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Mapping surface cover types using ASTER data

Dr Abdullah Mah
Earth Resource Mapping Pty. Ltd., Australia

ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is an imaging instrument that is flying on the NASA’s Terra satellite launched in December 1999. ASTER acquires 14 spectral bands and can be used to obtain detailed maps of land surface temperature, emissivity, reflectance and elevation. The specifications of the 14 ASTER spectral bands are mentioned in Table 1.

Table 1: 14 Spectral bands of ASTER
Spectral channel   Spectral range   Spatial resolution   Dynamic range   Comment
1 (Visible Green)   0.520 to 0.600um  15m   8 bit integer  Near Infrared has 2 
bands (3B and 3N) 
which are stereo pair 
images from which DEM 
can be generated. 

Orbit: 705km, 10:30am 
descending node, 
Sun-synchronous

Swath=60x 60 km
2 (Visible Red)   0.630 to 0.690um  15m   8 bit integer
3 (Near Infrared)   0.760 to 0.860um  15m   8 bit integer
4 (SWIR)   1.60 to 1.70um  30m   8 bit integer
5 (SWIR)   2.145 to 2.185um  30m   8 bit integer
6 (SWIR)   2.185 to 2.225um  30m   8 bit integer
7 (SWIR)   2.235 to 2.285um  30m   8 bit integer
8 (SWIR)   2.295 to 2.365um  30m   8 bit integer
9 (SWIR)   2.36 to 2.43um  30m   8 bit integer
10 (TIR)   8.125 to 8.475um  90m   12 bit integer
11 (TIR)   8.475 to 8.825um  90m   12 bit integer
12 (TIR)   8.925 to 9.275um  90m   12 bit integer
13 (TIR)   10.25 to 10.95um  90m   12 bit integer
14 (TIR)   10.95 to 11.65um  90m   12 bit integer


Each scene covers 60 x 60km and is daily capturing about 600 scenes. The recorded data exceed the specified signal to noise ratios (Yamaguchi et al, 2001). ASTER data has been used to map silicate and carbonate rocks (Hewson, et al, 2001), and has been used to carry out volcanic studies, urban studies, lithologic mapping, monitoring of coastal environments (Yamaguchi, et al, 2001).

The present paper used 2 dates (22 March, 2001 and 7 April, 2001) ASTER imagery of an area in ASIA to map surface cover types and to demonstrate change detection of vegetated areas.

The 14 ASTER spectral bands are saved into three groups; 3 bands with 15m resolution in VNIR, 6 bands with 30m resolution in SWIR and 5 bands with 90m resolution in TIR They are co-registered to WGS84 and a common NUTM zone.

Mapping surface cover types:

Water:
Reflectance feature of water at Visible Green and absorption feature at NIR (Lillesand and Kieffer, 1994) are used to map surface water. A threshold of 0.4 from the normalised difference ((Gr-NIR)/(Gr+NIR)) image between Visible Green and NIR of the ASTER 22 March 2001 image is used to map surface water. Threshold to map water may vary from a scene to a scene due atmospheric affect and pollution in water. Surface water mapped is shown in blue color in the figure below with a background of the ratioed image in grey scale.



Vegetation:
Normalised Difference Vegetation Index (NDVI) [(NIR-Gr)/(NIR+Gr)] ratio image is used to map vegetation. No atmospheric noise reduction nor decorrelation stretch was applied on the data in mapping the vegetation in image shown below. Threshold used is 0.00432 and may vary due to different type of vegetation from a scene to another. Vegetation is shown in green in the figure below with the ratioed grey scale image shown as background.




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