Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were acquired from the Bishop, CA area, located at the northern end of the Owens Valley, on July 30, 1987. Radiometrically-corrected AVIRIS data were flat-field corrected, and spectral curves produced and analyzed for pixels taken from both native and cultivated vegetation sites, using the JPS SPAM software program and PC-based spreadsheet programs. Analyses focussed on the chlorophyll well and red edge portions of the spectral curves. Results include the following: AVIRIS spectral data are acquired at sufficient spectral resolution to allow detection of blue shifts of both the chlorophyll well and red edge in moisture-stressed vegetation when compared with non-stressed vegetation; a normalization of selected parameters (chlorophyll well and near infrared shoulder) may be used to emphasize the shift in red edge position; and the presence of the red edge in AVIRIS spectral curves may be useful in detecting small amounts (20 to 30 pct cover) of semi-arid and arid vegetation ground cover. A discussion of possible causes of AVIRIS red edge shifts in respsonse to stress is presented

Defeo, N. J.

Elvidge, Christopher D.

Rock, B. N.

English

NASA Technical Reports Server

ASSESSMENT OF AVIRIS DATA FROM VEGETATED SITES IN THE OWENS 
VALLEY, CALIFORNIA 
ROCK, B. N., Complei Systems Research Center, University of New 
Hampshire, U. S. A.; ELVIDGE, C. D., Desert Research Institute, 
University of Nevada, U. S. A.; and DEFEO, N. J., Complex Systems 
Research Center, University of New Hampshire, U. S. A. 
ABSTRACT 
Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data 
were acquired for the Bishop, CA area, located at the northern 
end of the Owens Valley, on July 30, 1987. Radiometrically- 
corrected AVIRIS data were flat-field corrected, and spectral 
curves produced and analyzed for pixels taken from both native 
and cultivated vegetation sites, using the JPL SPAM software 
program and PC-based spreadsheet programs. Analyses focussed on 
the chlorophyll well and red edge portions of the spectral 
curves. Results include the following: AVIRIS spectral data are 
acquired at sufficient spectral resolution to allow detection of 
Ilblue shifts" of both the chlorophyll well and red edge in 
moisture-stressed vegetation when compared with non-stressed 
vegetation; a normalization of selected parameters (chlorophyll 
well and near infrared shoulder) may be used to emphasize the 
shift in red edge position; and the presence of the red edge in 
AVIRIS spectral curves may be useful in detecting small amounts 
(20-30% cover) of semi-arid and arid vegetation ground cover. A 
discussion of possible causes of AVIRIS red edge shifts in 
response to stress is presented. 
INTRODUCTION 
The detection of various types of stress in vegetation using 
the red edge portion of the vegetation reflectance curve (Chang 
and Collins, 1983; Collins et al., 1983; Horler et al., 1980; 
Rock et al., 1986, 1988; Westman and Price, 1988). The shift of 
the red edge to shorter wavelengths (ie. toward the blue end of 
the visible spectrum) has been termed the "blue shift," and has 
been shown to occur in plants exposed to a variety of stress 
agents, including trace metals, airborne pollutants, and drying. 
A similar blue shift of the maximum absorption by chlorophyll 
pigments in the visible red, termed the chlorophyll well, has 
also been observed in vegetation exposed to these stress agents. 
These blue shifts are very subtle and range from 2-15 nm. 
Datasets from airborne high-spectral resolution sensor 
systems have used blue shifts of red edge parameters (both red 
edge and chlorophyll wells) to detect and map areas of stressed 
vegetation (Collins et al., 1983; Milton et al., 1983; Rock et 
al., 1988). The airborne sensor systems used provided a large 
number of spectral bands, with spectral resolution on the order 
-- in situ high-spectral resolution reflectance data has focussed on , 
88 
https://ntrs.nasa.gov/search.jsp?R=19890012791 2020-03-20T02:34:41+00:00Z
c 
of a few nanometers. Rock et al. (1988) used an airborne system 
known as the Fluorescence Line Imager (FLI) , providing 288 
spectral channels covering the spectral region from 0.4-0.8 um, 
each 2.3 nm wide, to detect and map areas of forest decline 
damage in Vermont (USA) thought to be due to air pollution. The 
present study was conducted in part, to determine if AVIRIS 
datasets acquired in 210 spectral channels (for radiometrically- 
corrected data) in the spectral region from'0.4-2.4 um, each 9.8 
nm wide, would provide adequate spectral resolution to resolve 
red edge parameter blue shifts. 
