Imaging spectrometry for ecological applications.pdf

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Imaging JAG Volume 3 - issue 4 - 2001 spectrometry for ecological applications Paul J Curran Department of Geograph y, University of Southampton, Highfield, Southampton 5017 IBJ, UK (e-mail: P.Curran@soton.ac.uk) KEYWORDS: ecology, ref lectance spectra, foliar bio- chemical concentration, NIRS, geological remote sens- ing ABSTRACT Imaging spectrometry from aircraft or satellite borne sensors has many potential ecological applications. This paper reviews its use for th e remote sensing of foliar biochemical concentration, as this is an ecological application of remote sensing that is unique to imag- ing spectrometry. Attention is focussed on the developme nt of methodologies, drawing where relevant on theory and techniques from both outside and inside remote sensing. Examples from the fields of near infrared spectroscopy (NIRS) and geological remote sensing, along with an extensi ve reference list, p rovide an introduc- tion to some of the ecological opportunities offered by imaging spectrometry. INTRODUCTION Many environmental variables influence the signals that reach a remote sensor. If we wish to use these remotely sensed signals to estimate environmental variables then we need to ensure that the number of remotely sensed signals is greater than the number of environmental vari- ables that are causing those signals to vary [Verstraete et a/, 1996; Curran, 20011. The number of environmental variables can be reduced by holding some constant (eg, restrict analysis to one geographic region, correct for atmospher e) and the number of remotely sensed signals can be increased by recording many such signals in nar- row wavebands along the spectrum. We can then repeat this sampling at different locations, times, geometries of sensor and illumination and polarizations [Curran et al, 19981. In ecology attention has been directed towards increased spectral sampling because of great spectral variability, in the 0.7 pm to 2.5 pm range, for vegetated landscapes. However, the majorit y of this spectral vari- ability lies within just three to five ‘dimensions’ [Curran et al, 19981; one or two dimensions in visible (0.4-0.7 pm) wavelengths, one dimension in near-infrared (0.7- 1.3 pm) wavelengths and one or two dimensions in mid- dle-infrared (1.3-2.5 pm) wavelengths. If the remotely sensed signal in say, four to five well-placed wavebands could account for most of the spectral variability in veg- etated landscapes then why would we wish to record in many more wavebands? From an engineering point of view surely we would be better advised to maximise the signal-to-noise ratio, SNR [Smith Curran, 19991 or increase (ie, make finer) the spatial resolution [Curran, 19941 of our sensor, rather than spread the remotely sensed signal over many mo re than five wavebands? These concerns are reasonable if we are asking what I will call first and second level ecological questions. First level questions are a version of ‘what is the vegetation there?’ and once answered, second level questions are a version of ‘how much vegetation is there?’ Both of these questions can usually be answered using broadband sen- sors with wavebands positioned in the locations men- tioned above. For example, we may wish to use remote - ly sensed signals in red, near infrared and middle infrared wavebands to classify for est as a land cover class and then estimate the leaf area index of that forest [Boyd Curran, 1998; Boyd et a/, 20001. Such a use of remote sensing is possible because we are detecting gross spec- tral features that are primarily the result of changes in pigment absorption, within-leaf scattering and water absorption respectively. If we wish to progress further and ask a third level ecological question such as ‘what is the condition of that vegetation?’ then the information we need does no t reside w ithin four to five broad wave- bands. This information is within narrow spectral fea- tures that result from harmonics and overtones of absorptions by foliar biochemicals [Curran, 1989; Clark, 19991. To record these narrow spectral features we need a sensor with many narrow wavebands [Hill Megier, 19941. In other words, it is imaging spectrometry that enables us to ask ecological questions of the third kind. REMOTE SENSING OF FOLIAR BIOCHEMICAL CON- CENTRATION Accurate remotely sensed estimates of the foliar bio- chemical concentration of vegetation canopies have been used to aid the understanding of ecosystem function over a wide range of scales [Peterson et a/, 1988; Gholz et al, 1997; Dawson et al, 19991. This is because many biochemical processes, such as photosynthesis, respira- tion, evapotranspiration and decomposition are related to the foliar concentration of biochemicals such as chlorophyll, water, nitrogen, lignin and cellulose 305

