...
Because ISO19115 originated in a community that was specifically interested in spatial data (especially data acquired by remote sensing) a key content area is documentation of the location relative to the Earth (geospatial location) that the information in a resource is about, and how the information in the resource content is mapped to location relative to the Earth.
The model allows spatial and temporal extents to be documented independently, or coupled as a spatial-temporal extent for subject regions that are in different geospatial locations at different times, or has one or more sampling extents that were sampled at different (possibly multiple) times.
Spatial extent
Properties that specify the geospatial location. There are options to specify horizontal extent via a bounding box using a standard spatial reference system (WGS84 latitude and longitude), using a place name or identifier, or with a general geographic coordinate geometry. Vertical extent can be documented separately with a minimum and maximum depth or elevation defined in a specified spatial reference system.
The bounding box is far and away the most common method used, but has various problems. Many client applications have difficulty handling bounding boxes that cross the 180 degree E. longitude; the box could be separated into multiple boxes, but many clients only index the first bounding box provided. In many situations, a dataset may contain information about widely separated sampling sites, e.g. hot springs in New Zealand and California, or ecologic observations in coastal areas of Virginia, Hawaii, and Tanzania. These lead to unrealistic bounding boxes and irrelevant spatial search results.
The use of the generalized geometry model from the Geography Markup Language allows representation of virtually any kind of coordinate-referenced geospatial location, with multiple points, lines, polygons, multi part geometries, polygons with holes, etc., but again, there are no search clients I know of that will parse complex geometries and use for search.
These major application problems are not inherent in the ISO 19115-1 model, but rather in simplifications that are typically made in client applications accustomed to the simple one bounding box (or one centroid point…) approach.
Spatial representation
This section provides elements to document how the mapping between data and location is implemented in the dataset. It accounts for vector representations, grids, and georeferenced images.
Spatial Reference System
This section provides elements to document how numeric coordinate values are registered to Earth positions, accounting for datum and map projection.
Spatial resolution
Documentation of the smallest distance between locations that can be distinguished in the spatial data.
Portrayal catalog
This section provides a citation to a separate document that provides a description of how spatial data can be visualized, assigning e.g. colors to polygons that have particular attribute values, or defining color ramps to use for visualizing image data.
Temporal aspects
These are properties for documented the time interval during which the data were collected or to which the data are considered relevant. For some thematic domains, e.g. geology, history, or archeology, this might be some time in the past.
Temporal extent
The ISO19115-1 model uses the temporal model from another ISO standard (19108), and allows a variety of options for represent temporal extent. The basic concepts are time instants and time intervals. The bounds of intervals are instants. The temporal position of an instant can be a time coordinate (defined in some temporal reference system), a calendar date time (defined in some calendar system), or a named time ordinal era from an ordinal reference system. There are obviously a lot of details that must be considered, see OWL time for a good reading starting point. The most commonly used implementation is an ISO 8601 date or datetime, or time interval bounded by ISO8601 date or datetime.
Temporal resolution
This is the smallest interval of time that can be distinguished in the temporal fields in the dataset.
Usage, security and legal constraints
Documentation for limitations on use, distribution, reproduction, etc are represented as access or use constraints categorized by a controlled vocabulary with values like copyright, patent, license, private, unrestricted…, or as security constraints categorized by a controlled vocabulary with values like unclassified, restricted, confidential, secret, protected…. Security constraints Any of these constraints can be accompanied by a text statement, and scoped to a specific part of the described resource ('constraintApplicationScope'), or to specific parties ('releasability'). Applicable licenses are specified using citations.
Resource Content/function
The original ISO19115 specification was strongly oriented towards image or raster based remote sensing data, and the Content Information section included in the model reflects that interest. Documentation of entity and attribute information is sometimes mapped into the content information section, but the intentions of the committees designing the specifications was that description of relational or object oriented data structures would use a separate specification, ISO19110 'Feature Catalogue'. Unfortunately, there was not a normative XML schema to implement the ISO19110 conceptual model until recently (after the release of ISO19115-1), making its use ad hoc.