Although an attempt was made to acquire AVIRIS data for the 
forest decline damage sites in the northeastern United States, a 
lack of cloud-free weather conditions in the New England area 
during July, 1987 prevented such data acquisition. An AVIRIS 
overflight on July 30, 1987 did acquire excellent data for the 
northern end of the Owens Valley, CA, an area characterized as 
semi-arid, exhibiting extensive vegetation cover dominated by 
members of the Great Basin sagebrush community and saltbush 
community. Along river courses a number of tree species form 
dense canopies, and heavily irrigated areas support either 
agricultural crops such as alfalfa, or grass-dominated areas such 
as pastures, lawns, and a golf course. Based on a field visit, 
vegetated areas were selected from the flightline which 
represented three basic vegetation types: lush, green vegetation; 
dry, semi-arid vegetation; and cut, dried, vegetation which had 
been green prior to cutting. Preliminary analyses of AVIRIS data 
acquired for these vegetation types are presented. 
MATERIALS AND METHODS 
AVIRIS data used in this study are from flight line #lo, Run 
#009, Segment 03, and were acquired on July 30th, 1987 at 
approximately 11:36 AM Pacific Standard Time. The flight line was 
selected for study because of the diversity of vegetation types 
occurring near the town of Bishop, CA (37'22" X 118920'W) and 
the surrounding area. In addition, these data were selected 
because of their high quality. The AVIRIS data presented here are 
radiometrically corrected and have been flat-field corrected. 
The radiometrically-corrected data were obtained from the 
Jet Propulsion Laboratory in Pasadena, CA. The radiometric 
correction was accomplished by subtracting the dark current 
values from the raw data, equalizing the response of the 224 
individual detectors and resampling the wavelenghts to a uniform 
9.8 nm sampling interval. The radiometrically-corrected data 
contain 210 spectral bands with band centers ranging from 0.4000 
um (band 1) to 2.4482 um (band 210). The response of each 
detector is such that the same amount of reflected radiance in 
each of the bands would yield the same DN (digital number) value. 
A flat-field correction was applied to the data to remove 
the effect of solar irradiance. A grus pit (consisting of the 
weathered products of granitic rocks) was used as the flat field. 
For each spectral band, pixel values from all vegetated sites 
89 
were divided by pixel values from the grus pit. Because the grus 
pit contains no vegetation, all the spectral data from the 
vegetation at the other sites was preserved while the component 
of solar irradiance was eliminated. A secondary effect of flat- 
field correction is the removal of atmospheric absorption 
features common to both the grus pit and vegetation datasets. The 
resulting spectral curves, produced by plotting brightness values 
of the 210 bands for each pixel or group of pixels were 
comparable to laboratory- or field-acquired spectra of 
vegetation. The resulting brightness values have not been 
converted to reflectance as yet, but this would be possible since 
ground reflectance data have been acquired for the grus pit 
calibration targets included within the AVIRIS flightline. The 
curves presented here were plotted on a Macintosh Plus using 
Cricket Graph software*. All plots are averages of several pixels 
from the areas cited. 
It is difficult to compare red edge and chlorophyll well 
parameters from spectral curve plots because of variations in the 
height of the NIR (near infrared) plateau and the depth of the 
chlorophyll well characteristic of different vegetation types. If 
the data are normalized, differences in the position of the red 
edge and the extent of the blue shift can be more easily studied. 
