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# Why calculate slope and Aspect - PowerPoint PPT Presentation

Why calculate slope and Aspect?. S tudy the flow of water I dentify the habitats of plants Identify potential sites for urban growth D rainage patterns on digital elevation models (DEMs) E nvironmental modeling. Slope.

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### Why calculate slope and Aspect?

Study the flow of water

Identify the habitats of plants

Identify potential sites for urban growth

Drainage patterns on digital elevation models (DEMs)

Environmental modeling

• Slope measures the steepness of the surface at any particular location.

• Slope is often measured in degrees or in percent rise

• Aspect measures the direction of steepest slope for a location on the surface.

• It is usually measured in degrees, where 0 degrees is due north, 90 degrees is due east, 180 degrees is due south, and 270 degrees is due west.

• Using the elevations of the surrounding eight points in a 3 x 3 neighborhood.

• To calculate the slope and aspect for cell “F” needs elevations at point A, B, C, G, K, J, I, E

• ∆X = ((ZC + 2*ZG +ZK) – ZA+2*ZE+ZI))/8*cell size

• ∆Y= ((ZA + 2*ZB +ZC) – ZI+2*ZJ+ZK))/8*cell size

tan(Slope(F)) = squrt((∆x)2+ (∆Y)2)

tan(Aspect(F)) = ∆x / ∆Y

Aspects indicate the steepest downslope direction

Aspects are measured clockwise

Hydrologic Slope- Direction of Steepest Decent

• Assign a flow direction to each grid cell based on the neighboring cell with the lowest elevation.

Eight Direction Pour Point Model

Elevation Grid

Flow Direction Grid

• Flow accumulation is calculated from flow directions.

• It calculates the number of grid cells drain into an individual grid cell.

• The flow accumulation grid has a value for each cell that represents the number of the number of cells upstream from that cell.

• Cells with higher values will be located in drainage channels rather than hillsides or ridges.

• Watersheds are delineated from the flow direction layer. It is the area flowing into a given outlet or pour point.

• Flow accumulation threshold value defines the minimum or number of cells drainage area upstream necessary to define a stream.

Cell size = 100 x 100 m

Each cell represents 10,000 m2 in area.

If delineate the streams for a 10,000 cell flow accumulation threshold, the minimum drainage area upstream will be:

10,000 x 10,000 = 100,000,000 m2 or 100 km2