Geoscience Reference
In-Depth Information
3.1 Concept of D16
The designed D16 algorithm uses almost 5 9 5 pixel neighbour resolutions that
will provide 22.5 degree (22.5) for each neighbour direction as in Fig. 9 a, which
is half of the angle range for D8 method. This will produce 50 % increment in the
available flow direction options compared to the D8 method. However, due to the
restriction introduced, it might have less than 40 % chances on flow direction to
select these new additional eight directions.
The additional rules and equations are introduced to enhance the result of D8
algorithm. The new features or sub-algorithms embedded in D16 are:
1. Additional of eight new directions option from D8 algorithm.
2. Using D8 method for the first 3 9 3 pixel resolution for second outer pixels
3. Introduction of Intermediate Factor (IF) to restrict and filter the flow direction
to the eight new added directions, illustrated in Fig. 9 b.
4. Implementation of Sink and Stagnant formula.
3.1.1 Deriving D16 Algorithm Formulae
Since it is raster based, the array format in programing is the most suitable method
to illustrate the derived formulae. The main formulae involved in the computation
are:
r
X 1 X 2
h
i
Þ 2 þ Y 1 Y 2
Þ 2
Distance
:
ð
ð
ð 1 Þ
derived from Distance Weighted ; w ðÞ ¼1 = d p
:
(NCGIA 1988-2013 )
Height difference
: Z a ¼ Z n Z D16
ð 2 Þ
Intermediate Factor
: Height intermediate ; Z IF ¼ Z a þ Z b
ð
Þ= 2
ð 3 Þ
: if Z IF \ Z n ; the test is pass :
Stagnant
: Z a ð max1 Þ [ 0 : 001 and Z a ð max2 Þ \0 : 001
and Z a ð max3 Þ [ 0 : 0001 and Z a \ 0 : 3
ð 4 Þ
Sink
: Z a ¼ nil or Z a [ Stagnant ð 0 : 3 Þ
ð 5 Þ
The values for stagnant formulae ( 4 ) are not a constant numbers, it can be
varied depend on users or applications requirement of expected result. However,
the values of the stagnant formulae were fixed at certain constants in this study for
the purpose of general comparison testing on the swamp area aspect.
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