Detecting forest areas and crops using vegetation indices
2.0 Study area and data acquisition
The Bukit Kajang Forest Reserve and its surrounding agricultural areas which cover a ground area approximately 225 square km was selected as the study site. The areas is equivalent to 512x512 pixels of the TM image. The area is located approximately 25 km to the north-west of Raub, Pahang. This region is cultivated with major cover types such as oil palm, rubber, cocoa, banana and forest area. The location map of the study areas is shown in Figure 1.
Figure 1. Location map of study area
A Landsat Thematic Mapper ™ scene (WRS 127/57, A4) acquired on 15 June 1989 covering the study areas was used. This particular scene was chosen because it was the best quality data available and contains most of the major crops including forest areas. Other ancillary data such as Landuse maps, topographical maps, and related information were used in the study.
3.0 Data processing and calculation of VI
The image processing was carried out using the Intergraph IP225 system and the PCI EASI/PACE system available at the Centre for Remote Sensing, Universiti Teknologi, Malaysia. Contrast and linear stretching, band combination and image filtering were carried out to the satellite data to enhance vegetation area. The geometric rectification was performed using second order polynomial transformation with geometric rectification was performed using second order polynomial transformation with thirteen GCPs to sub pixel accuracy. The nearest neighbourhood resampling technique was then applied to the data since this technique will maintain the original DNs. Later the band combination of 5,4, and 3 was used because it has been found to be good for most forest and vegetation surveys where the absorptive and reflective properties of the vegetation are of importance.
The VIs were calculated by using the Complex Arithmetic Algorithm task on the Intergraph IP225 system.
The VI algorithms used were
RVI = NIR (1)
VIS
NDVI = ( NIR-VIS) (2)
( NIR + VIS)
PVI = √( RggNIR-RpNIR)2 = ( RggVIS-RpVIS)2 (3)
In the above equations , VI values are also obtained by replacing The NIR band with MIR band. For the PVI equation, the mean DNs for soil were found to be 47, 50, 72, 102, and 48 for TM bands 2,3,4,5, and 7 respectively. These values are assumed to be content throughout the areas since the soil in the areas is of the same type. All TM bands except TM1 and TM6 have been used to derive the VIs from equations (2) , (3) and (4).
Various band combinations were analysed and the results from some of the combinations are given in Tale 1. Figure 2 shows the PVI map of the study area from bands 5 and 2 of the Landsat TM.
Figure 2. Perpendicular Vegetation Index of study area from bands 5 and 2 of Landsat TM
Table 1: VI ranges for different types of vegetation and soil in study areas using selected band combinations
| VI |
FOREST |
OIL PALM |
RUBBER |
COCOA |
BARE SOIL |
| RVI ( TM4,3) |
2.73 – 3.47 |
3.29 -3.68 |
2.96-3.36 |
2.59 -3.39 |
1.07-1.80 |
| NDVI (TX4,3) |
0.48-0.55 |
0.53-0.57 |
0.50.0.54 |
0.44-0.55 |
0-0.27 |
| NDVI (TM5,2) |
0.24-0.31 |
0.30-0.34 |
0.39-0.43 |
0.36 -0.43 |
0.33 – 0.47 |
| PVI (TM4,3) |
24-31 |
31-37 |
27-34 |
25-39 |
0-15 |
| PVI (TM5,2) |
48-52 |
42-48 |
25-33 |
18-25 |
0-24 |
| PVI (TM5,3) |
49-59 |
45-50 |
28-36 |
27-37 |
0 – 27 |
