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Remote sensing: the use of polarized visible light (PVL) to estimate surface soil moisture
clpplied Geography (198 I), I. 41-53 0 1981 Butterworths
41
Remote sensing: the use of polarized visible light (PVL) to estimate surface soil moisture Paul J. Curran
Department of Geography, Unicersity of Reading, England
Abstract Sunlight is unpolarized: if it is reflected from a soil surface it will become partially polarized. The degree of polarization will be dependent upon the soil surface roughness and moisture content. The study of polarized visible light (PVL) reflectance has developed in the past two decades, from the use of a non-imaging photometer-polarimeter to record the surface roughness of laboratory samples, to the use of a camera and polarizing filter to estimate surface soil moisture in the field. It is demonstrated that PVL recorded at a high phase angle, from up to light aircraft altitudes, can successfully estimate surface soil moisture. This is possible regardless of solar angle. soil albedo or soil slope. over a wide but not umlimited range of surface soil moisture states, but only if surface disturbance and cloud cover are minimired. While PVL cannot, in general. rival currently available techniques for the remote sensing of surface soil moisture it is eminently suitable where an estimate of surface soil moisture is required at low cost for a small area,
Introduction
To sense remotely the surface soil moisture content of an area the size of a field, the well-established non-visible thermal or radar sensors are, in most cases, accurate but very expensive. Photography in visible and near visible wavelengths is a suitable alternative and is less expensive. However, although photographic multispectral sensing for surface soil moisture assessment is well documented, the use of polarized visible light (PVL) has received little attention. This review assesses the value of PVL for the estimation of surface soil moisture. The basis of the relarionship
between
PVL and surface soil moisture
Light waves from the sun are unpolarized and vibrate in a number of planes. When these waves are reflected from a soil surface the degree of polarization at a high phase angle (Fig. I) will be dependent upon several factors (Beckman 1961); one of the more important of these is surface soil moisture. For completely saturated soils where free water appears on the surface, light will not be scattered but will be reflected specularly in one vibrational plane and will therefore be completely polarized. For rough surfaced soils, below saturation, reflectance is not specular but diffuse and polarization is low but will increase as saturation is approached. The intensity of PVL reflected from a soil surface is therefore related to the moisture content of that soil surface; to the depth of a few microns, in theory, but found to be to a depth of about a centimetre in practice.
42
Remote sensing SENSOR SUN
ALTITUDE PHASE
SOLAR
SURFACE
ANGLE 1
Fig. I. Geometrical Developments
ANGLE
relationships
and terms.
in the use of PVL for measuring surface conditions
The polarization of visible light has many practical applications in fields as diverse as astronomy, optics, engineering and zoology. Aside from several 19th century lunar studies using PVL (Secchi 1860), research into the relationship between PVL reflectance and surface condition is a product of the last two decades and has followed four distinct development stages (Table 1). Tnhle I. Development Development stage
Stage I
stages in the use of PVL Era
for recording
surface
conditions
Objects of study
Equipment
Location
Non-imaging Non-imaging
Imaging
Laboratory Laboratory and field Laboratory and field Laboratory and field
Stage 2
Mid- 1960s Late 1960s
stage 3
Early 1970s
Artificial surfaces Artificial and natural surfaces Soils
Stage 4
Late 1970s
Soils
Non-imaging
Sfage one Laboratory-based experiments were first used to determine the relationships between surface texture and PVL reflectance; for example Coulson el al. (1965), Egan and Hallock (1966), Blau et al. (1967) and Chen et al. (1967). These studies concluded that while PVL could be used to distinguish objects with different surface roughness, further experiments were required before this technique could be used for the remote sensing of absolute surface roughness. Srage two Egan and his co-workers used PVL to study a number of natural surfaces, in both the laboratory and the field (Egan 1968; Egan et al. 1968; Egan and Hallock 1969). They
