Open Geospatial Consortium
Submission Date: 2017-08-17
Approval Date: 2017-10-16
Publication Date: 2018-01-08
External identifier of this OGC® document: http://www.opengis.net/doc/IS/hy-features/1.0
Internal reference number of this OGC® document: 14-111r6
Category: OGC® Implementation Standard
Editor: David Blodgett, Irina Dornblut
OGC® WaterML 2: Part 3 - Surface Hydrology Features (HY_Features) - Conceptual Model
Copyright © 2018 Open Geospatial Consortium
To obtain additional rights of use, visit http://www.opengeospatial.org/legal/
This document is an OGC Member approved international standard. This document is available on a royalty free, non-discriminatory basis. Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.
Document type: OGC® Standard
Document stage: Approved
Document language: English
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- 1. Scope
- 2. Conformance
- 3. References
- 4. Terms and Definitions
- 4.1. application schema
- 4.2. boundary (line)
- 4.3. catchment
- 4.4. catchment area
- 4.5. catchment divide
- 4.6. catchment network (topology)
- 4.7. (internal) catchment topology
- 4.8. channel
- 4.9. channel network
- 4.10. cross section (of a stream)
- 4.11. cross section (of a stream bed)
- 4.12. data
- 4.13. data set (also dataset)
- 4.14. data product
- 4.15. dendritic catchment
- 4.16. depression
- 4.17. domain feature
- 4.18. endorheic (drainage)
- 4.19. exorheic (drainage)
- 4.20. feature
- 4.21. flowpath (also flow path)
- 4.22. flow line (also flowline)
- 4.23. hydrographic network
- 4.24. hydrologic complex
- 4.25. hydrologic feature
- 4.26. hydro(-logic) location
- 4.27. hydro(-logic) nexus
- 4.28. hydrologic realization
- 4.29. hydrology
- 4.30. hydrometric feature
- 4.31. hydrometric network
- 4.32. hydrometry
- 4.33. indirect position
- 4.34. interior catchment
- 4.35. longitudinal section (of a stream)
- 4.36. longitudinal section (of a stream bed)
- 4.37. main stem (also mainstem)
- 4.38. mapping
- 4.39. named feature
- 4.40. nillable
- 4.41. referent
- 4.42. representation
- 4.43. river referencing
- 4.44. storage (of water)
- 4.45. stream
- 4.46. waterbody (also water body)
- 4.47. watercourse
- 5. Conventions
- 6. Clauses not Containing Normative Material
- 6.1. The abstract idea of the hydrology phenomenon
- 6.2. Catchment and realizations of the catchment concept
- 6.3. Catchment topology and hierarchy
- 6.4. Catchment network realization
- 6.5. Surface water networks
- 6.6. Referencing hydrologic locations along a river
- 7. Clause containing normative material
- 7.1. The HY_Features conceptual model
- 7.2. HY_Features conceptual model conformance
- 7.3. Hydro Feature concepts
- 7.4. Surface Hydro Feature concepts
- 7.5. Hydrometric Network concepts
- Annex A: Conformance Class Abstract Test Suite (Normative)
- Annex B: Code lists for the HY_Features model
- Annex C: HY_Features - AHGF Mapping
- Annex D: HY_Features - NHDPlus Mapping
- Annex E: HY_Features-INSPIRE Hydrography Theme
- Annex F: HY_Features- SANDRE Mapping
- Annex G: HY_Features - NHN Mapping
- Annex H: Revision History
- Annex I: Bibliography
The OGC Surface Hydrology Features (HY_Features) standard defines a common conceptual information model for identification of specific hydrologic features independent of their geometric representation and scale. The model describes types of surface hydrologic features by defining fundamental relationships among various components of the hydrosphere. This includes relationships such as hierarchies of catchments, segmentation of rivers and lakes, and the hydrologically determined topological connectivity of features such as catchments and waterbodies. The standard also defines normative requirements for HY_Features implementation schemas and mappings to meet in order to be conformant with the conceptual model.
