This topic describes the manner in which ArcGIS for Desktop supports the ISO TC 211 standards associated with geospatial metadata. Whether you are new to metadata, new to ISO metadata standards, or experienced with ISO metadata, this topic will help you understand how ArcGIS for Desktop produces valid ISO metadata XML documents.
ISO metadata standards
ISO metadata is not a single standard. It is a constellation of intertwined standards that concern different areas of metadata content and the manner in which that information is stored. Before discussing the level of support provided in ArcGIS for Desktop for these standards, it is essential to be clear about the names and numbers of the different standards and the aspect of an item's metadata they control.
Content standards
Some ISO standards are content standards in the same manner that the FGDC's Content Standard for Digital Geospatial Metadata (CSDGM) is a content standard. A content standard determines what information should be provided in an item's metadata. They indicate, for example, that you should provide a title and date for an item and if that content is mandatory.
The following standards are the primary flavors of ISO metadata that are important to the geospatial data community. Each standard was developed independently, on their own time line.
- ISO 19115:2003 Geographic Information -- Metadata and ISO 19115:2006 Technical Corrigendum 1
- ISO 19119:2005 Geographic Information -- Services and ISO 19119:2008 Amendment 1
- ISO 19115-2:2009 Geographic Information -- Metadata -- Part 2: Extensions for imagery and gridded data
There are also many supporting metadata standards that address key portions of metadata content. These standards are also developed and maintained on a different time line from the primary metadata standards.
- ISO 19110:2005 Geographic Information -- Methodology for feature cataloguing
- Data quality standards
- ISO 19113:2002 Geographic Information -- Quality principles
- ISO 19114:2003 Geographic Information -- Quality evaluation procedures
- ISO 19138:2006 Geographic Information -- Data quality measures
If you are new to ISO metadata, but familiar with the FGDC CSDGM standard, here is how the above ISO metadata content standards relate to the different sections of the CSDGM.
Implementation specifications
The format in which metadata content is exchanged is determined by an implementation specification.
For FGDC CSDGM metadata, the metadata parser utility, mp, defines the supported formats and validates their content. XML, SGML, and structured text formats are all accepted as inputs by the mp utility.
For ISO metadata, each content standard is associated with an implementation specification—a document and a set of XML Schemas that describe the XML format of the content. XML is the only supported format. Some implementation specifications are published as separate standards with their own number. The XML Schemas can be used to validate an XML document to determine if it complies with the specification.
The content standards described above are associated with the following implementation specifications as illustrated in the diagrams below.
- ISO 19139:2007 Geographic Information -- Metadata -- XML Schema implementation
- XML format for ISO 19115:2003 and 19115:2006 Technical Corrigendum 1
- ISO 19136:2007 Geographic Information -- Geography Markup Language (GML)
- XML format for time and geometry information that are used in ISO metadata
-
OpenGIS Catalogue Service Implementation Specification[Catalogue Service for the Web (CSW)]
- XML format for ISO 19119:2008 Amendment 1
- While ISO 19119 is an ISO metadata content standard, and XML Schema to implement that standard were developed, the schemas were only published in connection with the OGC CSW specification.
- ISO 19110:2011 Geographic Information -- Methodology for feature cataloguing Amendment 1, Annex E
- XML format for ISO 19110:2005
- ISO 19139-2:2012 Geographic Information -- Metadata -- XML Schema implementation -- Part 2: Imagery and gridded data XML Schema implementation
- XML format for ISO 19115-2:2009
As you can see from the dates shown for the different content standards and implementation specifications, all standards are not all created and revised on the same time line. Therefore, the implementation specification for a given content standard can't include all of the content from all associated standards.
What's next for ISO metadata standards
ISO metadata standards are revised periodically to ensure they remain viable. The following metadata content standards are currently being revised. This work will be completed in the near future.
-
ISO 19110 Geographic Information -- Methodology for feature cataloguing
The existing feature cataloguing standard is undergoing a review as part of the normal maintenance for existing standards. The resulting updated model will be referenced by the revised metadata standard ISO 19115-1.
-
ISO 19157 Geographic Information -- Data quality
The data quality model that was produced by the three original standards was directly included in ISO 19115. These standards are being combined into one new standard, ISO 19157. The existing model will be revised as part of the normal maintenance for existing standards. The resulting updated model will be referenced by, but not directly included in, the revised metadata standard ISO 19115-1. When ISO 19157 is finalized, the previous three data quality standards will be withdrawn.
