NHN Work Units
NHN datasets are produced and distributed per NHN Work Unit. The NHN Work Unit is actually a drainage area. The NHN Work Unit Limit feature delimits the drainage area covered by a NHN dataset. This feature is not part of the NHN Standard and Data Model. It has been created to define the NHN dataset's extent. It is distributed within NHN data products for reference purposes. This limit is actually a simple polygon (no hole), as opposed to a complex polygon. Apart from Sable Island (01EQ002) located offshore from the Nova Scotia coast, NHN Work Unit Limits (polygons) are contiguous between them and make up a complete territorial coverage without any gap and overlap. Altogether they currently represent more than 1150 units or territorial divisions covering the entire Canadian landmass. This coverage stretches from the Canada/USA International Boundary up to the Canadian territorial sea or to the 1:50,000 scale National Topographic System (NTS) tile limits along the Canadian coastline, as shown in white and blue on the following figure.
The territorial sea mentioned above is an area of the sea that has an outer limit extending up to 12 nautical miles measured seaward from the Canadian coast.
The original NHN Work Unit Limits were created based on the Water Survey of Canada Sub-Sub-Drainage Areas (WSCSSDA) and Fundamental Drainage Areas (FDA) from the Atlas of Canada. They are being modified and refined as better provincial source data is brought into the data production process following NHN partnerships between federal and provincial/territorial agencies. Each NHN Work Unit Limit is adapted to fit the source data used to generate NHN data. Where applicable, they are stretched seaward from the Canadian coast. As a result, NHN Work Unit Limits no longer correspond exactly to those from the WSCSSDA or the FDA. Therefore, it is important not to mistake them as WSCSSDA or FDA boundaries. Moreover, although the naming convention used for NHN Work Units (e.g. 07LEA00) is inspired and derived from WSCSSDA (e.g. 07LE) and FDA (e.g. 07LEA) names, it does absolutely not refer to WSCSSDA or FDA hydrometric stations nor to their boundaries. It is expected that NHN Work Unit Limits will evolve over time as source data is replaced or updated.
Of the some 1150 Work Units to produce, about 20 do not have a full or complete coverage in source topographic data. Missing data is mainly located in northern Northwest Territories and in Nunavut. Those remaining NHN Work Units will be produced according to the NHN Implementation Strategy and made available on the GeoBase portal as their source data is created.
For more information about NHN Work Units, refer to the National Hydro Network, Data Product Specifications, Distribution Profile, Edition 1.0, 2007-06-01, more specifically the "Feature Information / Application Schema" and the "Data Maintenance / Description" sections, as well as the NHN Index section.
Watershed and drainage area boundaries available in Canada can be downloaded from the following sites:
- GeoGratis portal for information on the Water Survey of Canada Drainage Areas and Drainage Areas from the Atlas of Canada. Search under the All Collections section: Atlas of Canada 1,000,000 National Frameworks Data, Hydrology - Drainage Areas.
- Agriculture and Agri-Food Canada portal for information on the Prairie Farm Rehabilitation Administration (PFRA) watersheds. Search under the Data section and then under Access Geospatial Products to get access to PFRA Watershed Boundaries .