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Imaging

JAG Volume 3 - issue4 - 2001

spectrometry for ecological applications

Paul J Curran

Department of Geography, University of Southampton, Highfield, Southampton 5017 IBJ, UK (e-mail: [email protected])

KEYWORDS: ecology, reflectance spectra, foliar bio-

chemical concentration, NIRS, geological remote sens-

ing

ABSTRACT

Imaging spectrometry from aircraft or satellite borne sensors has

many potential ecological applications. This paper reviews its use

for the remote sensing of foliar biochemical concentration, as this is

an ecological application of remote sensing that is unique to imag-

ing spectrometry. Attention is focussed on the development of

methodologies, drawing where relevant on theory and techniques

from both outside and inside remote sensing. Examples from the

fields of near infrared spectroscopy (NIRS) and geological remote

sensing, along with an extensive reference list, provide an introduc-

tion to some of the ecological opportunities offered by imaging

spectrometry.

INTRODUCTION

Many environmental variables influence the signals that

reach a remote sensor. If we wish to use these remotely

sensed signals to estimate environmental variables then

we need to ensure that the number of remotely sensed

signals is greater than the number of environmental vari-

ables that are causing those signals to vary [Verstraete et

a/, 1996; Curran, 20011. The number of environmental

variables can be reduced by holding some constant (eg,

restrict analysis to one geographic region, correct for

atmosphere) and the number of remotely sensed signals

can be increased by recording many such signals in nar-

row wavebands along the spectrum. We can then repeat

this sampling at different locations, times, geometries of

sensor and illumination and polarizations [Curran et al,

19981. In ecology attention has been directed towards

increased spectral sampling because of great spectral

variability, in the 0.7 pm to 2.5 pm range, for vegetated

landscapes. However, the majority of this spectral vari-

ability lies within just three to five ‘dimensions’ [Curran

et al, 19981; one or two dimensions in visible (0.4-0.7

pm) wavelengths, one dimension in near-infrared (0.7-

1.3 pm) wavelengths and one or two dimensions in mid-

dle-infrared (1.3-2.5 pm) wavelengths. If the remotely

sensed signal in say, four to five well-placed wavebands

could account for most of the spectral variability in veg-

etated landscapes then why would we wish to record in

many more wavebands? From an engineering point of

view surely we would be better advised to maximise the

signal-to-noise ratio, SNR [Smith Curran, 19991 or

increase (ie, make finer) the spatial resolution [Curran,

19941 of our sensor, rather than spread the remotely

sensed signal over many more than five wavebands?

These concerns are reasonable if we are asking what I

will call first and second level ecological questions. First

level questions are a version of ‘what is the vegetation

there?’ and once answered, second level questions are a

version of ‘how much vegetation is there?’ Both of these

questions can usually be answered using broadband sen-

sors with wavebands positioned in the locations men-

tioned above. For example, we may wish to use remote-

ly sensed signals in red, near infrared and middle infrared

wavebands to classify forest as a land cover class and

then estimate the leaf area index of that forest [Boyd

Curran, 1998; Boyd et a/, 20001. Such a use of remote

sensing is possible because we are detecting gross spec-

tral features that are primarily the result of changes in

pigment absorption, within-leaf scattering and water

absorption respectively. If we wish to progress further

and ask a third level ecological question such as ‘what is

the condition of that vegetation?’ then the information

we need does not reside within four to five broad wave-

bands. This information is within narrow spectral fea-

tures that result from harmonics and overtones of

absorptions by foliar biochemicals [Curran, 1989; Clark,

19991. To record these narrow spectral features we need

a sensor with many narrow wavebands [Hill Megier,

19941. In other words, it is imaging spectrometry that

enables us to ask ecological questions of the third kind.

REMOTE SENSING OF FOLIAR BIOCHEMICAL CON-

CENTRATION

Accurate remotely sensed estimates of the foliar bio-

chemical concentration of vegetation canopies have been

used to aid the understanding of ecosystem function

over a wide range of scales [Peterson et a/, 1988; Gholz

et al, 1997; Dawson et al, 19991. This is because many

biochemical processes, such as photosynthesis, respira-

tion, evapotranspiration and decomposition are related

to the foliar concentration of biochemicals such as

chlorophyll, water, nitrogen, lignin and cellulose

305

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Imaging spectrometry for ecological applications

[Running, 1990; Peterson, 1991; Goetz 81 Prince, 19961.