-
ISO 19115-1 Geographic Information -- Metadata -- Part 1: Fundamentals
This revision will make some important changes to the primary ISO metadata content standard and some new metadata elements will be added. Several modifications that were made by the North American Profile will be incorporated into this standard. It will also incorporate services metadata as defined in ISO 19119.
When revisions are completed for a content standard, its associated implementation specification must be updated accordingly. It is not clear at this time how the implementation specifications will be updated, particularly how ISO 19139 will be updated to incorporate the changes defined by ISO 19115-1.
-
ISO 19110 (2015?) Geographic Information -- Methodology for feature cataloguing, Annex E?
The existing implementation specification for ISO 19110 is included in the same document as the content standard, in Annex E that was added with the 2011 revision of that standard. When the revision is complete and the standard is updated, it is expected that Annex E or a similar section of the document will be updated accordingly and provide the revised implementation specification.
-
ISO 19157 (2014?) Geographic Information -- Data quality
The implementation specification for the previous three data quality standards—ISO 19113, ISO 19114, and ISO 19138—was included in ISO 19139. It is unclear if the implementation specification for ISO 19157 will be included as an annex in that standard's document in the same manner as ISO 19110, if it will be provided as a separate implementation specification with its own number such as ISO 19157-2, or if it will be included in the new implementation specification for ISO 19115-1.
-
ISO 19115-3 (2014?) Geographic Information -- Metadata -- Part 3: XML Schema implementation of metadata fundamentals
It is proposed that the implementation specification for ISO 19115-1 should be provided as the standard ISO 19115-3.
Many standards associated with 19115-1 are also being revised but are not on the same time line. The actual set of information that will be provided with ISO 19115-1 is unclear at this time.
One plan is to include ISO 19115-1 and all associated implementation specifications in a single standard, ISO 19115-3, in the same manner as they were included in the past with ISO 19139. However, after ISO 19139 was published, it was not updated to include updates that were made to the related standards. If this path is pursued, it is unclear if ISO 19115-3 would wait for the associated standards to be finalized, or if it would be released using the existing implementation specifications for the related standards. It is also unclear if this new implementation specification would include the information models provided by ISO 19139-2.
Two possible outcomes are illustrated below.
One problem with including the existing implementation specifications concerns ISO 19139-2 and if it must be updated to include all of the revisions associated with ISO 19115-1 before it can be included in an implementation specification such as ISO 19115-3. Another plan would be to create ISO 19115-3 as an independent implementation specification that only includes the updates defined in ISO 19115-1, and references all related specifications, as illustrated below.
Standards supported by ArcGIS for Desktop
The metadata created and exported when you use the ISO 19139 Metadata Implementation Specification metadata style in ArcGIS for Desktop is valid according to the content standard ISO 19115:2003 with the 2006 Technical Corrigendum and the implementation specification ISO 19139:2007.
When you use the INSPIRE Metadata Directive metadata style or the North American Profile of ISO19115 2003 metadata style in ArcGIS for Desktop, the metadata created is valid according to the content standard ISO 19115:2003 with the 2006 Technical Corrigendum. For the INSPIRE style, the metadata content is also valid according to the European community's INSPIRE Metadata Implementing Rules: Technical Guidelines based on EN ISO 19115 and EN ISO 19119 v1.2. For the North American Profile style, the metadata content is also valid according to the North American Profile of ISO 19115:2003 - Geographic information - Metadata (NAP - Metadata, version 1.2).
When you use the INSPIRE Metadata Directive metadata style or the North American Profile of ISO19115 2003 metadata style and metadata describing data is exported, the resulting XML file is valid according to the implementation specification ISO 19139:2007. When metadata describing a service is exported, it is valid according to the implementation specification OGC Catalogue Service for the Web (CSW); this specification includes XML schemas that can validate an ISO 19139 metadata document that includes service metadata descriptions.
The system used by ArcGIS 10.x to store content in the internal ArcGIS metadata format and export content to the appropriate XML format for the current ArcGIS metadata style using XSLT transformations is a flexible one that can readily adapt as new metadata standards become available. As the metadata content standards change, new metadata elements and pages for editing can be added to the ArcGIS metadata editor to collect more content. New metadata styles can leverage the new pages and use new XSLT transformations to produce XML files that satisfy the new implementation specifications.
