An outline is presented of the required spatial and spectral resolution needed for accurate vegetation discrimination and mapping studies as well as for determination of state of health (i.e., detection of stress symptoms) of actively growing vegetation. Good success was achieved in vegetation discrimination and mapping of a heterogeneous forest cover in the ridge and valley portion of the Appalachians using multispectral data acquired with a spatial resolution of 15 m (IFOV). A sensor system delivering 10 to 15 m spatial resolution is needed for both vegetation mapping and detection of stress symptoms. Based on the vegetation discrimination and mapping exercises conducted at the Lost River site, accurate products (vegetation maps) are produced using broad-band spectral data ranging from the .500 to 2.500 micron portion of the spectrum. In order of decreasing utility for vegetation discrimination, the four most valuable TM simulator VNIR bands are: 6 (1.55 to 1.75 microns), 3 (0.63 to 0.69 microns), 5 (1.00 to 1.30 microns) and 4 (0.76 to 0.90 microns)

Rock, B. N.

English

NASA Technical Reports Server

SPATIAL AND SPECTRAL RESOLUTION h_CESSARY _R
REMOTELY SENSED VEGETATION STUDIES
Barrett N. Rock
Jet Propulsion Laboratory
The following is an outline of the required spatial and spectral resolu-
tion needed for accurate vegetation discrimination and mapping studies as well
as for determination of state of health (i.e. detection of stress symptoms) of
actively growing vegetation.
A. Spatial Resolution
The spatial resolution required for accurate vegetation discrimination
and mapping is directly related to the complexity of the canopy being stu-
died. A relatively homogeneous canopy (crop plantings, conifer forests con-
sisting of one or two species, etc.) requires less spatial resolution for
accurate mapping than does a more heterogeneous canopy (conlfer/hardwood
mixes, multiple species of hardwoods, etc.) which contains greater scene
variation. The Lost River NASA/Geosat test site in eastern West Virginia
(ridge and valley province of the Appalachians) represents one of the most
complex areas for vegetation discrimination and mapping available in the
United States.
The Lost River test site is heavily forested (approximately 80%), repre-
senting a typical eastern mixed deciduous/evergreen (Oak-Hickory-Pine) cover
type. The non-forested areas of the sites are generally u_der cultivation
(lawns, pastures and field crops such as corn or hay) and thus surface reflec-
tance data are totally dominated by vegetation. The vegetation cover occur-
ring at Lost River is typical of the entire ridge and valley portion of West
Virginia.
As a product of the recently completed NASA/Geosat study, highly accurate
supervised vegetation classifications for the Lost River area have been pro-
duced. A total of seven forest tree classes (species or species associations)
are recognized with approximately 90% accuracy of class assignment. Data used
to produce these images are from Johnson Space Center's NS-001 airborne scan-
ner (Thematic Mapper Simulator), acquired with an instantaneous field of view
(IFOV) of 15 m (ii m resolution over center of the field). A recent over-
flight (October, 1981) has provided NSTL Thematic Mapper Simulator data for
the same area acquired with an IFOV of 30 m. In terms of accuracy of vegeta-
tion discrimination, these data provide a background for assessing the degree
of spatial resolution required for a given level of accuracy.
Degradation of NS-001 Lost River data (15 m IFOV), using a low-pass, 3 x
3 filtering produced data corresponding to that acquired with approximately 45
m resolution. A vegetation classification, produced using these degraded data
and the same training areas used for the 15 m data, proved less than
optimal. Although the produced image was never field checked, considerable
loss of usablelnformation resulted in less precise (i.e. detailed) vegetation
discrimination and mapping than is possible with the 15 m data. Clearly, 15 m
spatial resolution represents an upper limit necessary for accurate analysis
of a heterogeneous vegetation canopy,
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In study of crop type and maturation as well as stress symptoms in.plants
growing over mineralized soils (containing heavy metals such as copper, lead
and zinc), good success (Collins, 1978; Collins et al., 1981) has been
achieved using a 500 channel spectroradiometer with an 18 m IFOV. This spa-
tial resolution allows for detection of the so-called red-shift of .007 to