The HY_Features model is based on an abstract catchment feature type that can have multiple alternate hydrology-specific realizations and geometric representations. It supports referencing information about a hydrologic feature across disparate information systems or products to help improve data integration within and among organizations. The model can be applied to cataloging of observations, model results, or other study information involving hydrologic features. The ability to represent the same catchment, river, or other hydrologic feature in several ways is critical for aggregation of cross-referenced or related features into integrated data sets and data products on global, regional, or basin scales.
The following are keywords to be used by search engines and document catalogues.
ogcdoc, OGC document, hydrology, feature, identification, conceptual model, UML, implementation standard.
This standard defines the HY_Features common hydrologic feature model for use in identification of features as specific kinds of hydrologic features. It is intended to be used to document and share information about features that are the subject of hydrologic studies and reporting for a range of applications. HY_Features was originally commissioned to link hydrologic information across the scientific and technical programs of the World Meteorological Organization (WMO), and to assist WMO Members in discovering, accessing, and utilizing hydrologic data from different sources.
This standard is also intended to support the need for governance and guidance by national and international authorities. Aspects of the standard that support this goal are 1) canonical form, 2) implementation neutrality, 3) conformity to internationally recognized geographic information standards, and use of semantics inferred from terminology endorsed by the WMO and the UN Educational, Scientific and Cultural Organization (UNESCO).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.
Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.
iv. Submitting organizations
The following organizations submitted this Document to the Open Geospatial Consortium (OGC):
U.S. Geological Survey (USGS), USA
Federal Institute of Hydrology (BfG), Germany
CSIRO Land and Water, Australia
Bureau of Meteorology, Australia
French Geological Survey (BRGM), France
University of Texas at Austin, USA
INCLAM S. A., Spain
All questions regarding this submission should be directed to the editor or the submitters:
U.S. Geological Survey
Federal Institute of Hydrology (BfG)
Robert A. Atkinson
Australian Bureau of Meteorology
CSIRO Land and Water
University of Texas at Austin
French Geological Survey (BRGM)
French Geological Survey (BRGM)
vi. Relationship to the OGC® Abstract Specification
The HY_Features conceptual model, described in this standard, references concepts defined in the OGC Abstract Specification (OGC AS). Conformance to the OGC AS may be achieved via specialization of the relevant concept. An example of this kind of conformance is a geographic feature specialization that carries specific hydrologic feature attributes and associations. Conformance may also be achieved by asserting a correspondence or analogy between HY_Features concepts and OGC AS concepts. For example, the river reference system for referencing positions along a nominal flowpath corresponds to linear referencing along any linear feature element. This standard requires no changes to the current version of the OpenGIS Abstract Specification.
This OGC Implementation Standard defines a common conceptual feature
model for use in identification of features as typical features of the
hydrology domain using established models and patterns in use in the
Hydrology domain and endorsed by WMO and UNESCO such as those documented
in the "International Glossary of Hydrology".
This standard, along with expected future implementations of this
conceptual model, will form the third part of an intended 'OGC WaterML
2' suite of standards that groups water-related OGC standards. Following
Part 1: Timeseries, Part 2: Ratings, Gaugings and Sections, this
standard is titled Part 3: Surface Hydrology Features Conceptual
This document introduces the HY_Features conceptual model only. The
normative model is a machine-readable UML artifact published by the OGC
in conjunction with this document at: http://docs.opengeospatial.org/is/14-111r6/uml/. Clause 7 of
this document presents requirements classes (ways of conforming to the
conceptual model) and requirements for model implementations to meet in
order to be considered conformant with the conceptual model. Conformance
classes indicating how to demonstrate conformance to requirements are
presented in Annex A.
Future standards documents in this series are expected to specify
particular implementations of the conceptual model. For example, an
OWL/RDF implementation will define machine-processable entities and
relationships in the OWL schema language that implement the HY_Features
conceptual model elements. HY_Features implementations are expected to
support documentation and discovery of data as well as aid data
transformation efforts. For example, a discovery service such as a
catchment catalog might use HY_Features concepts to index services that
provide differing catchment representations as well as related water
quality and quantity observations. A catalog client could then use the
index for automated analysis and data product generation.