Esri personnel have participated in revisions to the ISO metadata standards described above and are closely monitoring changes to them. The revised standards will be supported in an ArcGIS for Desktop release after they are finalized. It is important to remember that implementation specifications are not typically available at the same time as the content standards. It is not possible to provide a complete metadata style for a new or revised standard until the final implementation specification is available. When a new metadata style is available, you can set ArcGIS for Desktop to use this new style; your existing ArcGIS metadata content remains the same.
Validating ISO metadata documents
Unfortunately, the mere existence of an implementation specification as a standard does not mean it is straightforward to validate XML documents that correspond to those specifications. This section will examine some of the anatomy of an XML document, how to validate an XML file, how the XML Schemas are published, and what it means to validate ISO metadata XML documents.
XML introduction
Extensible markup language (XML) was designed to be a flexible mechanism for storing and transporting data. In contrast, HTML was designed to display data. Neither XML nor HTML do anything. XML merely wraps data in tags. HTML wraps data in a set of predefined tags that browsers know how to display. An extensible stylesheet language transformations (XSLT) style sheet provides instructions for transforming the data in an XML document into something else such as an HTML document that can be displayed by a browser.
All XML documents must follow a basic set of rules—they must be well-formed. A well-formed XML document has correct XML syntax. For example, an XML document must have a root element, tags must be closed and properly nested, and attribute values must be quoted.
An XML document can optionally be valid. A valid XML document is a well-formed XML document that also conforms to the rules of a schema. A schema defines the structure of an XML document. For example, it can specify which elements are permitted to exist in the document, which elements can contain which other elements, which elements contain what type of data, which elements are mandatory, and the order in which elements must appear. An XML schema may be an XML Document Type Definition (DTD) or an XML Schema (XSD). An ISO metadata implementation specification provides a set of XML Schema (XSD) documents.
Some characters have special meaning in an XML document. For example, the greater-than (>) and less-than (<) characters are used to place tags around some text and create an XML element. If an element's text includes the plain greater-than character, this causes an error because this is interpreted as the start of a new XML element's tag. The presence of an ampersand (&) in an element's text also causes an error because the ampersand is used to denote special character references. All of the special characters described here, including the apostrophe (') and quotation mark ("), should be replaced by the appropriate entity reference: > for greater-than, < for less-than, & for an ampersand, ' for an apostrophe, and " for a quotation mark.
An XML document may have an XML declaration in the first line that defines the version of XML and the encoding used in the document. If a declaration is not present, the default XML version is 1.0. If an encoding isn't specified, it is assumed to be UTF-8 or UTF-16 as determined by the XML document's byte order mark. The ArcGIS for Desktop metadata editor and the metadata geoprocessing tools always produce a UTF-8-encoded XML document.
To learn more about XML and the syntax of an XML document, see http://www.w3schools.com/xml.
XML namespaces
Two XML Schemas may define elements with the same name but a different meaning. XML namespaces are used to avoid element naming conflicts when elements from different XML Schemas appear in the same XML document, where a different namespace is associated with each XML Schema.
A namespace is a prefix to an XML element's name, separated from the name by a colon such as <prefix:name>. A namespace is defined by providing an xmlns attribute and associating a uniform resource identifier (URI) with a string that will be used as the prefix such as xmlns:prefix=URI. Namespaces must be declared in the opening tag of an XML element where the prefix will be used. However, all namespaces used in a document are typically declared in the opening tag for the root element of the document. In the example below, two namespaces, a and b, are defined to distinguish between internal and external information:
<a:order xmlns:a="http://store.com/inventory" xmlns:b="http://store.com/customers">
<a:name>patio table</a:name>
<a:cost>650.00</a:cost>
<a:email>shipping@store.com</a:email>
<b:name>Jane Smith</b:name>
<b:email>customer@gmail.com</b:email>
</a:order>
A namespace's URI is not expected to be a valid Internet location. Even if a URI looks like a uniform resource locator (URL), its only purpose is to identify a set of XML element names.
If there is one namespace that is used more than the other namespaces, you can specify a default namespace for an XML document such as xmlns=URI. Specifying a default saves you from having to provide the same prefix for all elements associated with that namespace. Any XML element in the document that does not have a prefix is a member of the default namespace. The above example is repeated below, but this time with the URI for namespace "a" assigned as the default namespace:
<order xmlns="http://store.com/inventory" xmlns:b="http://store.com/customers">
<name>patio table</name>
<cost>650.00</cost>
<email>shipping@store.com</email>
<b:name>Jane Smith</b:name>
<b:email>customer@gmail.com</b:email>
</order>
In both examples, the XML element cost is associated with the namespace URI http://store.com/inventory. If you have an XSLT style sheet that transforms orders specified in this format into a document that can be printed, the same XSLT would process both of the above examples in the same manner and produce the same result. If the XML Schemas that define both sets of elements are identified, the above XML document examples could be validated—both of the above examples would be considered valid.