•010 _m in the chlorophyll absorption band in homogeneous cro_ canopies (this
red shift has been related to metal-lnduced stress).
Based on these results and the degree of accuracypossible in vegetation map-
ping at the Lost River test site, using 15 m resolution, it is my opinion that
an instrument delivering spatial resolution ranging from i0 to 15 m is
required for accurate geobotanlcal study in the future. While improved
spatial resolution may be possible, I am of the opinion that the increased
amount of data made available is likely unnecessary and may in fact create
additional problems related to excessive amounts of data (varying reflectance
data from separate portions of a slngle tree crown, for example) leading to
amplification of scene "noise."
B. Spectral Resolution
Broad-band spectral data (NS-O01) for the Lost River test site are avail-
able for two phenologlc periods; September (near the end of the growing season
for woody species) andOctober (period for fall foliage display, selected to
afford maximum species discrimination data). NS-001 bands 3 (0.63 - 0.69 _m),
4 (0.76 - 0.90 _m), 5 (1.00 - 1.30 _m), 6 (1.55 - 1.75 _m) and 7 (2.08 - 2.35
_m) are the most useful in providing accurate vegetation discrimination and
classification of forest species and species associations using October loll-
age display data.
The analysis of NS-001 spectral data acquired over the Lost River test
site provides insight into the utility of these broad-band data in mapping
densely vegetated terrain. _he work of Harold Lang (at JPL - Final Geosat
Report) shows that based on an evaluation mf interband correlatlonmatrices,
principal components analyses, and stepwlse dlscrlminant analyses for the
supervised vegetation classifications produced for the Lost River study, in
order of decreasing utility for vegetation discrimination, the four most
important NS-001VNIR bands may be ranked in the following order: band 6 (1.55
- 1.75 _m), band 3 (0.63 - 0.69 _m), band 5 (I.00 - 1.30 _m), band 4 (0.76 -
0.90 _m). The somewhat surprising absence of the bands at 0.45 - 0.52 _m
(band i) and 0.52 -0.60 _m Coand 2) is attributed to their high positive
correlation with the 0.63 - 0.69 _m band. The high ranking of the 1.00 - 1.30
Bm band is significant, since data will not be acquired in that wavelength
range by the Landsat-D _M. NS-001 bands at 0.52 - 0.60 _m (band 2), 0.63 -
0.69 Bm [band 3) an_ 0.76 - 0.90 _m [band 4) were of considerable value in the
process of vegetatio# discrimination and mapping since they may be used to
produce images whlc_mlmic aerial CIR photography, an image format with which
field botanists andphotointerpreters are familiar.
Certain spectral regions, when studied with hig h resolutlon, are seen to
contain a great many fine spectral features that may well provide useful data
for improved vegetati0n discrimination studies. Based on laboratory studies
(reflectance data from the Beckman 5240 spectrophotometer) the 0.80 to 1.30 _m
region contains the greatest amount of fine spectral structure (both reflec-
tance and absorbance features; note Table i), data which relate to leaf struc-
83
ture. Rhe 1.50 to 1.75 pm and 2.00 to 2.40 _m regions also contain fine spec-
tral structure, in this case relating to leaf moisture content (Gausman e___t
al___t.,1978; Tucker, 1980) and thus, state of health (senescence vs. active
growth). Table I lists areas of fine spectral structure in the VNIR, SWIR,
and TIR, which will be of use in both vegetation discrimination and stress
detection.
According to the work of Collins et al. (1981) and others, the spectral
region most affected by mineral-induced stress lles between .550 _m and .750
pm, with the symptomatic red-shift occurring on the far wing of the red chlor-
ophyll absorption band centered at approximately .680 pm (principal absorption
peak for chlorophyll-a molecules associated with the photosynthetic light
trap). As stated previously, the red-shift consists of a .007 to .010 _m
shift of the chlorophyll shoulder to slightly longer wavelengths (.690 to .700
_m).
In the study of phenologlc changes (ontogenetlc or maturation stages),
crop plants such as corn, wheat, and sorghum demonstrate a similar red-shift
in the chlorophyll-a absorption band (Collins, 1978). The red-shlft (again
•007 to .010 pm) occurs along the entire length of the absorption wing, but is
most pronounced on the chlorophyll shoulder centered at about .740 pm (ranging
from .700 to .750 pm). In both cases (mineral-induced stress and progressive
