Spectral characteristics of semic-arid plant communities using 128 channel airborne imaging spectrometer (AIS) data acquired on October 30, 1984. Both field and AIS spectra of vegetation were relatively featureless and differed from substrate spectra primarily in albedo. Unvegetated sand dunes were examined to assess spectral variation resulting from topographic irregularity. Although shrub cover as low as 10% could be detected on relatively flat surfaces, such differences were obscured in more heterogeneous terrain. Sagebrush-covered fans which had been scarred by fire were studied to determine the effect of changes in plant density on reflectance. Despite noise in the atmospherically corrected spectra, these provide better resolution of differences in plant density than spectra which are solar-corrected only. A high negative correlation was found between reflectance and plant cover in areas which had uniform substrates and vegetation types. A lower correlation was found where vegetation and substrates were more diverse

Rock, B. N.

Ustin, S. L.

English

NASA Technical Reports Server

N86-11622 
PRELIMINARY ANALYSIS OF AIS SPECTRAL DATA ACQUIRED FROM SEMI-ARID SHRUB 
COMMUNITIES IN THE OWENS VALLEY, CALIFORNIA 
SUSAN L. USTIN, Department of Botany, University of California, 
Berkeley, CA 94720, USA; BARRETT N. ROCK, Jet Propulsion Laboratory, 
Pasadena, CA 91109, USA 
ABSTRACT 
Spectral characteristics of semi-arid plant 
communities were examined using 128 channel AIS data 
acquired on October 30, 1984. Both field and AIS 
spectra of vegetation were relatively featureless and 
differed from substrate spectra primarily in albedo. 
Unvegetated sand dunes were examined to assess spectral 
variation resulting from topographic irregularity. 
Although shrub cover as low as 10% could be detected on 
relatively flat surfaces, such differences were obscured 
in more heterogeneous terrain. Sagebrush-covered fans 
which had been scarred by fire were studied to determine 
the effect of changes in plant density on reflectance. 
Despite noise in the atmospherically corrected spectra, 
these provide better resolution of differences in plant 
density than spectra which are solar-corrected only. A 
high negative correlation Was found between reflectance 
and plant cover in areas which had uniform substrates and 
vegetation types. A lower correlation l~as found where 
vegetation and substrates were more diverse. Thus 
shadows caused by substrate roughness or plant canopies 
also appear to have an important effect on brightness, 
making predictions from spectral characteristics diffi-
cult. Spectra obtained during periods of active plant 
growth may be more useful in vegetation studies. 
INTRODUCTION 
High spectral resolution imaging spectrometers such as AIS have 
potential for identification of minerals and plants based on the unique 
absorption features of their spectra. For ecologists, this may lead to 
new ways of studying and monitoring ecosystem processes, detecting 
environmental stresses, and measuring plant productivity. These 
instruments are also of considerable interest to exploration geologists 
working in arid and semi-arid regions, since in such areas the 
interpretative problems arising from mixed spectral reflectances should 
be minimal, due to the relatively low vegetation cover. 
lne primary objectives of our study are to evaluate the 
importai"ice of vegetation in mixed pixel AIS data from representative 
semi-arid shrub communities, to assess dynamic changes in spectral 
features of vegetation over the year, and to develop methods by which the 
spectra of vegetation and substrate may be separated. 
41 
https://ntrs.nasa.gov/search.jsp?R=19860002155 2020-03-23T06:31:28+00:00Z
.. 
Owens Valley, CA was chosen as a test site because it has a diverse 
combination of plant communities and substrate types. A creosote bush 
conmunity, typical of the warmer Mojave desert, extends into the 
southern end of Owens Valley on alluvial fan skirts from the Sierra 
Nevada to the west and the Inyo Mountains to the east. Vegetation on 
fans farther north is more typical of the Great Basin sagebrush 
community. Saltbush communities are found on the valley floor and on 
other alkaline soils. Major geologic units in the surrounding areas are 
Kesozoic granitic intrusives, Tertiary vQlcanics (primarily basalts), 
and Paleozoic sediments. Differences in plant density and species 
diversity exist within the various substrate types, offering the 
opportunity to examine the impact of vegetation on spectral character-
istics. 
AIS DATA AND CALIBRATION PROCEDURES 