Content information
Provides elements to describe a dataset represented as a coverage—a set of attributes on one or more variable dimensions; this approach is based on documenting gridded datasets like remote sensing data acquired from a satellite. The diagram below shows the content information elements implemented in ISO19115-3, which includes ISO19115-1 and ISO19115-2 elements.
Feature catalog
A feature catalog(ue) (ISO19110) represents datasets as a collection of 'features', each of which has one or more attributes. This approach maps nicely to entity/attribute data structures typical of relational databases, or to object-oriented data structures. The UML diagram below shows the feature catalogue model from ISO19110.
Application schema
The ISO 19115-3 implementation allows a citation to one or more documents describing the application schema for the resource. The scope note for Application Schema link object (MD_ApplicationSchema) reads "the application schema used to define and expose the structure of a resource, i.e. the model and/or data dictionary that represents the resource.", which can be interpreted as a document that describes a data structure or the operation of an application. In practice this element is rarely used in my experience. The application schema element is a property of the base MD_Metadata element; multiple values are allowed, but the model does not include a binding between and application scheme and a particular distribution.
Services
The service metadata (SV_ServiceIdentification) section is also used in some profiles to document service interfaces implemented to access one or more datasets. There is opportunity for ambiguity and confusion here as to whether the metadata record is about a service as a set of operations accessible via some invocation process, or whether the metadata record is about a dataset and the service provides one means of accessing the dataset (which seems to be the more common situation). The USGIN and INSPIRE metadata profiles recommend using MD_DistributionInformation for service distributions of a specific dataset in metadata records that are about that dataset.
Resource Distribution
Distribution section provides information on how to access a digital representation of the resource described by the metadata record. The ISO19115 model is structured to allow multiple distributions, with multiple distributors, formats, and transfer options associated with each distribution. Transfer options and formats can be bound to a particular distributor or not. The model is quite flexible, but this leads to multiple approaches to documenting distributions, see Resource Access links in the Challenges section, below.
Distributor
Identifies a party responsible for maintaining a distribution process via a CI_ResponsibleParty component (see Responsible party in the Components section, below), and information about any ordering process associated with the distributor, including fees, available date, instructions, expected processing turnaround time, and an extension point for specifying any other applicable options
Format
Provides a citation to a format specification, information about the physical medium used for distribution, and a free text specification of any file compression used.
Transfer options
Documentation for a particular access option, typically characterized by a specific linkage (URL), but might also document details of an offline distribution. Optional properties include the unit of distribution (free text), transfer size (real number), formats accessible at the URL (not in ISO19115(2006)), and the rate of occurrence of distribution. Multiple URLs and formats can be associated with each option. Properties that can be associated with each linkage (URL) are
- Name – a label used to present the link in user interfaces.
- Description – text describing what the link does
- Protocol – online message protocol to be used. Documentation provides examples http, ftp, file, http GET KVP, http POST.
- Application profile – free text specification of an application profile associated with the link.
- Function – controlled vocabulary to specify the function invoked by the link
- Protocol request (new in 19115-1). Documentation is unclear; free text to provide example http POST request content.
Technically, the linkage is not required to be a URL (http URI), but could be any locator string with a known dereferencing mechanism.
Lineage
Resource provenance is recorded by a collection of processing steps, each of which can have one or more associated sources (inputs), responsible parties, citations for related resources, a time stamp, a scope (resource part, space or time), and a description and rationale free text. Process steps can be ordered sequentially using the time stamps. ISO19115-2 adds additional elements; these are all implemented in ISO19115-3 XML, and are shown in the UML diagram below.
Usage
A collection of reports on usage of the resource can be included in a metadata record. These usage reports can include a time stamp, a responsible party for the usage report, various text fields documenting the usage, and links to citations with related information. The Usage element is shown in the UML diagram in the Basic Resource Documentation section, above.