The initial scope has been defined by the WMO Commission for Hydrology
(WMO-CHy): to facilitate data sharing within the hydrologic community of
the WMO Member countries and to improve the quality of data products
based on these data by defining hydrologic features to convey their
identification through the data processing chain "from measurement to
hydrological information" .
To enable semantic interoperability of hydrologic data and services, it
is necessary to agree on common concepts and methods. The HY_Features
model was developed in order to formalize concepts and relationships of
hydrologic and hydrographic features using the WMO/UNESCO "International
Glossary of Hydrology" as a starting point. One of HY_Features main
objectives is to provide a standard terminology for description of
relationships between hydrologic features that can be used to implement
data transfer formats. However, this conceptual model should be seen as
a building block toward a hydrographic data interchange standard,
but such a standard would require additional efforts.
HY_Features is meant to support linking data products across differing
applications and jurisdictions. To enable this, the concept of a
holistic catchment is defined and "hydrology-specific realizations" of
the catchment concept are modeled as typical hydrologic features. This
allows a particular catchment to be represented in different feature
data products for different purposes while still retaining its identity.
The HY_Features model provides a basis for common and stable references
that identify hydrologic features in a wide variety of applications:
to link hydrologic observations to their feature-of-interest, e.g. link a streamflow observation to the river or catchment being observed,
to allow aggregation of cross-referenced features into integrated data sets and data products on global, regional, or basin scales,
to enable systems to unambiguously link data between systems and domains,
to enable cross-domain or multi-discipline services to communicate through reference to standard concepts.
While HY_Features is intended to support a wide variety of applications,
it is narrowly focused on surface water hydrographic features. As such,
nuances of waterbodies and storages such as glaciers, snowfields, and
wetlands are left to future work. Additionally, temporal aspects of
hydrologic features are not addressed in this standard and would be
expected to be handled by implementations using HY_Features classes for
feature identification. Finally, as this standard precedes expected
encoding standards, implementation details, such as the structure and
handling of feature identifiers are out of scope.
This standard defines a conceptual model (normative) containing feature
type definitions that conform to the OGC General Feature Model (GFM)
(ISO 19101:2002 and ISO 19109:2006), expressed in the Geographic
Information Conceptual Schema Language (ISO 19103:2005) using the
Unified Modeling Language (UML). The GFM is a meta-model developed as a
general framework for features and their properties in the context of
geographic information. Feature types specific to an application domain
are defined as instances of the general feature metaclass with their own
sets of typical characteristics (property types and constraints) such as
attributes, associations, or operations.
As shown in Figure 1, the HY_Features conceptual model
defines instances of the general feature metaclass specific to the hydrology
domain. A general HY_HydroFeature type is defined to carry properties such as
identifier and name that all hydrologic features should have. Specific
feature types are then derived from HY_HydroFeature to reflect different
aspects of hydrology and carry properties specific to each aspect.
Figure 1 also illustrates that HY_Features features can
fill the role of features-of-interest or sampling features for hydrologic
observation data conforming to the Observation and Measurement (O&M) model
(ISO 19156:2011) or to hydro-specific O&M profiles such as in the OGC
WaterML 2' suite of water-related standards.
This standard defines a conceptual model for hydrologic features and
their fundamental relationships. Requirements for conformance with the
conceptual model are defined for two types of implementation targets: a)
implementation schemas for representation of hydrologic data or b)
implementation mappings between terms or concepts from two or more other
existing hydro data schemas based on common intermediary HY_Features
concepts. Any conformant implementations are required to satisfy the
requirements defined in Clause 7 of this document for the requirements
class(es) with which they assert conformance. This is true whether the
implementation is explicit in the case of a schema (such as an OWL/RDF
ontology or XML schema) or implicit in the case of common concepts for a
schema mapping (such as an XSLT rule). The accuracy or detail with which
an external term is mapped to a HY_Features common concept is beyond the
scope of this standard, as is specification of a particular concept
Annexes C-G of this document provide, nevertheless, informative mappings
to and from HY_Features for the following hydro data models:
These mappings are intended to provide a general understanding of the
correspondence between HY_Features concepts and existing conceptual /
physical models. They are informative with respect to any future mapping
Conformance of implementation targets with this standard shall be verified using all the relevant tests specified in Annex A (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be met to claim conformance are specified in the OGC Compliance Testing Policies and Procedures and the OGC Compliance Testing web site.
