• A so-called “hydrologic correction” has been applied to a regional digital elevation model (DEM) of our study area
  • this has enabled for the ability to automatically delineate contributing areas to any point in our jurisdiction.

Digital Elevation Model

Ground surface elevations were collected from the 2006 version of the Provincial (Ministry of Natural Resources) Digital Elevation Model (DEM). This is a 10x10m² DEM derived from 1:10,000 scale OBM base mapping based on photogrammetic elevation points and 5m contours where the photogrammetic elevation points did not exist. An up-scaled 50x50m² DEM is produced by merging the tiles shown below:

MNR 2006 Provincial DEM tiles shown in green MNR 2006 Provincial DEM tiles shown in green.

Elevation data were up-scaled by taking the average of known elevations occurring within every 50x50m² cell. The resulting grid is a 5000x5000 50m-uniform cell grid, with an upper-left-corner origin at (E: 545,000, N: 5,035,000) NAD83 UTM zone 17.

Hydrological “correction”

An automated topological topological analysis is performed on the DEM using the following methodologies:

  1. Automated depression filling (Wang and Liu, 2006) was applied to the DEM. This filtering of elevation data ensures that every grid cell has at least 1 neighbouring cell with an assigned elevation at or below the current cell’s elevation. This ensures that “drainage” is never impeded.
  2. While the above code works for most of the area, it will leave flat (zero-gradient) regions especially around lakes and wetlands. A fix by Garbrecht and Martz (1997) was added to ensure a consistent flow direction with negligible change to the corrected DEM.
  3. Flow paths are then computed based on the “D8” algorithm (O’Callaghan and Mark, 1984).

Cell slopes and aspects are computed using a 9-point planar regression from the cell’s elevation plus its 8 neighbouring grid elevations. The final product is here termed the Hydrologically corrected Digital Elevation Model (HDEM).

Manual adjustments

While automated hydrological correction is quite powerful when applied to the Provincial DEM, there are in rare places where the algorithm fails to capture mapped flow paths (usually in flatter rural regions close to embanked roads). Fortunately, these errors can be easily corrected by imposing flow directions using hand-drawn flow paths. Flow paths are save as polylines, where its vertices are ordered according to flow direction.

With the current version (v.2020), 10 flow corrections have been imposed and are saved in a set of shapefiles. There is at the moment 2 new flow path correction in queue and will be imposed for the next release. This is to say that this layer and its derivatives are continually being updated.