14
ERDC TR-05-1
instrument to measure the signal representing the downwelling irradiance. Natu-
rally, even in the best case, the measurement is only an approximation of the
downwelling irradiance on the ground owing to various atmospheric effects.
It was assumed that the vendors used Specim software tools to calculate at-
sensor-reflectance values for each pixel using the raw radiance images and the
downwelling solar irradiance data. The reflectance-calibrated data were then
available for analysis and classification.
Hyperspectral imagery was collected over four flights, each flight on a dif-
ferent day. The aircraft maintained an altitude of approximately 3050 m (~9842
ft) above ground level (agl). The nominal spatial resolution of the data is docu-
mented as 3 m (3- 3-m pixels = 9 m2 per pixel).
Parameters set within AISA operating mode B, as described above, produced
hyperspectral imagery with 38 spectral bands. The band center for the minimum
wavelength was at 461 nm, while the maximum band center was at 891 nm. The
width of each of the individual bands varied: bands 1 through 14 maintained a
band width of 4.86 nm, bands 15 through 32 maintained width of 5.2 nm, and
bands 33 through 38 at 5.32 nm.
The CCD output images were acquired at 12-bit radiometric resolution.
Therefore, assuming a minimum digital number (DN) value of 0 (i.e., no re-
flected electromagnetic radiation detected by the CCD), the maximum DN in the
raw imagery during acquisition was theoretically 4095. After reflectance calibra-
tion, the DN's were transformed to a signed 16-bit radiometric quantization. At-
mospheric conditions during image acquisition were not documented. The im-
ages show very few clouds; therefore, it is assumed that skies were clear.
The majority of the images were acquired in either an east-to-west or a west-
to-east flightpath. Several ancillary flightlines were acquired using either north or
south flightpaths. Figure 7 depicts each flightline and its default filename. In
general, the eastwest lines are numbered from 1 to 20, with path 1 at the north
extreme of the study site and path 20 at the south. It is assumed that the trailing
character in the filename (i.e., E, W, N, or S) indicates the direction the aircraft
was flying during acquisition.
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