All requirements-classes and conformance-classes described in this document are owned by the HY_Features standard.
The following normative documents contain provisions that, through reference in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.
OGC: OGC Abstract Specification Topic 1, Feature Geometry
(aka ISO 19107:2003, Geographic Information — Spatial schema), 2003
OGC: OGC Abstract Specification Topic 2, Spatial referencing by coordinates
(aka ISO 19111:2007, Geographic Information — Referencing by coordinates), 2007
OGC: OGC Abstract Specification Topic 5, Features
(aka ISO 19101:2002, Geographic Information — Reference Model), 2002
OGC: OGC Abstract Specification Topic 11, Metadata (aka ISO 19115-1:2014, Geographic Information — Metadata — Fundamentals), 2014
OGC: OGC Abstract Specification Topic 19, Geographic information - Linear referencing (aka ISO 19148:2012, Geographic Information — Linear referencing), 2012
OGC: OGC Abstract Specification Topic 20, Observations and Measurements (aka ISO 19156:2011, Geographic Information — Observations and Measurements), 2011
ISO: ISO/TS 19103:2005, Geographic Information — Conceptual schema language, 2005
ISO: ISO 19108:2006, Geographic Information — Temporal schema, 2006
ISO: ISO 19109:2006, Geographic Information — Rules for application schemas, 2006
ISO: ISO 19133:2005, Geographic Information — Location-based services — Tracking and navigation, 2005
This document uses the terms defined in Sub-clause 5.3 of [OGC 06-121r8], which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word SHALL (not "must") is the verb form used to indicate a requirement to be strictly followed to conform to this standard.
For the purposes of this document, the following additional terms and definitions apply.
Geometric representation of a (catchment) boundary, usually a geometric composite curve.
|This definition references the definition of a divide described as summit or boundary line .|
A physiographic unit where hydrologic processes take place. This class denotes a physiographic unit, which is defined by a hydrologically determined outlet to which all waters flow. While a catchment exists, it may or may not be clearly identified for repeated study.
This approach considers the catchment concept to be the basic unit
of study in hydrology, water resources management, and environmental
reporting. The approach is meant to be holistic, referring to the
continuous interaction of surface and subsurface waters within a
catchment, even if a particular representation of the catchment refers
to only surface or subsurface aspects of the catchment. Special subtypes
such as drainage basin and groundwater basin may be defined with a
The synonymous use of the terms catchment and catchment area in the
WMO/UNESCO International Glossary of Hydrology (which is the key
reference for the definitions in the HY_Features model) does not clearly
distinguish between the catchment concept and its geometric
representations such as catchment area, nor between catchment and its
possible specializations like drainage basin and groundwater basin.
To satisfy the need for a both a holistic definition of catchment and various refinements of it, the catchment feature type is refined by defining feature types that reflect distinct perspectives on the holistic catchment concept. In this standard, this type of refinement is referred to as hydrology-specific catchment realization. Also see clause 5.4 of this standard on the use of WMO terminology.
Two-dimensional (areal) hydrology-specific realization of the holistic catchment. Topologically, the catchment area can be understood as a face bounded by catchment divide and flowpath edges. The concept of a face bounded by edges is described in detail in the ISO topology model [ISO19107]. The catchment area is usually represented as a geometric surface, and the measure of the catchment area may be denoted as surface area.
One-dimensional (linear) feature that is a hydrology-specific realization of the holistic catchment. Topologically, catchment divide can be understood as an edge bounded by inflow and outflow nodes. The concept of an edge bounded by nodes is described in detail in the ISO topology model [ISO19107]. The catchment divide is usually represented as a geometric curve, or as a polygon ring feature.