Paul J. Curran
43
demonstrated, first, that PVL reflectance was maximized by smooth and wet surfaces and minimized by rough and dry surfaces and, second, that PVL reflectance was so highly correlated with moisture that it could be used to estimate surface moisture content. Stage three
PVL was first used to record surface soil moisture by Steg and Frost (197 1) and Stockhoff and Frost (I97 1). They demonstrated that high values of PVL reflectance, produced at phase angles above 80*, were highly dependent upon soil moisture (Fig. 2), with only slight dependence on clouds, atmospheric haze, soil type and surface slope. Percent 2
10
20
Polarisation 30
40
50
60
70
20-
Phase
angle, in degrees
Fig. 2. The degree of polarization (PVL) recorded using a photometer-polarimeter for a range of soil moisture contents and phase angles. (Source: Steg and Frost 197 I)
in the laboratory
By the end of the 1970s a camera and polarizing filter were replacing the non-imaging techniques. Using this method Curran (1978, 1979b) estimated a wide range of soil moisture conditions from ground, mast and aircraft altitudes. An example of the near-linear re!ationship between the surface moisture content of a peat soil and the intensity of PVL reflectance recorded from a light aircraft is given in Fig. 3. Future development stages are seen to be: first, the use of PVL to record the roughness of a range of natural surfaces, for example sea surface roughness (Curran 1979a) and vegetation cover (Curran 1981); and second, the use of PVL to estimate surface soil moisture in conjunction with other remote sensing techniques.
Remote sensing
44
L
l 0
C .o 5
0
3
.Y, b
l
0
0
B l
E u” 2 t a
0
1
l
0 100
200
300
Percent surface soil moisture Fig. 3. Per cent polarization (PVL) recorded for a range of surface soil moisture contents. The peat soil is recorded from a light aircraft using a camera with polarizing filter. (Source: Curran I9 78)
Equipment used to measure PVL in the field There are two types of equipment suitable for measuring PVL in the field. The first is a non-imaging photometer-polarimeter that records the intensity of PVL reflectance for a point on the soil surface; the second is a camera and polarizing filter that records the spatial distribution of PVL reflected from the soil surface.
Non-imaging:
Portable photometer-polarimeter
The portabie photometer-polarimeter is a battery-powered optical sensor; it consists of a photo-diode, a polarizer, a polarization analyser, spectral filters and optics. These are all enclosed in a !ight-weight housing suitable for mounting on to a tripod, a vehicle or an Piate 1. An examp!e of panchromatic, at minimum
and maximum.
light aircraft photography taken using a polarizing filter set The site is an area of peat extraction in central Somerset: note the
lighter image tone of the trees (central) and saturated polarization (PVL) imagery. (Source: Curran 1978)
zone (bottom left) in the maximum
MINIMUM
POLARISATION
SHAPWICK
HEATH,
SOMERSET
1977
MAXIMUM
22 “(Ii JUNE
POLARISATION
Remote sensing
46
aircraft. Incoming radiation passes through a quartz lens, a spectral filter, a rotating polarizer and then as an electric signai through a photomultipler and an alternator. The maximum and minimum light intensity (I) is recorded during each revolution of the polarizer; this occurs when the polarizer is respectively parallel and then at right angles to the ground surface. These values of maximum and then minimum intensity are first standardized against an internal reflectance standard and then recorded on magnetic tape or strip chart recorder (Egan 1968). The standardized values of maximum and minimum intensity (~1) are used to calculate per cent PVL using formula 1. Per
cent
pvL
=
ts1hax) - sI Cm31x
1oo
(Formula
1)
Is1 (max) + s.1 (min)l Imaging: Camera andpolarizingfilter The camera’s focal axis is set to face the sun and is angled towards the ground surface. Two exposures are made of the scene, both through a polarizing filter on to a panchromatic film. The operator obtains the record of maximum polarization by bringing the fine parallel lines on the polarizing filter in parallel with the ground surface. The second exposure has the filter set at right angles to the first, to obtain values of the maximum and then the minimum polarization (Curran 1978). An example of this type of photography is given in Plate 1. The opacity (tone) of the film negative is recorded quantitatively using a densitometer (Curran 1980). To suppress the variable effect of film exposure and processing, the calculation of per cent PVL (formula 2) includes standardization of the surface opacity (SuO) against the opacity of a scene standard (StO), for example a marker target or vehicle roof positioned within the scene. SuO (max)
