The TRIM Watershed Atlas Project

(A glossary of technical terms can be found at the bottom of this page)

The objective of the TRIM Watershed Atlas (TWA) project was to develop, using a fully automated process, a comprehensive database of watershed features derived from the existing TRIM 1:20,000 digital base map of British Columbia. The TWA has been realized using an object-relational database management system (ORDBMS) to produce and manage a database that contains:

• a single-line stream network,
• associated planimetric features,
• a hierarchy of watershed polygons (one per stream segment),
• watershed boundaries (heights-of-land),
• a non-gridded DEM, and
• links to associated attribute data held in other databases.

Stored in geographic coordinates (latitude, longitude, height) for seamless coverage, the single-line stream network is intelligent; e.g., if a point on a stream is selected, TWA can inform the user what is upstream or downstream. Functionality is also provided to generate gridded slope-elevation-aspect data from the TWA linework and DEM.

The TWA will support a wide variety of applications in various Provincial Ministries, Federal Departments, Crown Corporations, and private sector firms and agencies. Representatives from these groups were consulted regarding their intended uses of the TWA, during the design phase of this project.

One use of the TWA data will be to establish a common set of heights-of-land line work, which can be used for establishing the position of boundaries. The determination of a single set of line work will greatly reduce duplication of effort and discrepancies that currently arise during boundary interpretation by different groups. Additionally, a large number of hydrological applications will be supported by the TWA database. One method of access to the TWA will be through the Web, via an extensive range of display and query facilities. Example queries could include:

• from this point, into what stream segment will water flow?
• from this point on a stream, is there a downstream dam?
• from this point on a stream, what is the catchment area?

When the application requires more intensive analysis, users will be able to select and download data, in SAIF, for a given geographic region, and then work with this data locally (e.g., on a GIS).

The reason for using an ORDBMS in this project is to achieve tight integration of the spatial and attribute data, while enforcing rules to ensure that the data conform to high standards of quality, reliability and consistency. (The particular ORDBMS product chosen for the TWA project is IBM Informix Dynamic Server). The base dataset contains almost 200 million DEM points, several hundred thousand breaklines and associated planimetric features, and approximately 16 million stream segments. Achieving reasonable response times on spatial queries, and managing this large dataset reliably, require user-defined geometric data types, complex versioning schemes, and sophisticated spatial indexing.

The TRIM Watershed Atlas Project Glossary

TRIM: Terrain Resource Information Management. The digital base map of British Columbia. Nominal scale, 1:20,000. Dataset includes a non-gridded DEM.

watershed: A geographical area that drains into a particular stream, river, or other form of water body. A stream or river network will have an associated watershed. A river network watershed will be made up of a collection of sub-watersheds, one for each tributary and stream in the network. There is a hierarchical ordering to these watersheds.

digital base map: A metric (measurements can be made from it) representation of the ground, at a given scale, as seen from an aerial view. Typically, this representation includes: streams, lakes, roads, contours, buildings or groups of buildings, etc. In the past, base maps were a paper-based product, but now they are typically kept in digital form.

ORDBMS: Object-Relational Database Management System. In essence, a marriage of the SQL-based relational world and the object world. Allows the definition of new abstract data types (ADT), definition of classes of new complex data types using inheritance and polymorphism, rule definition (active database technology), and SQL-based access to the data including user-defined indexing. The ideal tool for building a spatial database.

single-line stream network: A directed acyclic graph defining which streams and lakes (represented as a network of inflows and outflows) drain into which other streams and lakes.

planimetric features: Geographic features whose two-dimensional representations have significance (unlike points and surfaces). These include all man-made features such as roads, buildings, fences, etc., as well as natural features such as streams, lakes, swamps, etc.

watershed polygons: Polygons describing the area on the ground from which water will flow into the defining stream system.

heights-of-land: A portion of a watershed boundary. Often used in defining the legal definition of a land parcel.

DEM: Digital Elevation Model. A series of points and breaklines (such as ridges) defining the Earth's surface. In TRIM, the DEM data consist of individual (x,y,z)-tuples, as well as some ridge lines, streams and certain other linear features.

attribute data: A characteristic of a planimetric feature. For example, whether a road is paved or not, the type of fish in a stream, the owner of a given well site, etc.

catchment area: The entire area from which drainage is received by a river system. It is also referred to as a basin. Any point on a stream will define an upstream catchment area, whereas watersheds are typically defined only at confluences (places where streams meet).

SAIF: Spatial Archive and Interchange Format. A language for modeling geographic data and a vendor-neutral format for archiving and distributing such data. Developed as a means of sharing spatial and spatiotemporal information. SAIF is designed to facilitate interoperability, particularly in the context of data exchange. SAIF follows a multiple inheritance, object-oriented paradigm.

GIS: Geographic Information System.

tight integration of spatial and attribute data: Traditionally, GIS architecture stores the geometric description of a feature separately from the attribute data. This leads to problems with referential integrity (keeping the correct version of the attribute data linked to the same version of the geometric data) and limits access to the data. Users can only access the data through the graphical user interface supplied by the GIS. True spatial databases provide user-defined complex data types which include both spatial and non-spatial data, eliminating referential integrity problems and access limitations.

spatial queries: Queries to a database or GIS in which the returned data are constrained by some spatial description (e.g., return all points within 100 meters from the well site at location (49:32:12.231N, 123:42:45.693W)).

geometric datatypes: The base types used to store the descriptions of features on the ground. Typically these include points, lines, and polygons, but can also include networks, trees, regular tilings, coverages, observation lists, etc.

versioning schemes: As new mapping is carried out in an area which has already been mapped, some features will be remapped. Versioning schemes are procedures for managing these changes and allowing reconstruction of the data at a particular point in time.

spatial indexing: A method of accessing geometric data that avoids sequential search by using the actual location (coordinates) as a key.