At present, we have had three flights over our study sites, on 
July 10 and October 30, 1984 and April 2, 1985. All flights obtained 128 
channel AIS data, most of them in the "rock" grating position mode, 1.2l-
2.40 j1m (Vane et al., 1983). For comparison, on some flights data from 
one line were recorded in the "tree" mode, ca. 0.85-2.10 llm. Thematic 
Mapper Simulator (NS001) imagery, Nikon black and white photographs, 
Zeiss color infrared images, and ground track videotapes were collected 
simultaneouslY· 
Data reported here are entirely from the October overflight. 
The data acquired in July are of questionable value due to a high 
concentration of smoke from a large brush fire in Sequoia National Park 
-to the west of Owens Valley. However, preliminary examination of these 
data supports the observations and conclusions of this report. 
The October 30, 1984 flight occurred at 1145-1245 hours PST, on a 
cloud-free day. Mean flight altitude was 5.25 km above the terrain, 
with an instantaneous field of view of about 10 m. Data were analysed on 
a VAX/IDIMS system using the SPAM (Spectral Analysis Manager) package 
developed b:, the Jet Propulsion Laboratory. Prior to data analysis, a 
solar irraCliance correction was applied to compensate for brightness 
variation with wavelength in the solar spectrum. We compared results of 
this COrrf!ction with those of two additional methods for removing 
atmospheric absorptance effects: a "flat field" method ratioing pixel 
reflectances by the mean reflectance obtained from a 32 by 50 pixel area 
judged to be spectrally and spatially homogeneous, and a theoretically 
based correction algorithm (J. Solomon, personal communication) which 
ratios each pixel reflectance by the mean reflectance along the flight 
line fol' each channel. 
RESULTS AND DISCUSSION 
Coinciding with the October overflight, we performed an 
experiment with a Collins Visible Infrared Intelligent Spectrometer 
(VIRIS) to examine the effect of spectral mixing at different distances 
above the plant and soil surfaces. When the spectrometer was close to 
the canopy, the spectral pattern was distinctively plant-shaped; 
however, this pattern was progressively damped out as distance increased 
(Us tin et al., 1985). This response appears to be due to shadows within 
the canopy branches and the increasing contribution of sand to the 
42 
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spectra. From this we conclude that the influence of vegetation in 
October AIS data would be most evident in albedo effects. 
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Fig. 1. AIS sand spectra with (a) flat field correction and (b) 
theoretically based atmospheric correction. Curve 1 is sunlit dune, 2 
shaded aspect, 3 a flat surface. 
Data from an unvegetated sand dune Legion show that topographic 
differences, such as changes in slope and aspect, confound interpreta-
tions of even atmospherically corrected spectra. Figure 1 shows data 
from the same 3-by-3 pixel areas using the flat field correction (la) and 
the atmospheric correction (lb). Curve 1 is from the sunlit side of a 
dune, curve 2 from the shaded side, and curve 3 from a flat surface 
normal to the AIS (curve 4 in la, from the shaded side of another dune, 
is not shown on 1b). From these and other curves, it appears that both 
correction methods yield comparable results, although the digital number 
(dn) values depend on the method of correction. The curves are 
featureless and essentially flat or slightly increasing across the 
spectrum, as expected for quartz sands. At another area which was 
studied in detail, we could distinguish a saltbush vegetation cover of 
only 10% (based on ground transects) from a non-vegetated uniform sandy 
surface of similar slope, because of brightness differences. However, 
these differences were within the dn range seen in Figure 1 on the two 
aspects of the same sand dune. During spring, greater divergence in 
spectral response curves between vegetation and substrate may provide a 
less ambiguous basis for their distinction. • 
Spectral data in Figure 1 and in Figures 2 and 3, from the Symmes 
Creek and Division Creek sites on fans on the west side of the valley, 
all show considerable noise, making identification of absorption 
features difficult or impossible. We found that omitting the water 
absorptance bands and replotting portions of the spectra (e.ga, 2.10-
2.40 \l.m) reduced the apparent noise in the atmospherically corrected 
spectra. Still less noise is evident in data to which only the solar 
correction was applied (Figure 2a), but the differences between spectra 
