|
The Basic Grid Analysis toolset consists of functions that are core to most basic Digital Elevation Model (DEM) analyses, and provide inputs to many of the more advanced TauDEM tools. The following table lists the tools in the Basic Grid Analysis toolset followed by a brief description of each.
| Tool | Description |
|---|---|
| Pit Remove | Identifies all pits in the DEM and raises their elevation to the level of the lowest pour point around their edge. Pits are low elevation areas in digital elevation models (DEMs) that are completely surrounded by higher terrain. They are generally taken to be artifacts that interfere with the routing of flow across DEMs, so are removed by raising their elevation to the point where they drain off the edge of the domain. The pour point is the lowest point on the boundary of the "watershed" draining to the pit. Filling pits is not essential if you have reason to believe that the pits in your DEM are real. |
| D8 Flow Directions | Calculates 2 grids. The first contains the D8 Flow directions which are defined, for each cell, as the direction of one of its eight adjacent or diagonal neighbors with the steepest downward slope. This is encoded as: 1 - East, 2 - North East, 3 - North, 4 - North West , 5 - West, 6 - South West, 7 - South, 8 - South East. The second contains the slope, as evaluated in the direction of steepest descent and is reported as drop/distance, i.e. tan of the angle. Flow direction is reported as "no data" for any grid cell adjacent to the edge of the DEM domain, or adjacent to a no data value in the DEM. In flat areas flow directions are assigned away from higher ground and towards lower ground using the method of Garbrecht and Martz (1997). The D8 flow direction algorithm may be applied to a DEM that has not had pits removed, but will then result in "no data" values for flow direction and slope associated with the lowest point of the pit. |
| D-Infinity Flow Directions | Assigns a flow direction based on the steepest slope of a triangular facet (Tarboton, 1997). Flow direction is defined as steepest downward slope on planar triangular facets on a block centered grid. Flow direction is encoded as an angle in radians counter-clockwise from east as a continuous (floating point) quantity between 0 and 2 pi. The flow direction angle is determined as the direction of the steepest downward slope on the eight triangular facets formed in a 3 x 3 grid cell window centered on the grid cell of interest. Tool also produces a slope grid using the D-infinity method. |
| D8 Contributing Area | Calculates a grid of contributing areas using the single direction D8 flow model. The contribution of each grid cell is taken as one (or when the optional weight grid is used, the value from the weight grid). The contributing area for each grid cell is taken as its own contribution plus the contribution from upslope neighbors that drain in to it according to the D8 flow model. If the optional outlet point feature is used, only the outlet cells and the cells upslope (by the D8 flow model) of them are in the domain to be evaluated. By default, the tool checks for edge contamination. This is defined as the possibility that a contributing area value may be underestimated due to grid cells outside of the domain not being counted. This occurs when drainage is inwards from the boundaries or areas with no data values for elevation. The algorithm recognizes this and reports "no data" for the contributing area. It is common to see streaks of "no data" values extending inwards from boundaries along flow paths that enter the domain at a boundary. This is the desired effect and indicates that contributing area for these grid cells is unknown due to it being dependent on terrain outside of the domain of data available. Edge contamination checking may be turned off in cases where you know this is not an issue or want to ignore these problems, if for example, the DEM has been clipped along a watershed outline. |
| D-Infinity Contributing Area | Calculates a grid of contributing area using the multiple flow direction D-infinity approach. D-infinity flow direction is defined as steepest downward slope on planar triangular facets on a block centered grid. The contribution at each grid cell is taken initially as the grid cell length (or when the optional weight grid is used, from the weight grid). The contributing area of each grid cell is then taken as its own contribution plus the contribution from upslope neighbors that have some fraction draining to it according to the D-infinity flow model. The flow from each cell either all drains to one neighbor, if the angle falls along a cardinal (0, pi/2, pi, 3pi/2) or ordinal (pi/4, 3pi/4, 5pi/4, 7pi/4) direction, or is on an angle falling between the direct angle to two adjacent neighbors. In the latter case the flow is proportioned between these two neighbor cells according to how close the flow direction angle is to the direct angle to those cells. When the optional weight grid is not used, the result is reported in terms of specific catchment area, the upslope area per unit contour length, taken here as the number of cells times grid cell length (cell area divided by cell length). This assumes that grid cell length is the effective contour length, in the definition of specific catchment area and does not distinguish any difference in contour length dependent upon the flow direction. When the optional weight grid is used, the result is reported directly as a summation of weights, without any scaling. Using the optional outlet feature results in only the area contributing to flow at the outlet points in the feature being accumulated. By default, the tool checks for edge contamination. This is defined as the possibility that a contributing area value may be underestimated due to grid cells outside of the domain not being counted. This occurs when drainage is inwards from the boundaries or areas with "no data" values for elevation. The algorithm recognizes this and reports "no data" for the contributing area. It is common to see streaks of "no data" values extending inwards from boundaries along flow paths that enter the domain at a boundary. This is the desired effect and indicates that contributing area for these grid cells is unknown due to it being dependent on terrain outside of the domain of data available. Edge contamination checking may be turned off in cases where you know it is not an issue or want to ignore these problems, if for example, the DEM has been clipped along a watershed outline. |
| Grid Network | Creates 3 grids that contain for each grid cell: 1) the longest upslope path length, 2) the total upslope path length, and 3) the Strahler order number. These values are derived from the network defined by the D8 flow model. The longest upslope path length is the length of the flow path from the furthest cell that drains to each cell. The total upslope path length is the length of the entire grid network upslope of each grid cell. Lengths are measured between cell centers taking into account cell size and whether the direction is adjacent or diagonal. Strahler order is defined as follows: A network of flow paths is defined by the D8 Flow Direction grid. Source flow paths have a Strahler order number of one. When two flow paths of different order join the order of the downstream flow path is the order of the highest incoming flow path. When two flow paths of equal order join the downstream flow path order is increased by 1. When more than two flow paths join the downstream flow path order is calculated as the maximum of the highest incoming flow path order or the second highest incoming flow path order + 1. This generalizes the common definition to cases where more than two flow paths join at a point. Where the optional mask grid and threshold value are input, the function is evaluated only considering grid cells that lie in the domain with mask grid value greater than or equal to the threshold value. Source (first order) grid cells are taken as those that do not have any other grid cells from inside the domain draining in to them, and only when two of these flow paths join is order propagated according to the ordering rules. Lengths are also only evaluated counting paths within the domain greater than or equal to the threshold. If the optional outlet point feature is used, only the outlet cells and the cells upslope (by the D8 flow model) of them are in the domain to be evaluated. |