that the visibilities are higher towards the 1000 m mountains to the southwest and lower towards the 50 m 
elevation of the populated regions to the northeast. 
3 - PRESSURE ELEVATION 
To compensate for well mixed gas absorption and molecular scattering in the atmosphere, an estimate of the 
surface pressure is required An algorithm has been developed to estimate the surface pressure (Green et al., 
1991b) based on the 760 nm oxygen band absorption in spectrum the AVIRIS measured radiance. Currently the 
algorithm uses a NLLSSF between the AVIRIS measured MODTRAN modeled in the region of oxygen band 
absorption. The oxygen absorption strength is calibrated to surface pressure using the oxygen band model in 
MODTRAN. Parameters constraining the pressure and the reflectance in the 760 nm spectral region are 
allowed to vary in the fit. Figure 5 shows the fit between the AVIRIS measured spectrum and the NLLSSF 
MODTRAN spectrum for the polo field in the region of oxygen absorption. The pressure can be presented as 
equivalent surface elevation for the entire Jasper Ridge data set. To a first order this AVIRIS estimated surface 
pressure height agrees with the topography for the region. 
Figure 5 : Estimation of surface pressure height from a NLLSSF algorithm applied to the 760 nm oxygen band 
absorption measured in the AVIRIS radiance spectrum for the polo field. 
4 - WATER VAPOR 
Over the most of the 400 to 2500 nm spectral range the strongest atmospheric gas absorber is water vapor. In 
the terrestrial atmosphere, water vapor varies both spatially and temporally up to an order of magnitude, to 
derive the water vapor abundance from AVIRIS radiance spectra algorithms have been developed (Conel et al., 
1988; Green et al 1989; Green et al., 1991a) band on the LOWTRAN 7 (Kneizys et al., 1987) and 
MODTRAN radiative transfer codes. The current algorithm uses a NLLSSF between a AVIRIS measured and a 
8 MODTRAN modeled radiance spectrum across the 940 nm water vapor absorption band. In addition to water 
vapor, parameters describing the reflectance magnitude, the reflectance spectral slope, and the surface leaf 
water absorption are used. Figure 6 shows the fit between the AVIRIS measured and the NLLSSF spectrum for 
the 940 mn water vapor absorption over the polo field. Over vegetated region leaf water absorption overlapping 
the 940 nm water vapor absorption must be compensated. Figure 7 shows the fit for the golf course and figure 8 
gives the required leaf water absorption to achieve this fit. 
11 
< 
E 
9 
O 
AVIRIS Measured Radiance 
s 
c 
MODTRAN Modeled Radiance 
Zero line 
Residual 
-1 
725 
750 
775 
800 
Wavelength (nm)