In the ISO metadata implementation specifications, each set of XML Schemas is associated with a three-letter string and a specific namespace URI. By convention, the three-letter string is always used as the prefix for XML elements defined in the associated XML Schemas. For example, most of the metadata elements defined in ISO 19115 are associated with the namespace URI http://www.isotc211.org/2005/gmd, as specified in the ISO 19139 document; these elements would use the prefix gmd if a default namespace is not specified in an XML document for this URI.
ISO metadata implementation specifications provide many XML Schemas and, therefore, define many namespaces and prefixes that could be used in an XML document. It is only necessary to include namespace and prefix declarations for elements that actually appear in an XML document.
Validating an XML document
An XML document must be well-formed. An XML document can optionally be valid. A valid XML document is a well-formed XML document that also conforms to the rules of a schema. A schema reference must be added to an XML document to validate it.
A reference to an XML DTD can be added following the XML declaration as illustrated below:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE order SYSTEM "http://store.com/order-format.dtd">
<order xmlns="http://store.com/inventory" xmlns:b="http://store.com/customers">
…
</order>
The XML document's root element is declared, followed by the location where the DTD is stored.
An XML Schema reference is added using attributes in the opening tag for the root element of the document, as illustrated below:
<?xml version="1.0" encoding="UTF-8"?>
<order xmlns="http://store.com/inventory" xmlns:b="http://store.com/customers"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://store.com/order-format.xsd">
…
</order>
First, the XMLSchema-instance namespace is declared. Then, the schemaLocation attribute specifies where the XML Schema is stored.
Various software tools are required to process XML documents. An XSLT engine or processor is required to perform the transformation defined in an XSLT style sheet. An XML parser is used to add, delete, read elements and attributes and their values, and perform various other operations on the structure of an XML document. A non-validating XML parser can check to make sure an XML document is well-formed. In addition to the other operations, a validating XML parser can also use a referenced XML schema to determine if the XML document is valid according to the rules specified in that schema.
Once an XML schema reference has been included in an XML document, a validating XML parser can check to see if the document is valid.
XML Schemas can be written to account for the majority of the rules defined in metadata content standards. However, they are not capable of assessing all of the rules defined in the ISO metadata content standard UML models. For example, with an XML Schema, an element's value can't be controlled by a sibling element's value.
Schematron is a rule-based language that can be used to assess an XML document using XPath queries and present plain-language errors if problems are found. Schematron can be used to complement XML schema validation to determine if rules defined in a content standard, but cannot be assessed by the schema, have been followed in an XML document. Because Schematrons are XML documents, they are typically transformed into an XSLT transformation; the resulting XSLT can then be used to validate an XML document using an XSLT processor. ISO metadata implementation specifications are not accompanied by Schematrons at this time; however, future specifications are likely to incorporate these capabilities.
ArcGIS for Desktop uses the Microsoft .NET Framework XML parser and XSLT processor to handle metadata. This technology exclusively supports XSLT 1.0. Therefore, only XSLT style sheets based on XSLT 1.0 can be run using ArcGIS for Desktopsoftware. Schematrons created based on XSLT 2.0 cannot be transformed into an XSLT style sheet that will run in ArcGIS for Desktop.
Validating metadata using ArcGIS for Desktop
The ArcGIS for Desktop metadata editor validates metadata internally by testing the metadata content managed by a specific page. The tests are based on the rules specified by the content standard and implementation specification associated with the current ArcGIS metadata style. All of the content standard's rules are accounted for, even the rules beyond those that can be tested in the implementation specification's XML schema. The metadata editor's table of contents quickly illustrates which pages have content that would be considered invalid for the metadata style.
It takes time to author good metadata content. The ArcGIS metadata editor allows you to save your content as is and come back later to finish it, even if your metadata is considered invalid at the time that you need to stop working on it.