Maintenance
A collection of reports about maintenance of the metadata record or the described resource can be included, each with documentation about the frequency of updates, data of last maintenance, scope of maintenance, responsible party for maintenance, and a text description of the maintenance activity. The Maintenance element is shown in the UML diagram in the Basic Resource Documentation section, above.
Related resources
Related resources are linked via a citation to a metadata record for the linked resource, or a citation to the linked resource itself. Each related resource instance has an association type from a controlled vocabulary with values like crossReference, source, largerWorkCitation, dependency, isComposedOf. An optional initiative type can be specified, with values from a controlled vocabulary that includes terms like campaign, collection, exercise, mission, sensor, platform, program, study, trial. Based on the values in this vocabulary, the initiative type can be used to categorize the kind of linked resource. The AssociatedResource element is shown in the UML diagram in the Basic Resource Documentation section, above. Note that in ISO19115(2006) this element is named MD_AggregateInformation.
Data Quality
Data Quality characterization was included in the 19115(2006) model, but in the 19115-1 model, the data quality section has been moved to a separate ISO specification ISO19157. The model for documenting data quality is very similar in both cases, with only minor modifications and additions in ISO19157. The data quality documentation consists of a set of reports. The reports are hard typed, with 16 types defined, as well as a stand alone quality report option that is a citation to an external document. The report types are grouped in the following way: Usability, logical consistency, temporal quality, thematic accuracy, and completeness. Each report includes one or more result values that can be a simple 'pass/no pass', a quantitative value, a coverage (ISO19157 amendment 1), or descriptive text; result values can have a time stamp and a scope. Optional properties include specification of the evaluation method, an identifier and name for the measurement made, and elements to characterize the confidence, representativity, and homogeneity of the reported quality.
Components:
There are several data objects that are used as the value type for elements in multiple parts of the model, and in some cases could also be used in multiple metadata records. Instances of these could be stored in a registry and referenced from multiple metadata records to improve efficiency and simplify maintenance of commonly used information.
Responsible party
CI_Party represents individuals (name or role) or organizations, and affiliations between individuals and organizations. A party is bound to a role in various 'responsibilities' relative to a described resource. This allows the same person or organization be play multiple roles like author, maintainer, publisher, owner, distributor, editor etc. (CI_RoleCode vocabulary). Contact information including telephone number, postal address, and e-mail address are linked to parties. ISO19115-1 Amendment 1 adds an identifier property for each party so that URI's can be associated with people and organizations. It also adds an extent (spatial or temporal) property on a responsibility assignment that can be used, for instance, to document areas that were mapped by different people in a geologic map dataset.
Responsible party is used in various contexts; various roles might be assigned to the party in any given context.
- Metadata point of contact
- Resource point of contact
- Cited responsible party
- Distribution contact
- Usage documentation
- Maintenance contact
Processing Step
Processing step descriptions (see Lineage, above) could be reused across multiple lineage descriptions. Verbatim reuse would preclude using the step time stamp to order steps in a particular process instance.
Citation
Citations are used in two roles:
- to provide the standard bibliographic citation content for the described resource
- to provide links or references to other resources in various roles, including additional documentation, sources that contributed data to create the resource, related resources, thesauri used for keyword vocabularies.
Citations in the second role might be used in multiple metadata records. The CI_Citation element is shown in the UML diagram with Responsible Party, above.
Extent
Named extent objects might be used in multiple metadata records. Extent objects can include one or more geographic, vertical or temporal elements. Geographic elements can be a bounding box specified by two corner point coordinates, a generic GML geometry, or an identifier object. Vertical elements have a lower an upper bounding value, and an optional specified reference system. Temporal elements can be specified by a time ordinal era (e.g. Jurassic, or Reign of Henry VIII), a calendar date and time, or a time interval specified by a begin and end calendar date time. Specific geographic elements can be bound to specific temporal elements.