Edge-node topology pattern of a set of catchments connected by their hydro nexuses. The topology pattern is derived from flowpaths, but ultimately reflects the inferred hydrologic connectivity between catchments and their hydro nexuses whether or not corroborated by geometric representations.
Topology pattern between hydrology-specific realization of a particular catchment. Example: Catchment area associating its catchment divide and the draining flowpath. This could include subsurface and atmospheric volumes, surficial catchment area, network of drainage channels and/or waterbodies, a linear flowpath, and an outlet (and possible inlet) node.
Natural or artificial waterway, clearly distinguished, which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water .
Connected set of depressions and channels that continuously or periodically contain water.
Section of a stream at right angles to the main (average) direction of flow .
transversal section of a stream bed in a vertical plane. This definition references the definition of a bed profile describing the shape of a stream bed in a vertical plane .
Data compiled and arranged into a set. Used in its concatenated form to indicate a specific dataset, or as data set, for non-specific sets of data.
Data set compiled for a specific purpose, e.g. for global dissemination using Web services.
Catchment in which all waters flow to a single common outlet. A dendritic catchment is permanently connected to others in a dendritic (tree) network, the most common drainage pattern of streams ultimately flowing into the ocean after joining together at confluences into larger and larger streams.
Landform lower than the surrounding land and partially or completely closed that is able to, but does not necessarily, contain water.
Draining [ultimately] into interior catchments .
One-dimensional (linear) feature that is a hydrology-specific realization of the holistic catchment. Topologically, flowpath can be understood to be an edge bounded by inflow and outflow nodes, and associated with left-bank and right-bank sub-catchment faces. The concept of an edge bounded by nodes is described in detail in the ISO topology model [ISO19107]. The flowpath is usually represented as a geometric curve.
A flowpath feature may form the "main stem" of the stream network flowing to the catchment outflow node from its inlet node(s).
With respect to the river referencing system described in this standard, the flowpath corresponds to the linear element 'that serves as the axis along which linear referencing is performed' as described in the OGC Abstract Specification Topic 19, Linear referencing [aka ISO 19148].
NOTE 3: Hydrologically, the flowpath references the path line __ described by a moving particle of water .
Geometric property of a flowpath, usually a geometric curve.
|This definition references the definition of a path-line  through using the synonym flow line to express the geometric property of a flowpath.|
Aggregate of rivers and other permanent or temporary watercourses, and also lakes and reservoirs [WMO, 2016].
|Not to be confused with the network of hydrological stations and observing posts.|
Collection of distinct hydrologic features forming a hydrologically connected system where the union of one or more catchments and a common hydro nexus is realized by multiple complexes of hydrology-specific topological elements. For example, a single catchment may be hydrologically realized as a face-edge complex of subcatchment areas and divides, or an edge-node network of flowpaths and hydro nexus nodes, and also as a dendritic edge-node network of either waterbodies or containing channels.
Feature of a type defined in the hydrology domain, whose identity can be maintained and tracked through a processing chain from measurement to distribution of hydrologic information.
Any location of hydrologic significance located "on" a hydrologic network that is a hydrology-specific realization of a hydrologic nexus. In a given dataset, hydro locations may or may not have an associated hydrologic nexus and associated catchment features. In such cases, hydro locations would typically be linearly-referenced to a defined set of catchments' flowpaths. Topologically, a hydro-location can be understood as an inlet or outlet node located at the end of a flowpath edge. The hydrologic location is usually represented as a geometric point.
|With respect to the river referencing system described in this standard, a hydro-location feature can correspond to either the referent that specifies a 'known, already referenced location on the linear element' as described in the OGC Abstract Specification Topic 19, Linear referencing [aka ISO 19148] or the reference location whose distance from a referent can be measured along a linear element.|
Conceptual outlet for water contained by a catchment. The hydro nexus concept represents the place where a catchment interacts with another catchment. Every catchment flows to a hydro nexus, conversely every location in a hydrologic system can be thought of as a hydro nexus that drains some catchment. Similar to catchments, hydro nexuses can be realized in several hydrology-specific ways.