Such remotely sensed estimates have also found applica-

tion in a wide range of ecosystems for estimating vege-

tation stress [Jago et a/, 19991; identifying species

[Martin et a/, 19981 and driving ecosystem simulation

models over large areas [Lucas Curran, 1999; Lucas et

a/, 2000]. The remote sensing of foliar biochemical con-

centration developed rapidly during the 1980s and 1990s

[Peterson Hubbard, 1992; Wessman, 1994; Curran

Kupiec, 1995; Wiilder, 1998; Treitz Howarth, 19991.

Emphasis during the 1990s was on the use of data from

airborne imaging spectrometers, such as the NASA’s

Airborne Visible/Infrared Imaging Spectrometer, AVIRIS

[Staenz, 1992; Curran, 19941 to estimate the biochemi-

cal concentration of tree canopies [Matson

et

a/, 1994;

Johnson et a/, 1994; Dungan et al, 1996; Johnson

Billow, 1996; Martin Aber, 19971. The research was

stimulated by both the NASA’s Accelerated Canopy

Chemistry Programme (ACCP) [Aber, 1994; Wessman,

1994; Smith Curran, 1995; 1996; Curran et a/, 19971

and parallel activities aimed at understanding the inter-

action of radiation with leaves [Fourty et al, 1996; Fourty

Baret, 19981, forest canopies [Danson Curran, 1993;

Danson, 19951 and leaves in forest canopies [Yoder

Pettigrew-Crosby, 1995; Kupiec Curran, 1995; Zagolski

et a/, 19961. In retrospect, a large component of this

understanding seems to have come from modelling

[Ustin et a/, 19991, where leaf models containing a bio-

chemical component [Jacquemoud Baret, 1990;

Dawson et a/, 1998al have been used on their own [Baret

Fourty, 1997; Ganapol et al, 19981 or have been cou-

pled with canopy models [Jacquemoud et a/, 1996;

Dawson et a/, 1999; Ganapol et a/, 1999; Clevers

Jongschaap, 20011. For example, such an approach has

been used to determine the seasonality of leaf chloro-

phyll content [Demarez et a/, 19991, the potential of radi-

ation measured by the Medium Resolution Imaging

Spectrometer (MERIS) on Envisat for the estimation of

canopy chlorophyll content [Dawson, 20001 and the

robustness of equations for the estimation of foliar bio-

chemical content [Gastellu-Etchegorry Bruniquel-Pinel,

20011.

Despite this increased understanding, the use of imaging

spectrometry for the estimation of foliar biochemical

concentration has fallen short of the potential envisaged

for it in the 1980s [Curran, 19891. The reasons for this

are both scientific and instrumental. A major scientific

limitation has been the influence of canopy

structure/architecture on remotely sensed spectra

[Daughtry et a/, 2000; Demarez Gastellu-Etchegorry,

ZOOO], as it was clear that reliance on field sties with

dense [Aber, 19941 or homogeneous [Jago et al, 19991

canopies was not a long term solution for the remote

sensing of foliar biochemical concentration. However,

two developments show great promise for minimising

JAG - Issue 4 - 2001

the canopy effect: first, the availability of reflectance

models that contain explicit descriptions of canopy struc-

ture/architecture [Dawson et al, 1998a; Asner, 1998;

Asner et al, 1999, Gastellu-Etchegorry Bruniquel-Pinel,

20011 and second, the availability of simultaneous LiDAR

data [Rees, 19991 that can be used to measure and in

part, correct for the influence of canopy structure/archi-

tecture on spectra. The instrumental limitations have

been the low SNR of imaging spectrometers and the lack

of a spaceborne imaging spectrometer. However noise in

imaging spectrometer data plummeted during the 1990s

and as a result the SNR of for example, AVIRIS increased

by 2-3 orders of magnitude from 1987-1997 [Green et

a/, 19981. Moreover, on 21 November 2000 a spaceborne

imaging spectrometer, Hyperion was launched on NASAs

Earth Observing (EO-1) satellite (http:/eoI.gsfc.nasa.gov/

Technology/Hyperion.html). This sensor is similar to

AVIRIS (220 bands, 30m spatial resolution compared to

224 bands, 20m spatial resolution of AVIRIS) and paves

the way for a series of future spaceborne imaging spec-

trometers (eg, CHRIS, SPECTRA).

If the remote sensing of foliar biochemical concentration

is to flourish then ecologists who use imaging spectrom-

eter data will need to be alert to (i) the very many chal-

lenges that still lie ahead [Peterson Hubbard, 1992;

Peterson, 20001 and (ii) techniques being developed for

the processing of spectra both outside and inside the

field of remote sensing.

DEVELOPMENTS OUTSIDE REMOTE SENSING: AN

EXAMPLE OF NIRS

The well-established research field of laboratory-based

near infrared spectroscopy (NIRS) involves the opera-

tional use of spectrometry in near-to middle-infrared

wavelengths [AOAC, 19901. NIRS methods were brought

to the attention of the remote sensing community by the

United States Department of Agriculture (USDA) which

used NIRS for the spectrometric assay of biochemicals in

forage crops [Williams Norris, 1987; Marten

et al,

19891. NIRS methods utilise an empirical multivariate

approach which assumes that a foliar spectrum is the dif-

ference between 100 percent reflectance and the sum of

the absorption features of each biochemical, weighted

by their concentration [Peterson et al, 1988; Curran et al,

19921. At its simplest this method uses stepwise regres-

sion to select wavelengths from across the whole of sev-

eral reflectance, or more usually, derivative reflectance

spectra [Dixit Ram, 1985; Tsai Philpot, 19981 that are

most strongly correlated with the biochemical concentra-

tion of foliar samples [Curran, 1989; Martin, 1994;

Jacquemoud Ustin, 20011. However, NIRS is far more

than this, as can be seen in NIRS textbooks [Burns

Ciurczak, 1992; Osborne et al, 19931, proceedings of the

various International Conferences on N/R Spectroscopy

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imaging spectrometry for ecological applications

[eg, Davies Williams, 19961, the Journal of Near

Infrared Spectroscopy and N/R News. Many of the appli-

cations reported in the NIRS literature relate, in some

way, to the food industry [Thyholta Isaksson, 1997;

Batten, 1998; Buning-Pfaue et al, 19981. For example,

quantifying cheese condition [Sorensen Jepsen, 19971,

detecting faecies on chicken carcasses [Peterson, 20001,

measuring sugar in sugar beet [Salgo et al, 19981, or esti-

mating kiwifruit softness [McGlone et al, 19971.

However, it is not a great leap of imagination to move

from the analysis of spectra produced by say, a laborato-

ry-based imaging spectrometer with a spatial resolution

of 5 nm [Martinsen et al, 19991 to the analysis of spectra

produced by an airborne or spaceborne imaging spec-

trometer.

The application of NIRS is wider than the food industry

and includes for example, operational tasks such as mon-

itoring production lines, to more interesting tasks such as

assay of blood outside and inside a patient, identifying a

cars make and model from paint left at the scene of an

accident, to predicting the trees that will be chewed by

elephants [Lister

et

a/, 1997; Sollinger Voges, 1997;

Cassis et a/, 1998; Heise et a/, 19981. Further examples of

NIRS applications are given in Table 1 and broadly-based

spectroscopy texts [eg, Clark, 1991; Duckworth, 1998;

Workman Springsteen, 19981.

TABLE 1. A selection of near infrared spectroscopy (NIRS)appli-

cations as reported in the Journal of Near Infrared Spectroscopy

volumes 1 [I9931 to 8 [ZOO01nclusive.

(Source: http://wwvv.nirpublications.com)

.

Est imat ion of a biochemical chemical

Salt in water; ammonia and propane in air; nitrogen in

soil, sheep guts and potatoes; sugar/starch in bananas,

fruit juice and oranges; nicotine in tobacco and ‘foreign

substances’ in milk.

.

Biochemical assay of

Beer, cereals, rapeseed, flax, goats milk, blood, faeces,

silage and compost.

.

Differentiation between

Olive oils, kiwi fruit, berries, high acid fruits, apple juices,

tree pulps, tobaccos, meats, plastics and crude oils.

Some of the techniques and applications of NIRS are

made possible by the large signal-to-noise ratio of labo-

ratory spectrometers and the homogeneity of the sam-

ples. However, there are three areas of NIRS that have an

immediate relevance here. The first is the biochemical

assay of leaves [Thygesen, 1994; Hallett et a/, 1997;

Methy et al, 19981, where laboratory-based spectra of

leaves have been used both for the estimation of key

foliar biochemicals, such as nitrogen [Joffre et al, 1992;

Aber et a/, 1994; Newman et a/, 1994; Gillon et a/,1999]

and for the development of methods relevant to remote

sensing [Card et a/, 1988; Curran et a/, 1992; Periuelas et

JAG Volume 3 - Issue 4 - 2001

a/, 1995; Gitelson Merzlyak, 1997; Ponzoni

Goncalves, 19991.

The second is the formulation of theory [Wozny et al,

20001 and the development of techniques [Berzaghi et

a/, 20001 for spectral analysis. Many NIRS techniques can

be transferred with little modification to the analysis of

remotely sensed spectra, whether it be for calibration

[Sinnaeve et al, 19941, variable selection [Westad

Martens, 20001, mixture modelling [Hlavka et a/, 19971

or some aspect of regression analysis [Montalvo et a/,

19941.

The third is the development of NIRS as part of the field

of chemometrics [Otto, 19991 where software well-suit-

ed to the analysis of vegetation spectra has been devel-

oped (eg, Unscrambler 7.6, http://www.como.com). The

interface between NIRS and issues such as signal pro-

cessing, artificial intelligence, spectral databases and

genetic algorithms provides an area of particular interest

for a remote sensing audience [Morgan, 1991; Geladi

Dabakk, 1995; Massart et al, 19981. The most fertile of

these is probably the use of neural networks to train

spectral classifiers [Hana et a/, 1997; Dawson et al,

1998b].

However, a note of caution:

Even though N/R began as a classical spectroscopy, its

growth was marked by pragmatism. That is, its char-

acteristics or effects were observed and exploited

long before we ever true/y understood why it really

worked [McClure et a/, 1991, p. 14A].

NIRS will continue to be a valuable source of techniques

for ecological remote sensing but to understand the

processes involved we will undoubtedly need to return to

first principles and leaf and canopy reflectance models

[Ustin et a/, 19991.

DEVELOPMENTS INSIDE REMOTE SENSING: AN

EXAMPLE FROM GEOLOGICAL REMOTE SENSING

NIRS techniques based on stepwise regression are robust

and effective; however, they are not ideal due to (i) the

risk of overfitting (usually when many more wavebands

than samples are used); (ii) the selection of wavelengths

that are non-causal [Baret Fourty, 19971, or are located

in the absorption feature of a biochemical to which the

biochemical of interest is correlated [Curran et a/, 1992;

Grossman et al, 19961 and the (iii) lack of generally

accepted procedures for both data preprocessing to

enhance absorption features and standardisation to min-

imise the effect of spectral variability that is independent

of biochemical concentration. To overcome at least some

of these problems we need look no further than geologi-

cal remote sensing [Clark, 1991; Kruse et a/, 1993;

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Imaging spectrometry for ecological applications

JAG Volume 3

-

Issue 4 - 2001

Schowengerdt, 19971, in particular the methodologies

proposed by Kokaly Clark [I9991 and used by Kokaly

[2001], that benefited from continuum-removal, cakula-

tion of band depth and normalisation. In these method-

ologies a continuum was fitted to the reflectance spectra

in order to represent absorptions other than the one of

interest [Clark Roush, 19841. The value associated with

a wavelength was then expressed, not as reflectance or

the first derivative of reflectance, but as band depth nor-

malised to the waveband at the centre of the absorption

feature @NC) or the area of the absorption feature (BNA).

2100 nm

Absorption eature

BAND DEPTH NORMAL ED TO BAND DEPTH AT THE CEN-

TRE OF THE ABSORPTION FEATURE (BNC)

This measures the depth of the waveband of interest

from the continuum line, relative to the depth of the

waveband from the continuum line at the centre of the

absorption feature.

BNC = (l-(R/Ri))/(l-(R,/Ri,))

where BNC is band depth normalised to the centre; R is

reflectance of sample at waveband of interest; Ri is

reflectance of continuum line at waveband of interest; R,

is reflectance of sample at absorption feature centre and

Ri, is reflectance of continuum line at absorption feature

centre.

BAND DEPTH NORMALISED TO AREA OF ABSORPTION FEA-

TURE BNA)

This measures the depth of the waveband of interest

from the continuum line, relative to the area of the

absorption feature.

0.8 ’

I I I

’ 0.2

2000

2100 2200

Wavelength (nm)

FIGURE 1. Diagrammetric illustration of continuum-removal

and band depth calculation for waveband (IJ in the 2100 nm

absorption feature.

BNA = (1 - R/Ri))/A)

where BNA is band depth normalised to area and A is

area of absorption feature.

These are illustrated for waveband h in Figure 1, where R

is 48.9 percent, Ri is 52.4 percent, R, is 41.5 percent and

Ri, is 49.5 percent. Therefore, band depth is 0.07 relative

reflectance at wavelength h and 0.16 relative reflectance

at the centre of the absorption feature. Given that the

area of the absorption feature (A) is 19.13 relative

reflectance nm-1 then for waveband h BNC is 0.41 and

BNA is 0.004.

Kokaly Clark [I 9991 developed and tested their

methodologies on a large data set collected as part of

the ACCP. This comprised laboratory spectra and bio-

chemical assays of three biochemicals (nitrogen, lignin,

cellulose) for seven sites and a total of 744 samples. They

used three absorption features (centred around 1730

nm, 2100 nm and 2300 nm) and the procedures outlined

above. With this approach they obtained high values of

R2 between spectral data and biochemical concentration

(for example, R2 for nitrogen ranged from 0.75 to 0.94).

These methodologies have been tested independently on

a smaller foliar dataset (n=68-70) from one site but for

twelve biochemicals [Curran

et a/

20011. Stepwise

regression on first derivative spectra resulted in large R2

(maximum 0.90, average 0.74) and a small root mean

square error (RMSE) as a percentage of the mean (mini-

mum 0.46, average 8.24) between estimated and

observed foliar biochemical concentrations. Stepwise

regression on bands normalised to band depth at the

centre of the absorption feature (BNC) or the area of the

absorption feature (BNA) resulted in even larger R2 (max-

imum 0.94 and 0.96, average 0.80 and 0.84, respective-

ly) and small RMSE as a percentage of the mean (mini-

mum 0.24 and 0.12, average 6.68 and 4.86, respective-

ly) between estimated and observed foliar biochemical

concentrations. In this case the most accurate methodol-

ogy for the estimation of foliar biochemical concentra-

tion is well-known in geological remote sensing [Clark,

19991 and was stepwise regression normalised to the

area of the absorption feature.

The original scientific rationale for imaging spectrometry

was geological [Peterson, 20001 and throughout the

1990s the geological agenda drove many developments

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R SUM

La

spectrometrie imageante a partir d’avions ou de satellites

equip de capteurs a plusieurs applications ecologiques poten-

tielles. Le present article passe en revue son utilisation pour la

teledetection de concentration biochimique foliaire, puisqu’il

s’agit la d’une application de teledetection unique pour la spec-

trometrie imageante. L’attention est concentree sur le develop-

pement de methodologies, faisant appel selon les cas a la theo-

rie et aux techniques de la teledetection interieure et exterieure.

Des exemples des champs de spectroscopic du proche infrarou-

ge (NIRS) et de la t l detection geologique, avec une liste de

reference extensive, fournissent une introduction a certaines des

opportunites ecologiques offertes par la spectrometrie imagean-

te.

R SUM N

La espectrometria de imdgenes tomadas por sensores aeroporta-

dos o satelitdrios se presta para muchas aplicaciones potenciales

en el area de la ecologia. Este artfculo hate una resetia de su

uso con fines de sensoramiento remoto de la concentraci6n

foliar en compuestos bioquimicos. La atencion se concentra en

el desarrollo de metodologias, con enfasis en teoria y tecnicas

tanto desde el angulo de la teledeteccion coma fuera de ello.

Como introduction a algunas de las oportunidades ofrecidas por

la espectrometria de imdgenes, se muestran ejemplos tomados

en 10s campos de la espectroscopia del infrarrojo cercano (NIRS)

y de la teledeteccion geologica

312