Challenges:
The ISO metadata model is comprehensive and flexible at the cost of some interoperability problems introduced by allowing multiple approaches to representing the same information. There are a few concepts that are not well accounted for by the model as well. This section discusses a few of those issues.
xlink:href as Identifiers
The ISO xml implementation allows substitution of an 'anchor' element for any characterString value in an instance document. The anchor element adds the xlink attributes, which are not included in the characterString implementation. Some data providers use these xlinks to specify URIs corresponding to the provided string value, particularly for keywords, individual names, and organization names. This works well in the context of a community profile, but is not a general solution documented in the specification. ISO19115-1 fixes this problem for individual and organization.
Resource categorization
The 2006 version of ISO19115 categorized resources with a 'hierarchyLevel' property; in the 19115-1 version, this has become 'resourceScope', but both properties are populated by a controlled vocabulary with values like software, dataset, service, model, collection (the 19115-1 vocabulary has many useful additions over the original version). Both models also allow a free text label that is related to the resource type, 'hierarchyLevelName' in the case of 19115 (2006), or metadataScope/name in the case of 19115-1. For many purposes this resource categorization is too restrictive, and the use of the scope properties is inconsistent because of varying interpretation of the intention.
Related resources
Mix of hard typing (e.g. for Additional documentation or Browse graphic) and soft typing (MD_AssociatedResource/associationType) for linking to other resources. Semantics of associations are limited by a controlled vocabulary with values like crossReference, largerWorkCitation, source, dependency, revisionOf. Sources for resources that are aggregations (compilation, syntheses, reviews…) can be documented through links in the lineage (provenance) or as related resources. The URLs and URIs for linked resources are buried in nested citation elements.
Entity and attribute information
Content information, feature catalog (ISO19110), or application schema (a linked document) all are possible approaches to describing the structure of a described data set. The Content Information approach is probably the most widely used at this point because it was implemented in the ISO19139 XML implementation of ISO19115(2006)
Resource Access links
There are multiple options for documenting online linkage to a resource, including operation/ onlineResource, distribution/transferOption/onlineResource, distribution/distributor/ distributorTransferOption/onlineResource or identificationInformation/citation/onlineResource (note these are conceptual paths, not full xml xpaths).
Spatial Extent
Extent geometry can be represented as bounding box or generic GML geometry objects. The generic GML geometry allows practically any kind of coordinate description of location, ranging from a point to multipart polygons with holes. Most client applications use the bounding box, and although the model allows multiple geometry elements, most client applications only index the first geometry providedExtent objects can include one or more geographic, vertical or temporal elements. Geographic elements can be a bounding box specified by two corner point coordinates using a standard spatial reference system (WGS84 latitude and longitude), a generic GML geometry, or an identifier object. Vertical elements have a lower an upper bounding value, and an optional specified reference system. Temporal elements can be specified by a time ordinal era (e.g. Jurassic, or Reign of Henry VIII), a calendar date and time, or a time interval specified by a begin and end calendar date time. The model allows spatial and temporal extents to be documented independently, or coupled as a spatial-temporal extent for subject regions that are in different geospatial locations at different times, or has one or more sampling extents that were sampled at different (possibly multiple) times.
Spatial extent
Spatial representation
This section provides elements to document how the mapping between data and location is implemented in the dataset. It accounts for vector representations, grids, and georeferenced images.
Spatial Reference System
This section provides elements to document how numeric coordinate values are registered to Earth positions, accounting for datum and map projection.
Spatial resolution
Documentation of the smallest distance between locations that can be distinguished in the spatial data.
Portrayal catalog
This section provides a citation to a separate document that provides a description of how spatial data can be visualized, assigning e.g. colors to polygons that have particular attribute values, or defining color ramps to use for visualizing image data.
Temporal aspects
These are properties for documented the time interval during which the data were collected or to which the data are considered relevant. For some thematic domains, e.g. geology, history, or archeology, this might be some time in the past.
Temporal extent
The ISO19115-1 model uses the temporal model from another ISO standard (19108), and allows a variety of options for represent temporal extent. The basic concepts are time instants and time intervals. The bounds of intervals are instants. The temporal position of an instant can be a time coordinate (defined in some temporal reference system), a calendar date time (defined in some calendar system), or a named time ordinal era from an ordinal reference system. There are obviously a lot of details that must be considered, see OWL time for a good reading starting point. The most commonly used implementation is an ISO 8601 date or datetime, or time interval bounded by ISO8601 date or datetime.
