NHN Implementation Strategy
After the NHN Standard was nationally adopted in August 2004, the team from the Centre for Topographic Information Sherbrooke (CTIS) was given the mandate to implement the NHN. The latter is implemented under the leadership of the National Hydro Network Project from the Earth Sciences Sector Contribution to GeoBase Program, Natural Resources Canada (NRCan). Discussions to produce NHN data in partnership were then engaged with some provincial/territorial partners. Agreements have been concluded with British Columbia, Manitoba and Yukon.
The agreement with British Columbia was assigning the responsability of the NHN data production to the province, whereas the other agreements gave this responsibility to CTIS. While British Columbia was developing its own NHN data production process based on using provincial source data, an initial production process was also developed and used by CTIS to produce NHN data in about 40 drainage areas or NHN Work Units elsewhere in the country. It was realized during this initial production that it was difficult to process (assemble or update) all NHN data at once. The most important problems noted were the large volume of features per NHN Work Unit, numerous references to other features (via identifiers), and the presentation of the same information/geometry in different features (e.g. banks vs waterbodies, obstacle features vs obstacle events).
Moreover, the emergence of new constraints required a review of the initial NHN Implementation Strategy. The need for data update, a quick access to a national NHN coverage by users, and a simplified production process to facilitate partner involvement are convincing examples of such constraints. The new NHN Implementation Strategy is now based on the following elements:
- New coverage of 10-20-metre SPOT imagery as a source for updating;
- Implementation by stages (completeness levels);
- Simplified updating model.
The new Medium Resolution Imagery (MRI/GeoBase) project launched in 2005 will fulfill NHN updating needs. The purpose of this project is to cover Canada in a five-year period (2005-2010) with cloud free SPOT orthoimages having a 10-metre resolution for the panchromatic band and 20-metre resolution for multispectral bands. This project is funded by all provinces and territories, several federal agencies and the GeoConnections federal program. The arrival of this new source of information allows the integration of the hydrographic network national update into the new NHN Implementation Strategy.
The new strategy aims at providing users a quick access to NHN data. To meet this objective, an implementation-by-stage approach, called Completeness Level (CL) Approach, was developed. In this new approach, levels are defined and completed in successive production phases, thus making intermediary results available to users. According to this approach, each drainage area or NHN Work Unit will have its own specific level. Each level is backward compatible regarding data content. This implies that a superior level contains all of the data content from a lower level and more. Thus, in regards to content, a lower level is always a subset of a superior one.
This approach was developed in consultation with provincial and territorial partners across the country. This way, users obtain quicker access to NHN data. They also get a clear description of the completeness level status for each drainage area or NHN Work Unit. Four Completeness Levels (CL1 to CL4) were defined under this approach. The NHN Implementation Strategy will progressively allow reaching the required content and structure of the NHN. The NHN Completeness Levels section describes the characteristics of the four NHN completeness levels distributed on GeoBase.