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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?

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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**• Slope measures the steepness of the surface at any particular location. • Slope is often measured in degrees or in percent rise**Aspect**• 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.**Raster based GIS grid model**• 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**Calculate east-west gradient:**• ∆X = ((ZC + 2*ZG +ZK) – ZA+2*ZE+ZI))/8*cell size • Calculate north-south gradient: • ∆Y= ((ZA + 2*ZB +ZC) – ZI+2*ZJ+ZK))/8*cell size**Slope & Aspect**tan(Slope(F)) = squrt((∆x)2+ (∆Y)2) tan(Aspect(F)) = ∆x / ∆Y**Aspect**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.**Flow Directions**Eight Direction Pour Point Model Elevation Grid Flow Direction Grid**Flow Accumulation**• Flow accumulation is calculated from flow directions. • It calculates the number of grid cells drain into an individual grid cell.**Flow Accumulation**• 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.**Watershed Delineation**• Watersheds are delineated from the flow direction layer. It is the area flowing into a given outlet or pour point.**Stream network delineation**• 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

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