Temporal resolution
This is the smallest interval of time that can be distinguished in the temporal fields in the dataset.
Usage, security and legal constraints
Documentation for limitations on use, distribution, reproduction, etc are represented as access or use constraints categorized by a controlled vocabulary with values like copyright, patent, license, private, unrestricted…, or as security constraints categorized by a controlled vocabulary with values like unclassified, restricted, confidential, secret, protected…. Security constraints Any of these constraints can be accompanied by a text statement, and scoped to a specific part of the described resource ('constraintApplicationScope'), or to specific parties ('releasability'). Applicable licenses are specified using citations.
Resource Content/function
The original ISO19115 specification was strongly oriented towards image or raster based remote sensing data, and the Content Information section included in the model reflects that interest. Documentation of entity and attribute information is sometimes mapped into the content information section, but the intentions of the committees designing the specifications was that description of relational or object oriented data structures would use a separate specification, ISO19110 'Feature Catalogue'. Unfortunately, there was not a normative XML schema to implement the ISO19110 conceptual model until recently (after the release of ISO19115-1), making its use ad hoc.
Content information
Provides elements to describe a dataset represented as a coverage—a set of attributes on one or more variable dimensions; this approach is based on documenting gridded datasets like remote sensing data acquired from a satellite. The diagram below shows the content information elements implemented in ISO19115-3, which includes ISO19115-1 and ISO19115-2 elements.
Feature catalog
A feature catalog(ue) (ISO19110) represents datasets as a collection of 'features', each of which has one or more attributes. This approach maps nicely to entity/attribute data structures typical of relational databases, or to object-oriented data structures. The UML diagram below shows the feature catalogue model from ISO19110.
Application schema
The ISO 19115-3 implementation allows a citation to one or more documents describing the application schema for the resource. The scope note for Application Schema link object (MD_ApplicationSchema) reads "the application schema used to define and expose the structure of a resource, i.e. the model and/or data dictionary that represents the resource.", which can be interpreted as a document that describes a data structure or the operation of an application. In practice this element is rarely used in my experience. The application schema element is a property of the base MD_Metadata element; multiple values are allowed, but the model does not include a binding between and application scheme and a particular distribution.
Services
The service metadata (SV_ServiceIdentification) section is also used in some profiles to document service interfaces implemented to access one or more datasets. There is opportunity for ambiguity and confusion here as to whether the metadata record is about a service as a set of operations accessible via some invocation process, or whether the metadata record is about a dataset and the service provides one means of accessing the dataset (which seems to be the more common situation). The USGIN and INSPIRE metadata profiles recommend using MD_DistributionInformation for service distributions of a specific dataset in metadata records that are about that dataset.
Resource Distribution
Distribution section provides information on how to access a digital representation of the resource described by the metadata record. The ISO19115 model is structured to allow multiple distributions, with multiple distributors, formats, and transfer options associated with each distribution. Transfer options and formats can be bound to a particular distributor or not. The model is quite flexible, but this leads to multiple approaches to documenting distributions, see Resource Access links in the Challenges section, below.
Distributor
Identifies a party responsible for maintaining a distribution process via a CI_ResponsibleParty component (see Responsible party in the Components section, below), and information about any ordering process associated with the distributor, including fees, available date, instructions, expected processing turnaround time, and an extension point for specifying any other applicable options
Format
Provides a citation to a format specification, information about the physical medium used for distribution, and a free text specification of any file compression used.
Transfer options
Documentation for a particular access option, typically characterized by a specific linkage (URL), but might also document details of an offline distribution. Optional properties include the unit of distribution (free text), transfer size (real number), formats accessible at the URL (not in ISO19115(2006)), and the rate of occurrence of distribution. Multiple URLs and formats can be associated with each option. Properties that can be associated with each linkage (URL) are
- Name – a label used to present the link in user interfaces.
