A threshold ΔNDVI value needed to be carefully decided upon to successfully capture true changes in NDVI between the two images from noise. It was decided upon a change threshold of two standard deviations from the mean to represent a true change in NDVI value. This was calculated within Idrisi. The two NDVI images were overlaid and the Idrisi OVERLAY function was selected to subtract the earlier image from the later image:
This would create a new image that included all change between pixels, no matter how small. A sensible threshold needed to be used in order to establish true changes in NDVI values from noise.
The image properties were inspected within Idrisi and a histogram generated.
The mean and standard deviation of the image would be utilised to assess the amount of change. A threshold of two standard deviations either side of the mean was used to represent true change and the data was reclassed into three categories using the RECLASS module. In the example above, the mean value is 0.009 and the standard deviation is 0.047. The three classes are therefore:
Increase in NDVI: > 0.103
No Change: Between -0.085 and 0.103
Decrease in NDVI: < -0.085
This process was completed for all of the change epochs and the data was inspected to ensure that this was a correct threshold that accurately captured change.
The 304 Austin census tracts are displayed below. The size and amount of change are given in kilometres squared. Negative values represent vegetation increase and positive values represent urban growth
Tract No.
|
Census tract size (sq/km)
|
|
Change: 88-95 (sq/km)
|
Change: 95-03 (sq/km)
|
Change: 03-10 (sq/km)
|
1
|
3.85
|
0.3339
|
0.774
|
-0.0297
|
2
|
12.24
|
-0.225
|
2.2689
|
-0.2448
|
3
|
14.71
|
0.162
|
-0.8244
|
0.2718
|
4
|
24.38
|
0
|
-0.0999
|
1.188
|
5
|
12.63
|
0.0126
|
0.063
|
0.621
|
6
|
5.70
|
0.0315
|
0.1134
|
0.1773
|
7
|
6.08
|
-0.0432
|
0.0099
|
0.909
|
8
|
43.01
|
0.6147
|
3.0807
|
0.567
|
9
|
10.32
|
-0.2466
|
3.0618
|
1.0773
|
10
|
3.68
|
-0.2745
|
0.7578
|
0.1368
|
11
|
3.09
|
0.1647
|
0.5157
|
0.0261
|
12
|
4.03
|
0.189
|
0.9936
|
0.1719
|
13
|
7.48
|
-1.3689
|
1.2294
|
-0.1719
|
14
|
8.61
|
-0.8064
|
0.5661
|
2.0889
|
15
|
6.16
|
0.2313
|
1.2051
|
1.0701
|
16
|
4.72
|
0.0747
|
0.5229
|
0.18
|
17
|
4.17
|
0.009
|
0.4149
|
0.4077
|
18
|
1.59
|
0.0981
|
0.2061
|
0.1305
|
19
|
4.45
|
0.2043
|
0.3186
|
0.747
|
20
|
2.62
|
0.1854
|
0.243
|
0.0243
|
21
|
145.97
|
-0.0243
|
0.0432
|
1.0341
|
22
|
4.74
|
-0.0774
|
0.6696
|
0.3114
|
23
|
15.65
|
-0.8163
|
0.3852
|
0.0603
|
24
|
15.79
|
-0.6462
|
0.9261
|
0.5283
|
25
|
8.85
|
0.0774
|
0.2223
|
1.5327
|
26
|
16.81
|
-0.1485
|
0.7371
|
0.5373
|
27
|
4.58
|
0.0045
|
0.9675
|
0.0837
|
28
|
5.85
|
-0.0054
|
0.657
|
0.7353
|
29
|
9.91
|
0.6462
|
4.023
|
0.2331
|
30
|
1.50
|
-0.0009
|
0.1413
|
0.0747
|
31
|
13.28
|
-0.8505
|
0.6246
|
0.2772
|
32
|
27.88
|
-0.1017
|
0.0459
|
0.7614
|
33
|
15.46
|
-0.0207
|
0.4707
|
0.1368
|
34
|
1.57
|
0.0018
|
0.0657
|
0.2655
|
35
|
3.06
|
-0.0126
|
0.4158
|
-0.009
|
36
|
2.01
|
-0.1332
|
0.2754
|
0.0648
|
37
|
2.63
|
-0.0243
|
0.1224
|
-0.2196
|
38
|
4.38
|
-0.0783
|
0.6219
|
0.0567
|
39
|
2.50
|
-0.0234
|
0.1899
|
0.0189
|
40
|
9.29
|
0.0144
|
1.629
|
0.144
|
41
|
3.00
|
0.0153
|
0.2421
|
0.0567
|
42
|
2.78
|
-0.0423
|
0.7974
|
0.2349
|
43
|
15.44
|
0.0171
|
0.7128
|
1.4004
|
44
|
4.56
|
0.1053
|
0.0576
|
0.4068
|
45
|
10.56
|
-0.2034
|
0.3339
|
0.8748
|
46
|
61.53
|
0
|
0
|
0
|
47
|
79.83
|
0.0576
|
0.3087
|
0.0774
|
48
|
137.22
|
0
|
0
|
0
|
49
|
77.32
|
0.2853
|
3.8214
|
1.4211
|
50
|
65.34
|
-0.1053
|
1.3572
|
0.2997
|
51
|
27.86
|
0.0027
|
-0.0018
|
0.3186
|
52
|
33.80
|
-0.3339
|
0.5688
|
0.0207
|
53
|
45.57
|
-0.0756
|
0.0819
|
0.5265
|
54
|
21.50
|
0.2214
|
0.0603
|
0.5436
|
55
|
10.93
|
0.1854
|
0.4167
|
1.8504
|
56
|
1.38
|
-0.0567
|
0.0333
|
0.1179
|
57
|
1.61
|
-0.027
|
-0.0252
|
0.1737
|
58
|
5.40
|
0.2214
|
0.5868
|
0.9774
|
59
|
3.52
|
0.1422
|
0.243
|
0.4041
|
60
|
5.95
|
-0.0567
|
0.603
|
1.6587
|
61
|
4.87
|
0.4014
|
0.3744
|
0.4365
|
62
|
7.19
|
0.819
|
1.0719
|
0.2187
|
63
|
7.19
|
-0.3204
|
0.8496
|
1.0629
|
64
|
4.92
|
-1.0062
|
0.7767
|
0.1494
|
65
|
10.06
|
0.2484
|
1.3887
|
1.6461
|
66
|
1.50
|
-0.243
|
0.1962
|
0.0666
|
67
|
3.87
|
-0.7587
|
0.3627
|
0.1332
|
68
|
5.02
|
-0.27
|
0.7038
|
0.2349
|
69
|
10.54
|
0.1251
|
0.3258
|
-0.0585
|
70
|
3.21
|
0.072
|
0.7821
|
0.2232
|
71
|
2.60
|
-0.1035
|
0.1566
|
0.0765
|
72
|
2.95
|
0.0486
|
0.252
|
0.1188
|
73
|
16.30
|
-1.4499
|
3.9825
|
0.3348
|
74
|
14.30
|
-0.8037
|
2.8269
|
0.7695
|
75
|
13.25
|
-0.8532
|
2.0376
|
0.7992
|
Share with your friends: | 