maturation) the red-shlft may be related to a decrease in chlorophyll produc-
tion.
The instrumentation used by Collins (1978) in his study of crop canopies
was an airborne 500 channel spectroradiometer with .0014 pm spectral resolu-
tion, collecting data in the .400 to I.I00 pm spectral region. Collins noted
that the red-shift can be measured with .010 pm-wide spectral bands centered
at .745 _m and .785 pm. _hese two bands plus one Centered at .670 pm provide
enough information to identify a variety of crop plants as well as detect the
symptomatic red-shift. Tucker and Maxwell (1976) have demonstrated that wide
spectral bands of .050 pm or more do not provide adequate information in the
700 to 750 Pm range for detection of the chlorophyll absorption edge.
C. Summary
Good success has been achieved in vegetation discrimination and mapping
of a heterogeneous forest cover in the ridge and valley portion of the Appala-
chians using multispectral data acquired with a spatial resolution of 15 m
(IFOV). A sensor system delivering I0 to 15 m spatial resolution is needed
for both vegetation mapping and detection of stress symptoms.
Based on the vegetation discrimination and mapping exercises conducted at
the Lost River site, accurate products (vegetation maps) are produced using
broad-band spectral data ranging from the .500 to 2.500 pm portion of the
spectrum. In order of decreasing utility for vegetation discrimination, the
four most valuable NS-001VNIR bands are: 6 (1.55-1.75 pm), 3 (0.63-0.69 _m),
5 (1.00-1.30 _m) and 4 (0,76-0.90 _m). Spectral data acquired in the 1.00-
1.30 pm range is essential. Finer spectral detail from certain wavelength
regions (0.80-1.30 pm, 1.50-1.75 _m, and 2.00-2.40 pm) will supply additional
useful information for vegetation mapping and monitoring, provided that 10-15
m spatial resolution is possible.
84
References
Collins, W. 1978. Remote sensing of crop type and maturity. Photogrammetric
Engineering and Remote Sensing. 44(1):43-55.
Collins, W., S. H. Chang, and J. T. Kuo. 1981. Detection of hidden mineral
deposits by airborne spectral analysis of forest canopies. Final Report
to NASA, Contract NSG-5222.
Gausman, H. W., D. E. Escobar, J. H. Everltt, A. J. Richardson, and R. R.
Rodriguez. 1978. Distinguishing succulent plants from crop and woody
plants. Photogrammetric Engineering and Remote Sensing. 44(4):487-491.
Tucker, C. J. 1980. Remote sensing of leaf water content in the near infra-
red. Remote Sensing of Environment. 10:23-32.
t
Tucker, C. J., and E. L. Maxwell. 1976. Sensor design for monitoring vegeta-
tion canopies. Photogrammetric Engineering and Remote Sensing. 24(11):
1399-1410.
85
TABLE I- FINE STRUCTURE OF PLANT
SPECTRA IN THE VNIR, SWIR AND TIR
Type of
Wavelength (_m) Feature Value
.440-.500 Absorbance Detection of changes in chlorophyll/carotenold ratios
(related to stress).
•650-.700 Absorbance Detection of chlorophyll states as well as tannin and
anthocyanln content. Initial stress detection.
•700-.750 Reflectance Senescence detection. Dead or dormant vegetation.
.800-.840 Absorbance Possibly related to leaf anatomy and/or state of hydration.
•865 Reflectance Height of feature may be useful in species discrimination.
•940-.980 Absorption Shifts in this minor water absorption band may be useful in
species discrimination and determination of hydration state.
1.060-1.100 Reflectance Shifts in peaks may be related to leaf anatomy and/or
morphology. May be useful for species discrimination.o_
1.140-1.220 Absorbance Shifts in this minor water absorption band may be useful in
species discrimination and determination of hydration state.
1.250-1.290 Reflectance Height of this feature very useful for species discrimination
of senescent forest species. A ratio of this feature with the
one at 1.645 pm offers a good indication of moisture content
and thus stress.
1.630-1.660 Reflectance An indication of moisture content of leaf. May also be an
indicator of variation in leaf anatomy. May be useful for
species discrimination. An indicator of leaf moisture
content when used as a ratio with the 1.270 _n data above.
2.190-2.300 Reflectance An indicator of moisture content. May also be of value in
species discrimination.
3.000-5.000; Reflectance Little is known concerning the value of thermal IR data in
8.000-14.000 the study of vegetation. This is an area that n_ds
further study.


Spatial and spectral resolution necessary for remotely sensed vegetation studies

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