If a given hydro nexus does not have a known hydrology-specific realization or is undetermined, it is termed 'nillable' in this standard. For example, a hydro nexus exists in the form of flow to the subsurface or atmosphere but may be undetermined and unrepresented within implementations focused on surface water hydrology and would not be included or referenced.
A hydrologic feature type that reflects a distinct hydrology-specific perspective of the catchment or hydro nexus feature types. Shares identity and catchment-nexus relationships with the catchment or nexus it realizes but has hydrologically determined topological properties that express unique ways of perceiving catchments and hydrologic nexuses. Distinct from representation in that it is a refinement of the holistic catchment, allowing for multiple geometric representations of each hydrologic realization.
Science that deals with the waters above and below the land surfaces of the Earth, their occurrence, circulation and distribution, both in time and space, their biological, chemical and physical properties, their reaction with their environment, including their relation to living beings. 
Feature of a type which denotes a physical structure intended to observe properties of a hydrologic feature.
|This definition references the definition of a hydrometric station at which data on water in rivers, lakes or reservoirs are obtained on physical and chemical properties of water . A hydrometric feature may be a composite station configured by arranging several monitoring components. Used to sample a hydrologic feature, a hydrometric feature may be considered a sampling feature as defined in the ISO Observation model. A sampling feature is described in general in ISO 19156: Observation and Measurement, the special monitoring point of hydrologic observation is described in the OGC WaterML 2.0: Part 1- Timeseries standard .|
Aggregate of hydrologically connected monitoring stations situated on and used for hydrologic observation of a feature such as a catchment or hydrographic network. This definition references the definition of a synonymous hydrological network consisting of hydrological stations and observing posts situated within a catchment in such a way as to provide the means of studying its hydrological regime .
Science of the measurement and analysis of water including methods, techniques and instrumentation used in hydrology .
Position expressing the location of a feature relative to the known location of another feature. Indirect position requires a logical axis, bounded by the feature being placed and the feature being used as a reference, along which the position can be determined. Common examples in hydrology are mileposts along a river referencing the river source and/or mouth, and the placement of monitoring stations referencing already located stations.
Catchment in which all waters are collected and drainage is endorheic; an interior catchment does not drain to other catchments. This definition is rooted in the blind drainage pattern of water collecting in sinks or lakes not connected to streams .
Vertical section of a stream in longitudinal direction. This definition is rooted in the definition of a longitudinal section along a channel at its center line , but generalized for all types of vertical section along a line.
Longitudinal section of a stream bed in a vertical plane. This definition references the definition of a bed profile describing the shape of a stream bed in a vertical plane .
Main course along which water flows in a catchment excluding tributaries. For any identified catchment, the flowpath connecting inflow and outflow locations would typically correspond to or follow the main stem, but the main stem is conceptually broader than the catchment flowpath concept of HY_Features.
Establishing a semantic relationship between concepts in different information models using a formalism that specifies how elements from a source information model may be transformed into elements of a target model. Every pair of N models generally require a separate (2-way) mapping for each source concept (a total of N!/[2(N-2)!] mappings). Mappings that in contrast involve transformation by way of a common concept can be more efficiently expanded to more than two models as each additional model only requires mapping once into the set of common concepts (N mappings).
Feature identified by a name. Hydrologic features may have multiple names depending on the cultural, political or historical context.
Nillable is meant in this standard to signify that a feature property logically exists but may not be determined in a given implementation.
Feature with a known location being used as a reference to locate another feature on the logical axis that stretches between the two. The (indirect) position of a new location is expressed as the distance along that linear element from the known referent to the feature being placed.
Any process-able data, data set, or data product which characterizes a given feature concept.
