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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
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
A
E
I
M
B
F
C
G
D
H
J
N
K
O
L
P
1. Calculate east-west gradient:
∆X = ((ZC + 2*ZG +ZK) – ZA+2*ZE+ZI))/8*cell size
2. 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
A(80) B(74) C(63) D(89)
E(69) F(67) G(56) H(78)
I(60)
J(52) K(48) L(60)
M(59) N(49) O(47) P(40)
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
32
64
16
120
103
102
1
164
8
132
128
4
2
110
Flow Direction Grid
110
100
107
2
4
8
2
4
8
1
4
16
Flow Accumulation
120
103
164
132
102
110
110
100
107
•Flow accumulation is
calculated from flow
directions.
•It calculates the
number of grid cells
drain into an individual
grid cell.
Flow Accumulation
120
103
16
0
0
0
132
102
110
0
3
0
110
100
107
0
7
0
•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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