ArcGIS for Desktop supports a wide range of customers with different requirements. Most organizations prefer to create metadata following a community standard. However, some organizations have different internal requirements and their metadata documents would be considered invalid according to a metadata standard's XML schema. ArcGIS allows these organizations to produce metadata according to their own requirements. As a result, when an item's metadata content is exported to a standard's XML format, the XML file produced will not include an XML schema reference.
When an item's metadata content is complete, you can optionally test to see if it is valid according to the XML schema for the metadata standard associated with the metadata style. This can be accomplished using the Validate button in the Description tab, which runs the Validate Metadata geoprocessing tool. The tool parameters are set appropriately for the current metadata style when its dialog box opens. The settings associated with the ISO 19139 metadata style are illustrated below.
This tool first exports the item's ArcGIS metadata content to a metadata standard's XML format—in this example, the ISO 19139 XML format. Then an XML schema reference is inserted into the exported XML document based on the information provided in the tool's parameters. The example below shows how the xsi:schemaLocation attribute is produced based on the information provided in the example above.
<?xml version="1.0" encoding="UTF-8"?>
<MD_Metadata xmlns="http://www.isotc211.org/2005/gmd"
xmlns:gco="http://www.isotc211.org/2005/gco"
xmlns:gts="http://www.isotc211.org/2005/gts"
xmlns:srv="http://www.isotc211.org/2005/srv"
xmlns:gml="http://www.opengis.net/gml"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.isotc211.org/2005/gmd
http://www.isotc211.org/schemas/2005/metadataEntity.xsd">
…
</MD_Metadata>
If appropriate, the target namespace must be declared in the xsi:schemaLocation attribute to correspond with the XML Schema's target namespace. The target namespace indicates which elements in the XML document conform to the specified XML Schema and will be validated. Other XML Schemas, such as the FGDC CSDGM schemas, may not require a target namespace to be specified.
Finally, the exported XML file with the xsi:schemaLocation attribute is validated using the .NET Framework XML parser. Any warning or error messages provided by the .NET Framework are reported in the tool's messages. ArcGIS for Desktop reports the error messages exactly as they are produced.
ISO metadata XML Schema repositories
There are many official locations or repositories where different ISO metadata implementation specification XML schemas have been published. All specifications have not been published in all places. This makes it difficult to determine which schemas should be used to validate an XML document that is said to comply with a specific standard. This dilemma is described in the INSPIRE Metadata Implementing Rules v1.2 document in section 2.1.2, "ISO Schemas Location".
During the time line over which different implementation specifications have been developed, GML has evolved from an OGC standard to an ISO standard. GML content is only used in ISO metadata XML documents to record times and geometric shapes as needed. The older version of GML was originally used in ISO metadata XML Schemas, and one namespace was used in the GML and ISO metadata XML Schemas for this version. The newer version of GML has a different namespace.
While the changes to the GML standard make little or no difference in the metadata XML documents produced according to ISO metadata content standards, the change to the GML namespace complicates the manner in which these XML documents are validated. An XML Schema specifies the target namespaces that will be validated. If an XML document uses namespace G1 and an XML Schema targets namespace G1 for validation, the document will validate if its content and structure are correct. If the XML document uses namespace G1 and an XML Schema targets namespace G2, the document will not validate even if the content and structure of the document are correct. Typically, if the XML document's namespace is altered from G1 to G2, the document will then validate successfully as long as it uses content that is the same in both G1 and G2.
The different ISO metadata XML Schemas published in different locations all reference different versions of the GML XML Schemas and, therefore, target different GML namespaces. When considering which XML Schemas to use for validation, an important factor is which GML namespace that XML Schema targets. Below is a list of the different repositories where ISO metadata XML Schemas can be found; the GML namespace they use is also indicated.
-
http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_19139_Schemas/
http://standards.iso.org/ittf/PubliclyAvailableStandards/is the ISO repository for all publicly available standards. This is the schema location mentioned in the ISO 19139 standard document as the official set of schemas. These schemas were updated to use the new GML 3.2.1 namespace: http://www.opengis.net/gml/3.2. They refer directly to the ISO 19136 version of the GML 3.2.1 schemas available from this same ISO schema repository.
This location does not include XML schemas for other published ISO metadata standards such as ISO 19119, ISO 19110, or ISO 19139-2. The only other TC 211 XML Schemas published to this location are for ISO 19135-2 Geographic information - Procedures for item registration -- Part 2 XML schema implementation. Therefore, only an XML document that includes ISO 19115 metadata content and uses the new GML namespace can be validated with these XML Schemas.