from different areas within sites are less than in spectra corrected for 
both solar and atmospheric effects (Figure 2). 
43 I 
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Fig. 2. AIS spectra (a) without atmospheric correction and (b) 
with atmospheric correction. Spectra 1 and 2 are from an area which had 
been burned, spectra 3 and 4 the non-burned vegetation. 
1.21 1.35 t.!Il I.. 1.80 1.!15 2.10 2.211 2.40 
lM'IElENGTH("ICIOISI 
Fig. 3. AIS spectra from surfaces which differ in plant cover 
and substrate. Spectrum 1 is from burned alluvium, 2 burned basalt, 3 
non-burned alluvium, 4 non-burned basalt, and 5 non-vegetated basalt. 
Despite the greater noise in data with the atmospheric 
correction, they sHow clear differences related to vegetation cover. At 
Symmes Creek (Figure 2), alluvial substrates al:'e essentially homogeneous 
and spectral differences relate to an abrupt change in vegetation 
density due to a brush fire which removed the plant cover in 1977. The 
upper two curves in 2a and 2b are from the burned area, which has a total 
plant cover of 23% . including litter. The lower two curves are from the 
surrounding undisturbed sagebrush community, which has 46% cover. 
Reflectance differences between these two areas, assuming the curves are 
flat and using the dn value at 1.21 pm from randomly chosen 3-by-3 pixels 
(n-18 and 15 on and off the burn, respectively), are statistically 
significant (ANOVA, p(.OOOl). 
44 
We also measured plant density on and off burns at two other 
sites near Symmes Creek. Burns at these sites were much older (pre-
1947) and had greater plant densities, but adjacent unburned areas did 
not vary significantly in cover. Twelve to 15 spectral curves were 
obtained from randomly chosen 3-by-3 pixels within each of these areas. 
As described above, curves from each area were averaged and compared with 
plant cover, which ranges from 23 to 48 %. A linear regression of mean 
dn from all three sites with mean total plant cover produced a negative 
correlation coefficient of 95 % (n-6). Thus, shrub cover has a 
consistent effect in lowering substrate albedo, and the amount of change 
is related to plant density. 
At Division Creek, a similar sagebrush community occurs on both 
a granitic alluvial substrate and a basalt flow. In 1982, afire removed 
the vegetation across both substrates. Curves in Figure 3 are 
representative of 12-16 reflectance spectra from each of these surfaces. 
The upper curve (1) is from the burned alluvial surface, which has a 9 % 
plant cover, nearly all of which consists of herbaceous dicot litter at 
this season. The next curve (2) is from the burned basalt, which has a 
surprisingly high reflectance. It has a high cover, 52 %, almost all of 
which is grass litter. Curves 3 and 4 are from the non-burned areas on 
the alluvial and basalt substrates, which support a similar. shrub 
community with covers of 64 % and 45 %. The lowest curve (5) is from a 
non-vegetated basalt outcrop. Although there is a good correlation of 
dn with cover (71 %), it is lower than that from the other burns. This 
probably reflects the greater substrate roughness on the basalt flow and 
differences in physiognomy (shrub vs. herbaceous). Thus, although the 
amount of plant cover is clearly the major det~rminant of reflectance, 
shado~'1s caused by plant canopies and substrate roughness may also be an 
important source of albedo variation. The presence of vegetation 
reduces the differences in reflectance between the two substrates, thus 
masking any geologic absorption features. Although this feature is 
somewhat difficult to detect in curve (1), other curves from the burned 
alluvial surface clearly show a hydroxyl absorbance of presumed 
substrate origin, at 2.19 ~m, and which was confirmed by field VIRIS 
spectra. This feature is absent in all AIS data from surfaces with 
greater plant cover. 
REFERENCES 
Ustin, S.L., Rock, B.N., and Woodward, R.A. 1985. Analysis of 
substrate and plant Hpectral features of semi-arid shrub 
communities in the Owens Valley, California. Proc. Int. Symp. 
on Remote Sensing of Environment. Fourth Thematic Conference: 
Remote Sensing for Exploration Geology (in press) 
Vane, G., Goetz, A.F.R., and Wellman, J.B. 1983. Airborne imaging 
spectrometer: a new tool for remote sensing. Proc. 1983 
International Geoscience and Remote Sensing Symposium. pp. FA-
4, 6.0-6.5 
45 


Preliminary Analysis of AIS Spectral Data Acquired from Semi-arid Shrub Communities in the Owens Valley, California

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