- Description – text describing what the link does
- Protocol – online message protocol to be used. Documentation provides examples http, ftp, file, http GET KVP, http POST.
- Application profile – free text specification of an application profile associated with the link.
- Function – controlled vocabulary to specify the function invoked by the link
- Protocol request (new in 19115-1). Documentation is unclear; free text to provide example http POST request content.
Technically, the linkage is not required to be a URL (http URI), but could be any locator string with a known dereferencing mechanism.
Lineage
Resource provenance is recorded by a collection of processing steps, each of which can have one or more associated sources (inputs), responsible parties, citations for related resources, a time stamp, a scope (resource part, space or time), and a description and rationale free text. Process steps can be ordered sequentially using the time stamps. ISO19115-2 adds additional elements; these are all implemented in ISO19115-3 XML, and are shown in the UML diagram below.
Usage
A collection of reports on usage of the resource can be included in a metadata record. These usage reports can include a time stamp, a responsible party for the usage report, various text fields documenting the usage, and links to citations with related information. The Usage element is shown in the UML diagram in the Basic Resource Documentation section, above.
Maintenance
A collection of reports about maintenance of the metadata record or the described resource can be included, each with documentation about the frequency of updates, data of last maintenance, scope of maintenance, responsible party for maintenance, and a text description of the maintenance activity. The Maintenance element is shown in the UML diagram in the Basic Resource Documentation section, above.
Related resources
Related resources are linked via a citation to a metadata record for the linked resource, or a citation to the linked resource itself. Each related resource instance has an association type from a controlled vocabulary with values like crossReference, source, largerWorkCitation, dependency, isComposedOf. An optional initiative type can be specified, with values from a controlled vocabulary that includes terms like campaign, collection, exercise, mission, sensor, platform, program, study, trial. Based on the values in this vocabulary, the initiative type can be used to categorize the kind of linked resource. The AssociatedResource element is shown in the UML diagram in the Basic Resource Documentation section, above. Note that in ISO19115(2006) this element is named MD_AggregateInformation.
Data Quality
Data Quality characterization was included in the 19115(2006) model, but in the 19115-1 model, the data quality section has been moved to a separate ISO specification ISO19157. The model for documenting data quality is very similar in both cases, with only minor modifications and additions in ISO19157. The data quality documentation consists of a set of reports. The reports are hard typed, with 16 types defined, as well as a stand alone quality report option that is a citation to an external document. The report types are grouped in the following way: Usability, logical consistency, temporal quality, thematic accuracy, and completeness. Each report includes one or more result values that can be a simple 'pass/no pass', a quantitative value, a coverage (ISO19157 amendment 1), or descriptive text; result values can have a time stamp and a scope. Optional properties include specification of the evaluation method, an identifier and name for the measurement made, and elements to characterize the confidence, representativity, and homogeneity of the reported quality.
Components:
There are several data objects that are used as the value type for elements in multiple parts of the model, and in some cases could also be used in multiple metadata records. Instances of these could be stored in a registry and referenced from multiple metadata records to improve efficiency and simplify maintenance of commonly used information.
Responsible party
CI_Party represents individuals (name or role) or organizations, and affiliations between individuals and organizations. A party is bound to a role in various 'responsibilities' relative to a described resource. This allows the same person or organization be play multiple roles like author, maintainer, publisher, owner, distributor, editor etc. (CI_RoleCode vocabulary). Contact information including telephone number, postal address, and e-mail address are linked to parties. ISO19115-1 Amendment 1 adds an identifier property for each party so that URI's can be associated with people and organizations. It also adds an extent (spatial or temporal) property on a responsibility assignment that can be used, for instance, to document areas that were mapped by different people in a geologic map dataset.
Responsible party is used in various contexts; various roles might be assigned to the party in any given context.
- Metadata point of contact
- Resource point of contact
- Cited responsible party
- Distribution contact
- Usage documentation
- Maintenance contact
Processing Step
Processing step descriptions (see Lineage, above) could be reused across multiple lineage descriptions. Verbatim reuse would preclude using the step time stamp to order steps in a particular process instance.