-
http://schemas.opengis.net/iso/19139/20070417/
This is one of the locations where the ISO 19139 schemas are available from the OGC Schema repository. These were updated to use the new GML 3.2.1 namespace. These are the same XML Schemas that are published to repository #1; however, they refer directly to the GML 3.2.1 schemas available from this same OGC schema repository.
This location does not include XML schemas for any other published ISO metadata standards such as ISO 19119, ISO 19110, or ISO 19139-2. Therefore, only an XML document that includes ISO 19115 metadata content and uses the new GML namespace can be validated with these XML Schemas.
-
http://schemas.opengis.net/iso/19139/20060504/
This is another location where the ISO 19139 schemas are available from the OGC Schema repository. These schemas use the older GML 3.2.0 namespace: http://www.opengis.net/gml. These are the same XML Schemas published to repositories #4, #5, and #6. These schemas reference a copy of the older GML XML Schemas that are provided in this same location.
This location also includes XML Schemas for ISO 19119. Therefore, an XML document that includes both ISO 19115 and ISO 19119 metadata content and uses the older GML namespace can be validated with these XML Schemas.
-
http://schemas.opengis.net/csw/2.0.2/profiles/apiso/
This is another location where the ISO 19139 schemas are available from the OGC Schema repository. These schemas use the older GML 3.2.0 namespace. These are the same ISO 19139 schemas that are published to repositories #3, #5, and #6. These schemas reference a copy of the older GML XML Schemas that are provided in this same location.
This location also includes XML Schemas for ISO 19119. Therefore, an XML document that includes both ISO 19115 and ISO 19119 metadata content and uses the older GML namespace can be validated with these XML Schemas.
This repository is associated with the OGC Catalogue Service for the Web (CSW) specification, which provides a mechanism for querying and providing ISO metadata documents. Any metadata catalog that implements the CSW protocol is based on XML schemas that use the older GML 3.2.0 namespace.
-
http://www.isotc211.org/schemas/2005/
This is the original location where the ISO 19139 XML Schemas were published before they were available from the official ISO repository described in #1. The HTML page provided at this location has links to ISO metadata XML Schemas that take you to repository #6, not repository #1. While you do not see a directory of XML schemas in this location as for the other repository locations, the original XML schemas provided here remain available if you provide the full path to the XML Schema files.
These schemas use the older GML 3.2.0 namespace. These are the same ISO 19139 schemas published to repositories #3, #4, and #6. These schemas reference a copy of the older GML XML Schemas provided in this same location.
This location does not include XML schemas for any other published ISO metadata standards such as ISO 19119, ISO 19110, or ISO 19139-2. Therefore, only an XML document that includes ISO 19115 metadata content and uses the new GML namespace can be validated with these XML Schemas.
-
This is another location where the ISO metadata XML Schemas are available from the TC 211 committees. The ISO 19139 XML Schemas were published here when it was thought to be a good idea to make the schemas available at the same URL as the namespaces associated with ISO 19139. This location has become a repository where XML Schemas are posted while they are works in progress. However, the HTML page available from the URL provided as repository #5 directs people to this location for some ISO metadata XML Schemas instead of to repository #1.
The ISO 19139 XML schemas available in this location use the older GML 3.2.0 namespace. These are the same ISO 19139 schemas published to repositories #3, #4, and #5. These schemas reference a copy of the older GML XML Schemas that are provided in this same location.
This repository also provides XML Schemas for other ISO metadata standards. It includes three different versions of the ISO 19110 XML Schemas: the XML schemas for ISO 19139-2, the XML schemas for ISO 19135-2, and XML schemas to support ISO 19145 Geographic information -- Registry of representations of geographic point location. It does not include the XML Schemas to support ISO 19119. The ISO 19110 XML Schemas also use the older GML namespace. The ISO 19139-2 XML Schemas reference the version of the ISO 19139 XML Schemas published in repository #1 and use the newer GML 3.2.1 namespace.
Given the wide variety of choice, it is far from clear which official set of XML schemas should be used to validate metadata.
Many international ISO metadata profiles associate their content with different repositories listed above. The only real difference between them is the version of the GML namespace they use. The actual content of the schemas are identical in every other respect. The INSPIRE document suggests that you should validate your metadata using the set of schemas based on the specific version of GML appropriate for your situation. This holistic approach to metadata validation makes a lot of sense.
