The NHN model incorporates two representations of water. The first is a logical representation of water using a linear network detailing the movement of surface water, the Hydro Network, and the second is a more traditional cartographic representation of water-related phenomena, the Hydrographic Network.

The NHN model is based on the Linear Reference System (LRS), which allows the association of an undefined number of variables to a unique, stable geometry. In the LRS each object is divided into two parts: the geometry, which indicates the position of the entity without describing it in detail, and the attributes (events), which indicate information that applies to the line or geometry. The LRS system is flexible, so that users can add their own attributes to a single, common geometry.

The NHN model is divided into 4 packages: the Hydro Network, Hydro Events, Hydrographic Network, and the Metadata.

The Hydro Network is the basis for the NHN model. It consists of the lines and junctions that represent surface water. These lines and junctions are divided into five entity classes, which make up the geometry part of the LRS. These classes are as follows:

1. Network Linear Flow Lines, which show the movement of surface water in a single dimension.
2. Bank Lines, which outline internal waters such as the outline of a lake or watercourse.
3. Coast Lines, which delimit land from sea.
4. Delimiters, which are theoretical lines that delimit different but contiguous bodies of water, that outline the presence of tidal water, that show the theoretical limit of coast lines, that separate bodies of water according to provincial or territorial divisions, and that show the outline of work units.
5. Hydro Junction, which are intersection that are always connected to one of several elements of the network. and that show the beginning or end of a network line.

The Hydro Events package describes the Hydro Network using the LRS. This package is also divided into five classes. These classes are as follows:

1. Anthropogenic Events, comprised of structures built by people to facilitate access to a hydro resource or to regulate water levels. For example dams, docks, lock gates, and boat ramps.
2. Obstacle Events, which apply strictly to the Network Linear Flow Line category of the Hydro Network and which include all elements that disturb or impede the natural flow of water. For example waterfalls, rapids, rock bars, and reefs.
3. Flow Property Events, which are found on all Network Linear Flow Lines in the Hydro Network and which describe the flow line properties in terms of the speed of water flow and waterway usage. For example, canals, conduits, drainage ditches, lakes, reservoirs, and watercourses.
4. External Geometry Events, which consist of only points, and which situate an intersection between the Hydro Network and one or many events that are not in the NHN so that users can integrate information from a separate database to the NHN. For example a user could identify a geometry representing a pipeline (not in the NHN) that connects with a Network Linear Flow Line in the Hydro Network at a particular.
5. External Events, which are essentially place holder lines and points so that a user can add his or her own relevant events that are not integrated to the NHN and stored in another database, by populating this class of events. from another data base.

The Hydrographic package consists of a series of classes that represent water-related phenomena in a more traditional way. This package represents surface water geometry as closely as possible to the geometry of bodies of water on the Earth's surface. As in the Hydro Events package, there are anthropogenic entities such as dams and docks, and obstacle entities such as falls and rapids. However, in the Hydrographic package, the graphical representation of the phenomenon is more detailed. In the Hydrographic package, obstacle and anthropogenic entities are represented by points, lines, and polygons. The Single Line Watercourse class has a Line geometry that represents canals and rivers. The Waterbody class has polygon geometry and represents lakes and reservoirs. The Island class is also represented using polygon geometry, as shown in the image below.

Representation of classes of the Hydro Events package on the Hydor Network

Representation of classes of the Hydrographic package

The Metadata package consists of information describing the data itself. There are two classes of metadata: Polygon Metadata and Object Metadata.

Further documentation describing the official NHN Standard is available in PDF format on the GeoBase Web site.>

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