Therefore, the data were normalized as follows. For each spectral 
curve, the value corresponding to the lowest spectral band in the 
chlorophyll well was subtracted from every spectral band in the 
curve. For all vegetated sites along the flight line, the lowest 
value occurred at either spectral band 29 (band center = 0.6744 
um) or 30 (band center = 0.6842 um). The second step in the 
normalization involved scaling the curve so that channel 38 (band 
center = 0.7626 um) on the NIR plateau had a value of two. With 
these two points on each curve pinioned, the resulting curves 
clearly show relative red edge positions. 
The native vegetation occurring in the Bishop, CA area include 
the following dominant types. In July, lush, green trees and 
large shrubs occur along stream banks and areas of damp soil, 
while semi-arid, non-green or partially green sagebrush/saltbush 
shrubs occupy drier sites. Common tree species are cottonwoods 
(Populus) and willows (Salix) and these occur in dense stands 
where there is enough soil moisture to support their growth. At 
drier sites, only low desert shrubs are found, consisting of 
either sagebrush (Artemisia tridentata) or rabbit brush 
(Chrvsothamnus nauseosus). The rabbit brush characterizes 
disturbed areas (pasture lands) while the sagebrush is more 
typical of open range lands that are largely undisturbed. In 
drier sites characterized by soils high in salt levels, a small 
desert shrub known as saltbush (Atriplex spp.) is common. During 
the summer months, sagebrush retains its leaves, while rabbit 
brush and saltbush may lose their leaves. 
Along portions of the AVIRIS flightline three major types of 
* Reference to specific products is made for clarity, and does 
not constitute an endorsement of these products. 
90 
cultivated vegetation occurs: large areas of alfalfa (Medicaao 
sativa) grown in rotation (some fields with green, actively 
growing plants, and other fields with cut, drying plants); a 
large golf course of green, well-watered grasses and trees 
(cottonwoods); and irrigated pastures of green grass. The alfalfa 
fields were of great interest in this study because they provided 
large, homogeneous areas of either green or dried plants of the 
same species. Because of their rectangular shape, they could be 
easily identified in the AVIRIS data. The harvesting of alfalfa 
results in rows of cut plants alternating with partly green 
stubble and exposed soil. 
Radiometrically-corrected AVIRIS data are strongly 
influenced by both variation in solar illumination and by 
atmospheric absorption features. Figure 1 presents such AVIRIS 
spectral data acquired from the golf course. Note the decreasing 
brightness with increasing band numbers (wavelength) due to the 
drop in solar illumination intensity, as well as absorption by 
atmospheric water, centered at bands 57 (0.9488 um) , 76 (1.1350 
um) , and 102 (1.3898 um) . The absorption feature located at band 
38 (0.7626 um) is likely due to atmospheric molecular oxygen, 
while the strong absorption centered at band 29 (0.6744 um) is 
due to chlorophyll in the vegetation. This feature is referred to 
herein as the chlorophyll well, and is followed by a sharp rise, 
the red edge, located between bands 30 and 37. The broad 
absorption in bands 1-12 (0.4000-0.5078 urn) is due to both 
chlorophyll and carotenoid pigments. 
Raw spectral data for the grus pits used for flat-field 
correction are presented in Figure 2. Many of the same absorption 
features seen in Figure 1 are also seen in Figure 2 (features at 
bands 38, 57, 76, and 102), indicating that they are common 
atmospheric components. A broad absorption seen centered at band 
150 (1.8602 um) is due to atmosphereic water, while the strong 
feature at band 2 (0.4098 um) is likely due to iron oxide found 
in the grus. Causes of the absorption features at bands 89 
(1.2624 um) and 165 (2.0072 um) have not yet been identified. 