A TC 211 XML Management Committee has been formed to try to resolve the consistency problems with the different XML schema repositories outlined above. The committee wants to create a new repository that will become the official XML Schema repository for all ISO TC 211 standards and remove the existing XML schemas from the other repositories when the new site is created. However, the XML Schemas provided from the OGC repositories will likely remain. The new repository does not exist yet.
XML Schemas and ArcGIS for Desktop
The ArcGIS for Desktop metadata system was designed to validate metadata using official schemas hosted on the web by standards organizations, because those are the official schemas. Schemas are not packaged into and distributed with the software because they would not be the official schemas.
When the ISO 19139 Metadata Implementation Specification metadata style is used, ArcGIS for Desktop uses the XML Schemas from repository #5 to validate ISO 19139 metadata documents. This repository was selected because it was the first one that provided access to a stable set of official XML Schemas, it predated the repository described in #1, and because these schemas are associated with the CSW standard. Because ArcGIS for Desktop has been using the XML schemas from repository #5 to validate metadata, ArcGIS for Desktop customers have been able to create ISO 19139-compliant metadata in a stable environment. Metadata exported with this style uses the older GML 3.2.0 namespace—http://www.opengis.net/gml—and will validate successfully using any of the ISO 19139 XML Schemas that use the GML 3.2.0 namespace (#3, #4, #5, #6).
Beginning with ArcGIS 10.2.1 for Desktop, when the ISO 19139 Metadata Implementation Specification GML 3.2 metadata style is used, the XML Schemas from repository #1 are used to validate ISO 19139 metadata documents. Use this metadata style instead if it is important to export metadata that uses the GML 3.2 namespace, and validate metadata using this alternate set of XML Schemas. Metadata exported with this style uses the newer GML 3.2.1 namespace—http://www.opengis.net/gml/3.2— and will validate successfully using any of the ISO 19139 XML Schemas that use the GML 3.2.1 namespace (#1, #2).
When the Validate Metadata tool's dialog box opens, you can change the settings it uses to validate an item's metadata and use a different set of XML Schemas instead of the schemas that are associated with the current metadata style. You can also make this change permanent by creating a custom metadata style that always uses your preferred settings. For example, you could validate metadata using a set of XML Schemas that is available on your organization's internal network instead of the schemas provided on the internet by the TC 211 committee. Information about how to create a custom metadata style is provided with the ArcGIS Metadata Toolkit, which can be downloaded from the Esri support site.
If you choose to validate the XML documents exported by ArcGIS for Desktop using the wrong set of ISO 19139 XML Schemas, validation will fail. If you change the GML namespace declaration in the exported XML documents by adding or removing /3.2 to or from the end of the declaration, the files will validate successfully with the other set of XML Schemas.
ArcGIS for Desktop will be updated to export ISO 19139 XML documents for a different repository of XML Schemas and validate metadata with those schemas when the TC 211 XML Management Committee creates a new, stable repository that includes all XML schemas. The new repository should provide access to all of the ISO metadata implementation specifications in one, reliable location.
A final word about XLinks
XLink is a W3C recommendation that supports querying and referencing portions of an XML document. The XLink XML Schemas have been incorporated into ISO TC 211 standards. However, they have never been widely adopted by the software community as a whole. Internet browsing applications do not support them natively. No XML software produced by Microsoft supports it except in the manner that it supports any XML Schema.
While XLinks are permitted by the ISO 19139 XML schemas, they are entirely optional. Therefore, there is no requirement to use XLinks in an ISO 19139-compliant XML file.
XLinks are valuable when people interact with an XML document because it is easy for people to interpret and understand the meaning of an association between two sections of an XML document. If you create XML documents from scratch by typing in XML data, XLinks can be very useful for saving time, improving maintenance costs, and preventing basic data entry errors.
ArcGIS for Desktop uses a different method to resolve these same metadata maintenance problems. Sections of a metadata document that are reused many times are saved into a separate XML file. These XML fragments can be loaded into an item's metadata at a later time when they are needed. This capability effectively offers the same benefit as XLinks, though in a different manner.
When metadata is exported to the ISO 19139 format, a complete ISO 19139 document is always generated. This result is by design—it ensures a complete metadata document is available and can be read and validated unambiguously outside of the organization without relying on any specialized software. The ISO 19139 metadata XML files exported from ArcGIS for Desktop use XLinks sparingly, only when necessary, and when this is the best method for handling content.