Citation
Citations are used in two roles:
- to provide the standard bibliographic citation content for the described resource
- to provide links or references to other resources in various roles, including additional documentation, sources that contributed data to create the resource, related resources, thesauri used for keyword vocabularies.
Citations in the second role might be used in multiple metadata records. The CI_Citation element is shown in the UML diagram with Responsible Party, above.
Extent
Named extent objects might be used in multiple metadata records.
Challenges:
The ISO metadata model is comprehensive and flexible at the cost of some interoperability problems introduced by allowing multiple approaches to representing the same information. There are a few concepts that are not well accounted for by the model as well. This section discusses a few of those issues.
xlink:href as Identifiers
The ISO xml implementation allows substitution of an 'anchor' element for any characterString value in an instance document. The anchor element adds the xlink attributes, which are not included in the characterString implementation. Some data providers use these xlinks to specify URIs corresponding to the provided string value, particularly for keywords, individual names, and organization names. This works well in the context of a community profile, but is not a general solution documented in the specification. ISO19115-1 fixes this problem for individual and organization.
Resource categorization
The 2006 version of ISO19115 categorized resources with a 'hierarchyLevel' property; in the 19115-1 version, this has become 'resourceScope', but both properties are populated by a controlled vocabulary with values like software, dataset, service, model, collection (the 19115-1 vocabulary has many useful additions over the original version). Both models also allow a free text label that is related to the resource type, 'hierarchyLevelName' in the case of 19115 (2006), or metadataScope/name in the case of 19115-1. For many purposes this resource categorization is too restrictive, and the use of the scope properties is inconsistent because of varying interpretation of the intention.
Related resources
Mix of hard typing (e.g. for Additional documentation or Browse graphic) and soft typing (MD_AssociatedResource/associationType) for linking to other resources. Semantics of associations are limited by a controlled vocabulary with values like crossReference, largerWorkCitation, source, dependency, revisionOf. Sources for resources that are aggregations (compilation, syntheses, reviews…) can be documented through links in the lineage (provenance) or as related resources. The URLs and URIs for linked resources are buried in nested citation elements.
Entity and attribute information
Content information, feature catalog (ISO19110), or application schema (a linked document) all are possible approaches to describing the structure of a described data set. The Content Information approach is probably the most widely used at this point because it was implemented in the ISO19139 XML implementation of ISO19115(2006)
Resource Access links
There are multiple options for documenting online linkage to a resource, including operation/ onlineResource, distribution/transferOption/onlineResource, distribution/distributor/ distributorTransferOption/onlineResource or identificationInformation/citation/onlineResource (note these are conceptual paths, not full xml xpaths).
Spatial Extent
Extent geometry can be represented as bounding box or generic GML geometry objects. Although the model allows multiple geometry elements, most client applications only index the first geometry provided. The bounding box is the most common method used, but has various problems. Many client applications have difficulty handling bounding boxes that cross the 180 degree E. longitude; although the box could be separated into multiple boxes if only the first bounding box provided is indexed, this does not produce the desired result. A dataset might contain information about widely separated sampling sites, e.g. hot springs in New Zealand and California, or ecologic observations in coastal areas of Virginia, Hawaii, and Tanzania. These lead to unrealistic bounding boxes and irrelevant spatial search results if a single bounding box is constructed to include all sites.
The use of the generalized geometry model from the Geography Markup Language allows representation of virtually any kind of coordinate-referenced geospatial location, with multiple points, lines, polygons, multi part geometries, polygons with holes, etc., but there are no search clients I know of that will parse complex geometries and use them for search.
These problems are not inherent in the ISO 19115-1 model, but rather in simplifications that are typically made in client applications accustomed to the simple one bounding box (or one centroid point…) approach.
File formats
Binding specific formats to specific distribution endpoints is not simple and there are not clear conventions on how to do it. The various places to document file formats (MD_format, distributorFormat, or distribution/MD_Format) lead to various interpretation and interoperability challenges.
...