Referencing along a river applied to place a feature on a (linear) waterbody feature. The feature location is specified as an indirect position expressed as distance along the watercourse on which the feature is to be placed.
|A (hydrologic) feature of interest which is located along the locating one-dimensional flowpath between inflow and outflow nodes, corresponds to the linear referencing described in the OGC Abstract Specification Topic 19, Linear referencing 'specifying a location relative to a linear element along that element' [ISO 19148].|
Impounding of water in surface or underground reservoirs, for future use. 
|NOTE: Storage refers to a body of water from the perspective of a usable water resource. The management of a reservoir itself, as a human action with the objective of efficient and sustainable use of the resource, is not in the scope of the conceptual model.|
Water, generally flowing in a natural surface channel, or in an open or closed conduit, a jet of water issuing from an orifice, or a body of flowing groundwater .
Mass of water distinct from other masses of water .
Natural or man-made channel through or along which water may flow , including large interstices in the ground, such as cave, cavern or a group of these in karst terrain.
This section provides details and examples for any conventions used in the document. Examples of conventions are symbols, abbreviations, use of XML schema, or special notes regarding how to read the document.
The normative provisions in this specification are denoted by the URI
All requirements and conformance tests that appear in this document are denoted by partial URIs which are relative to this base.
WMO Commission for Hydrology
Geography Markup Language
Global Runoff Data Centre (GRDC)
OGC Hydrology Domain Working Group
International Organization for Standardization
Open Geospatial Consortium
Web Ontology Language
Unified Modeling Language
- WaterML 2
working title for the indented suite of water-related OGC standards
WMO Information System
WMO Integrated Global Observing System
World Meteorological Organization
eXtensible Markup Language
Most diagrams that appear in this specification are presented using the Unified Modeling Language (UML) static structure diagram, as described in Sub-clause 5.2 of the OGC Web Services Common Implementation Specification (OGC 04-016r2). UML classes are named in UpperCamelCase and property names in lowerCamelCase.
The HY_Features model uses—as far as possible—terminology recommended by the WMO Commission for Hydrology for use in the WMO Member countries. The key reference is the "International Glossary of Hydrology"  a joint publication of the WMO and the UNESCO (hereinafter referred to as WMO/UNESCO glossary of hydrology). Wherever appropriate, terms from this glossary are applied to the feature concepts in this standard to capture meaning and contextual relationships. The synonym approach widely used in this glossary is interpreted as signifying that synonymous terms are not necessarily synonymous in every respect. Differences in terminology were explored through reconciling the explicit definitions documented in the glossary with usage reflected in various data sets and products. The accepted terms were augmented with the relationships inferred from other terminology in order to define complex terms that do not clearly distinguish between a specific logical concept and its geometric representation or between a general term and its specific conceptual meaning. The definitions used in the conceptual model described in this standard may be understood as a conceptual refining and/or narrowing of the definitions given in the WMO/UNESCO Glossary of Hydrology.
Some words have been avoided because of their disparate uses in various implementation schemes. The words "reach" and "mainstem" are examples that are used to refer to such a variety of collections of features, that they have been avoided. Similarly, the term "watershed" is used and defined differently depending on its context and has been avoided. Unfortunately, it is inevitable that the terminology in HY_Features will conflict with one or more schemes or implementations of hydrologic features. In these cases, great care will need to be taken and the specific feature type names from HY_Features should be used when necessary.
Though rooted in the definitions given in the WMO/UNESCO Glossary of Hydrology, many of the hydrologic features in this standard have been defined more specifically for relevance to the surface water domain. Some requirements defined in this standard refer to the Scoped Name concept of ISO 19103: Conceptual Schema and any such Scoped Name implementations should reflect if possible a name endorsed by the WMO.
The HY-prefix used in the UML model follows the ISO naming conventions for UML elements. There is no explicit requirement to use this same name in a corresponding implemented term but conformance will require that each such term be clearly connected back to its corresponding conceptual model term.
Processes that continuously deplete and replenish water resources cause or result in a wide range of phenomena that are the subject of monitoring, modeling and reporting in hydrology and related sciences. This standard conceptualizes these distinctly named or otherwise uniquely identified real-world hydrologic phenomena as hydrologic features.