCitation), [`HumanObservation`](http://rs.tdwg.org/dwc/terms/HumanObservation), [`MachineObservation`](http://rs.tdwg.org/dwc/terms/MachineObservation), [`Taxon`](http://rs.tdwg.org/dwc/terms/Taxon)) the record is about.
Many Darwin Core terms (`properties`) are defined as being associated with another term (a `class`). For example, [`scientificName`](http://rs.tdwg.org/dwc/terms/scientificName) and [`Taxon`](http://rs.tdwg.org/dwc/terms/Taxon) are both Darwin Core terms, but `scientificName` is a property organized within the `Taxon` class. When constructing schemas that take advantage of classes in structures, implementors SHOULD maintain the property/class organization for the terms whenever possible (refer to the grouping of the term within a class in the [Quick Reference Guide](../../terms/). To promote reuse, Darwin Core provides a set of xml schemas to use as the basis of additional schemas:
- [Terms XML Schema](tdwg_dwcterms.xsd) - property term definitions as typed global elements and named groups for all terms for a given class to be referenced. The schema makes use of substitution groups `anyClass`, `anyProperty`, `anyIdentifier` and `anyXYZTerm` for each class, e.g. `anyTaxonTerm`. This is the schema upon which the [Simple Darwin Core XML Schema](tdwg_dwc_simple.xsd) is based.
- [Class Terms XML Schema](tdwg_dwc_class_terms.xsd) - class term definitions as typed global elements with subelements referencing all corresponding property terms via their substitution group.
Classes SHOULD be used in a normalized way to avoid deep nesting. An [XML schema](tdwg_dwc_classes.xsd) is provided to freely mix any Darwin Core Class in a global list and allow them to reference each other using the respective class identifier terms.
Following is an example of using a normalized class-based schema to represent two related specimen dwc:Occurrences from one dwc:Event. In this example a Western garter snake collected by Gordon W Gullion in 1949 was found to have eaten a Coastal giant salamander. Note the reuse of class instances by referring to the identifiers declared in the instances of those classes:
Here is an example demonstrating area count observations for dwc:Events on two different days at the same dcterms:Location. Note that we omit the dwc:Identification class here as there is no identification-related data and link directly to the dwc:Taxon via the `dwc:taxonID`:
