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Drainage Modeling

The automated analysis system uses a drainage model based on a physical approach. It works by simulating a drainage runoff physically. Each pixel gets an initial water value of one, and for each pixel the path of this simulated fluid is calculated using a steepest descent approach. Drainage along this path is accumulated and if it meets another path already found, its drainage is added to that path downstream of where it met. In this approach, drainage of every pixel in the image is determined. In addition, every pixel in the image has a value that represents the drainage accumulation (area draining) into that pixel. Thresholding the results at different accumulation values yields the drainage delineation that is used for this system. 

Since this is a physically-based approach rather than a pure topographical (shape only) approach, it has the advantage of not only delineating the channels, but providing a measure of the drainage area flowing through them. This is believed to be very important for accurate stream ordering calculations. However, the presence of noise and/or data artifacts still can cause problems. Channels can be interrupted; because of noise or artifacts, even high flow drainage channels can have nowhere to flow. To correct for these difficulties, an algorithm is used for the filling of depression areas in the data.Single pixel pits are filled by raising the height of the single pixel pit to a position halfway between the lowest and the next lowest neighbor.

The drainage modeling produces an accumulated drainage map, Strahler branch ordering map and a watershed map. These maps are used by the knowledge bases in conjunction with the elevation data. The adjacent tables indicate the attribution associated with the computed drainage model.

An example of a Strahler branch order drainage plot overlaid on synthetic shaded relief. Blue is lowest order, with green, orange, and red representing increasingly higher orders.


Network Properties



Drainage Density

Mean channel length/unit area

Stream Frequency

Number of stream segments by order

Bifurcation Ratio

Avg. ratio of Order N to Order N+1

Channel Slope

Slope in drainage channels

Relief Properties




Elev diff between highest and lowest

Relief Ratio

Basin relief / max basin length

Ruggedness Nbr

Basin relief * drainage density

Volume/Area Ratio

Measure of basin irregularity

Watershed Properties




Fundamental Measure

Perimeter Length

Fundamental Measure

Circularity Ratio

Total basin area / circle area with same perimeter

Elongation Ratio

Diameter of same area circle / max basin length

Form Factor

Basin area / basin length squared


Basin length squared/basin area

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