Flat-field corrected spectral curves are presented in Figure 
3 for the golf course, green alfalfa, and an area of low rabbit 
brush and grass (approximately 20-30% cover). Note that the flat- 
field correction has removed the effects of variable intensity of 
solar illumination and of atmospheric absorptions. Artifacts 
introduced into these curves by the flat-field correction method 
include a shift of one band in the position of the chlorophyll 
absorption maximum and the apparent lack of absorption by 
chlorophylls and carotenoids in the visible blue (bands 1-11), 
due to the strong iron oxide absorption seen in the grus. The 
overall results are most encouraging however, since the flat- 
field corrected curves for the golf course and alfalfa look very 
much like typical vegetation curves acquired with field and 
laboratory spectrometers. 
91 
A more detailed presentation of flat-field corrected visible 
and near infrared AVIRIS data is seen in Figure 4. Spectral 
curves for cottonwoods and green alfalfa show maximum absorption 
due to chlorophyll (the chlorophyll well) at band 30 (0.6842 um) 
while the chlorophyll well for drying alfalfa is located at band 
29 (0.6744 um) . Rock et al. (1988) report a five nanometer blue 
shift in chlorophyll well position with increased damage in 
spruce (Picea) forests as measured by the airborne FLI, although 
a similar blue shift of only two nanometers was measured in situ. 
Westman and Price (1988) report a similar two nanometer blue 
shift in chlorophyll well position determined in situ for pines 
(Pinus) experimentally exposed to drying, acid misting, and 
ozone. 
It is difficult to assess the true nature of the blue shift 
of the chlorophyll well reported here. Westman and Price (1988) 
attribute the blue shift seen in pine induced by drying to 
denaturing of chlorophylls. The blue shift seen in spruce has 
been correlated with decreases in the amount of chlorophyll b 
relative to chlorophyll a (Rock et al., 1988). The blue shift 
seen in Figure 4 may also be due to increased amounts of soil and 
stuble exposed by the cutting of alfalfa. 
Note that the variations in amplitude of both red (bands 28- 
30) and NIR (bands 38-90) reflectance seen in the Figure 4 curves 
makes evaluation of the red edge position difficult. The red edge 
of green alfalfa appears shifted to shorter wavelengths when 
compared with the red edge of the cottonwoods. This apparent 
shift is due to the greater NIR reflectance seen for the green 
alfalfa, an effect which can be removed by normalizing the two 
curves. Figure 5 presents normalized red edge curves for the golf 
course, cottonwoods, and green alfalfa. This approach 
demonstrates that the red edge parameters for all three green 
vegetation types are very similar. 
When the red edge data for the cut alfalfa are normalized to 
those of the green vegetation types (Figure 6) a distinct flshiftff 
is seen such that the red edge curve for the drying vegetation is 
separated from the others. This blue shift of the red edge of cut 
alfalfa is very similar to that seen in normalized FLI data 
acquired from the forest decline sites (Rock et al., 1988). 
Regardless of the cause of the blue shift (alteration of plant 
pignents, increased reflectance from soil/stubble, etc.), it is 
clear that AVIRIS data have been acquired with sufficient 
spectral resolution to detect such spectral fine features 
associated with stress in vegetation. 
The presence or absence of the red edge feature in spectral 
curve data may be useful in detecting trace amounts of vegetation 
in semi-arid regions. Although not presented in detail here, 
subtle changes in reflectance amplitude across the red edge 
region are seen in AVIRIS data for the sagebrush and rabbit brush 
areas of the flightline (see Figure 3). An investigation which 
relates the occurrence of such amplitude changes with percent 
ground cover and vegetation type will be needed to establish the 
92 
value of using red edge data for the purpose of trace vegetation 
detection and quantification. 
ACKNOWLEDGEMENTS 
The research reported here has been supported by the Earth 
Processes Branch of the National Aeronautics and Space 
Administration (NASA) and was carried out at the Jet Propulsion 
Laboratory (JPL), Pasadena, CA. Special thanks are given to Gregg 
Vane, Lisa Barge, and Alex Zak of JPL for their assistance in 
this effort. 
REFERENCES 
Chang, S.H:, and Collins, W. 1983. Confirmation of the airborne 
biogeochemical mineral exploration technique using laboratory 
methods. Economic Geolocry 78:723-736. 
