Image Processing Reference
In-Depth Information
atmospherically corrected using a similar approach to that described above for
ASTER data. Data for September 19, 2000, or for date ranges including this day
(Table
12.1
), were downloaded for comparison with spatial metrics calculated from
the ASTER classified data. The MODIS subsets were georeferenced to the UTM
coordinate system using nearest-neighbor resampling. The data number (DN) values
for each dataset were then converted to the correct units using appropriate scaling
factors in a commercial image-processing software environment.
12.2.3
Land Cover Classification of ASTER Data
The 15 m/pixel VNIR ASTER data (bands 1 - visible green, 2 - visible red, and
3 - near infrared) were used as the initial base data for land cover classification.
These bands were chosen as they provide the highest spatial resolution. The six 30
m/pixel shortwave infrared bands were also assessed for their usefulness in land
cover classification, but no significant improvement in discrimination of urban
classes was observed. This observation is in agreement with the results of Zhu and
Blumberg (
2002
), who found that the VNIR ASTER bands performed better
than the shortwave bands in classification of the Beer Sheva, Israel urban area. The
classification approach presented here is broadly similar to work previously per-
formed for the Phoenix metropolitan area using Landsat TM data by Stefanov et al.
(
2001b
), and was also used in an associated multiscalar investigation of ASTER
and MODIS data for urban landscape structure characterization (Stefanov and
Netzband
2005
).
An initial minimum distance to means (MDM; Jensen
1996
) supervised classi-
fication was performed on the ASTER mosaic using 16 classes: Desert Soil, Low
Vegetation; Desert Soil, Vegetated; Bedrock; Fluvial Sediments; Bare Soil; Fallow
Agricultural Soil; Water; Canopied Vegetation; Grass; Riparian Vegetation; Active
Agricultural Vegetation; Mesic Built Materials; Xeric Built Materials; White
Rooftops; Blue Rooftops; and Asphalt. The term “mesic” refers to land cover types
with significant vegetation in the form of grass, shrubs, and canopied woody plants.
Xeric land cover types are typified by little to no grass or shrub cover and open-
canopy plant types with significant bare rock and soil (i.e. similar to equatorial
deserts). The MDM classification was run using each training sub-area as a separate
class, followed by aggregation of the results into the original 16 classes.
The ASTER mosaic was also used to calculate a Normalized Difference
Vegetation Index (NDVI: Botkin et al.
1984
; Tucker
1979
). This index highlights
actively photosynthesizing vegetation by comparing reflectance values in the visible
red (low for vegetation) and near infrared (high for vegetation) bands. The index is
calculated using:
(Band
3
−
Band )
2
NDVI
=
Band )
(12.1)
(Band
3
+
2
