12
Figure 3.5 shows the catchments area which generated from Digital Elevation Model DEM. Delineating process obtained three large watersheds, which overlapped
with Telaga Warna Nature Reserve, as follows: Ciliwung, Cigundul and Cibeet watersheds. However, the area study is limited on the catchments area part of
watershed that has an upper stream region on nature reserve area. Figure 3.5.b. shows that area study has eight catchments that have upper stream region on nature reserve
area.
Figure 3.5. Delineating catchments area
B. Slope Classes and Length Generation
The length and the gradient of the slope have a major influence on the amount of soil erosion that can be occurred. The DEM was generated using surfacing method used
to produce length and gradient of the slope. The length and gradient of slope are illustrated in the figure below:
The example of procedure: interval contour 25 m rise and distance 50 m run, so length of slope is 55.9 m Pythagoras Equation and gradient of slope is 50 .
tan =
run rise
rise
run
Percent of slope =
100 x
run rise
Length of slope
Degree of slopegradient =
Digital Elevation Model
13
n
11
n
12
n
1k
n
1+
n
21
n
22
n
2k
n
2+
n
k1
n
k2
n
kk
n
k+
n
+1
n
+2
n
+k
n Row total
Column total Reference
C lass
if ic
at ion
C. Classification Accuracy Analysis
Essentially, the procedure of accuracy assessment Congalton and Green, 1999; USGS, 2006b can be summarized into two general procedures, i.e.: sampling design and
the assessment of accuracy parameters. a. Sampling Design
Sampling definition
Sampling unit and size total area in observation: 9629.23 ha, including watershed
Sampling technique b. Assessment of the Accuracy Parameters
Figure 3.6. Possible Accuracy Categories of Classification The accuracy parameters involve in building error matrix, estimating accuracy
parameters such as total accuracy, user’s accuracy, and producer’s accuracy Congalton and Green, 1999, KHAT statistic Kappa estimator and its variance. Error matrix
describes the number of sample units collected from reference data assigned to three possible accuracy categories, i.e.: correctly classified, misclassified and unclassified
categories Figure 3.7..
Figure 3.7. Error Matrix Miscl
assified and unclassified category is usually known as commission error user’s accuracy and omission error producer’s accuracy respectively. Misclassified occurs
when an area is included in an incorrect category, but omission error occurs when an area
14
is excluded from the category to which it belongs Congalton and Green, 1999. The design of error matrix is illustrated by Figure 3.7.
The following equations are used to calculate the accuracy parameters Congalton and Green, 1999:
Total Overall Accuracy
Pˆ
:
n n
P
k i
ii
1
ˆ ;
Producer’s Accuracy
A
Pˆ
j jj
j A
n n
P
ˆ
User’s Accuracy
U
Pˆ
:
i ii
Ui
n n
Pˆ
Other accuracy parameter used is Kappa parameter, which estimated by computing KHAT statistic. KHAT value is a measure of agreement accuracy based on the
difference between actual agreement in the error matrix the accuracy between classification and reference data as indicated by the major diagonal and the chance
agreement as indicated by the row and column totals. In short, it is a measure how well the classification agrees with reference data Congalton and Green, 1999. The equation
of KHAT statistic is given below.
k i
i i
i k
i i
k i
ii
n n
n n
n n
n K
1 2
1 1
ˆ
The advantage of using KHAT statistic is that confidence interval around KHAT can be computed. Additionally, it has normal distribution. Therefore, the hypothesis whether
the significance of the agreement between the classification and there reference data is greater than 0 can be formulated and tested. The equation used to compute variance of
KHAT statistic is shown in the following equations Congalton and Green, 1999: n
ii
: diagonal elements of error matrix at i-th row and i-th column i : the i-th row of error matrix
n : total number of sample units
j A
Pˆ : Produce r’s accuracy at j-th class
n
jj
: diagonal elements of error matrix at j-th row and j-th column n
+j
: total number of sample units in j-th column
i U
Pˆ : Produce r’s accuracy at i-th class
n
ii
: diagonal elements of error matrix at i-th row and i-th column n
+i
: total number of sample units in i-th column
15
4
2 2
2 4
2 1
3 2
3 2
1 1
2 2
1 1
1 4
1 1
2 1
2 1
1 1
ˆ var
n K
;
k i
ii
n n
1 1
1
;
k i
i i
n n
n
1 2
2
1
;
k i
i i
ii
n n
n n
1 2
3
1
;
k i
k j
i j
ij
n n
n n
1 1
2 3
4
1
The test statistic for testing the significance of KHAT statistic from a single error matrix is expressed by:
ˆ var
ˆ
1
K K
Z
The null hypothesis is rejected when Z ≥ Z
α2
, where α2 is the confidence level of
the two-tailed Z test.
D. Land Cover Change Analysis