Collins, W., Chang, S.H., Raines, G., Canney, F., and Ashley, F. 
1983. Airborne biogeochemical mapping of hidden mineral 
deposits. Economic Geolocrv 78 :737-749. 
Horler, D.N.H., Barber, J., and Barringer, A.R. 1980. Effects 
of heavy metals on the absorbance and reflectance spectra of 
plants. International Journal of Remote Sensinq 1:121-136. 
Milton, N.M., Collins, W., Chang, S.H., and Schmidt, R.G. 1983. 
Remote detection of metal anomalies on Pilot Mountain, Randolph 
County, North Carolina. Economic Geolocrv 78:605-617. 
Rock, B.N., Hoshizaki, T., and Miller, J.R. 1988. Comparison of 
- -  in situ and airborne spectral measurements of the blue shift 
associated with forest decline. Remote Sensing of Environment 
24:109-127. 
Rock, B.N., Vogelmann, J.E., Williams, D.L., Vogelmann, A.F., and 
Hoshizaki, T. 1986. Remote detection of forest damage. 
Bioscience 36:439-445. 
Westman, W.E., and Price, C.V. 1988. Spectral changes in 
conifers subjected to air pollution and water stress: 
experimental studies. IEEE Transactions on Geoscience and Remote 
Sensinq GE-26:11-21. 
93 
600 
400 
200 
0 
0 100 BAND INHBER 
200 
Figure 1. Radiometrically-corrected AVIRIS data plotted as DN vs. 
band numbers for pixels selected from the golf course. Band 
numbers discussed in text are labelled. 
600 
400 
200 
0 
0 100 
B A m  ImIBER 
200 
Figure 2. Radiometrically-corrected AVIRIS data plotted as DN vs. 
band numbers for pixels selected from the grus pits used as 
calibration targets. Band numbers discussed in text are labelled. 
94 
4 
VI B 3  
2 Q OREENALFALFA c r * RABBITBRUSH 
E * ooLFcouRsE m 
m 2  
z 
5 
m 
c 
W 
0 
0 100 
BAND NUMBER 
200 
Figure 3. Flat-field corrected AVIRIS data plotted as relative 
brightness vs. band numbers for pixels selected from fields of 
green alfalfa, the golf course, and an area of low-density 
vegetation dominated by rabbit brush. 
1.4 
.I, COTTomfoODs 
* "QREENALF. 
* GREENALFALFA 
0.6 - 
0.4 - 
0 20 $0 60 80 100 
0- luneER 
Figure 4. Flat-field corrected visible and near infrared AVIRIS 
data plotted as brightness values vs. band numbers for pixels 
selected from stands of cottonwoods and fields of green and cut 
alfalfa (labelled "nongreen alf."). Note the difference in both 
shape and position of the chlorophyll wells f o r  green alfalfa and 
cottonwoods (centered at band 30) and drying alfalfa (centered at 
band 29). 
95 
20 30 50 
Figure 5 .  Normalized red edge AVIRIS da ta  f o r  three types of 
green vegetation: t h e  golf course,  cottonwoods, and a l f a l f a .  Note 
t h e  red edge pos i t i on  is very s i m i l a r  f o r  a l l  three types.  
2 
0 GWCWRSE 
* COTTONWOODS 
1 * NOTGR. WW 
+ GREENALFALFA 
0 
20 40 B m D  llLEneER 30 
50 
Figure 6 .  Normalized red edge AVIRIS da ta  f o r  three types  of 
green vegetat ion (golf  course,  cottonwoods, and green a l f a l f a )  
and c u t ,  drying a l f a l f a  ( l a b e l l e d  lvnot g r .  a l f a l f a v v ) .  Note t he  
change i n  pos i t ion  of t he  red edge of t he  c u t  a l f a l f a  r e l a t i v e  t o  
t h e  red edge of the  green vegetat ion.  


Assessment of AVIRIS data from vegetated sites in the Owens Valley, California

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