Per cent PVL =
(
SuO (min)
St0 (max)
- St0 (min) i x 100 SuO (max) SuO (min) + St0 (min) 1 1 St0 (max)
(Formula
2)
The relationship between PVL and surface soil moisture The degree of correlation between PVL reflectance and surface soil moisture is known to be dependent upon the user’s observation of, and allowance for, (i) sensor characteristics, (ii) scene characteristics and (iii) inherent limitations of the technique. Unless adequate attention is paid to these three factors the degree of correlation between PVL reflectance and surface soil moisture is likely to be low. For example, using PVL, Curran (1978, 1979b) obtained an explained variance (13) of only 42 per cent when flying in suboptimal conditions. This increased to 84 per cent under more favourable sensor and scene conditions. Sensor characteristics The user-controlled characteristics of the sensor that affect the degree of correlation between PVL reflectance and surface soil moisture are, first, the wavelengths recorded by the sensor and second the geometry of the sensor; its phase, look and solar angle and its altitude (Fig. I).
Paul J. Curran
47
(a) Wavelengths. Polarized short visible wavelengths, although more sensitive to surface conditions than long visible wavelengths, are greatly reduced in intensity by atmospheric scatter. Therefore, the polarization of the whole visible spectrum from 400 nm to 700 nm is usually recorded (Chen ef al. 1967). An example of the relationship between PVL reflectance, recorded in five wavebands at a height of 10 metres, and the surface moisture of a peat soil is given in Fig. 4; the correlation between PVL reflectance and surface soil moisture is noted to be high when using the whole of the visible spectrum.
LIGHT . . . . . . . . .. . . . --_______
white b,“e green orange yellow red
a-
2-
__--__-___ _______
__--Ot
0
I
I
I
-_________ I
,
50
,
__---
-----_________---1
I
1
,
,
,
100
,
1
,
I50
,
,
1
,
-I
200
Percent Surface Soil Masture
Fig. 4. Per cent polarization (PVL) recorded for a range of surface soil moisture contents in the field, from an altitude of 10 metres, using a camera with polarizing filter. (Source: Curran 1978)
(b) Phase angle. The term phase angle is used to define the angle between the sun and the sensor in a vertical plane (Fig. 1). Each surface type has a polarization maximum over a specific range of phase angles. The polarization maxima for soils is recorded over a wide range of phase angles, from around 90” for smooth surfaced soils to around 1 IO0 for rough surfaced soils. This wide phase angle range ensures that the relationship between PVL reflectance and surface soil moisture changes little with the small changes in imaging geometry (Fig. 2) that occur during every photographic mission, thus making PVL a suitable technique for remotely sensing undulating terrain from an unstable light aircraft platform. (c) Solar and look angle. The solar angle varies diurnally and seasonally; to maintain a phase angle of between 90’ and 110“ the look angle will have to move from very high on winter mornings to very low at midday during the summer (Fig. 1). Steg and Frost (1971) have shown that the actual solar and look angles are unimportant providing the phase
Remote sensing
48
angle remains within the phase angle observation that only the illuminated and therefore the increase in shadow polarization once the measurement is
range of soils (90°-1 loo). They attribute this to the portion of the soil contributes to the measured signal at low solar angles has little effect on the degree of standardized against a scene standard.
(6) Altitude of rhe sensor. The relationship between PVL reflectance and surface soil moisture is unique for each sensor altitude. This is a result of first, atmospheric scattering that decreases the intensity of PVL reflectance recorded with altitude (Egan 1968), as in Fig. 5. and second the problem of sampling, as an increase in sensor altitude results in the integration of a progressively larger and more variable area of soil within the area sampled by PVL (Curran 1978). An increase in sensor altitude therefore decreases the amount, while increasing the variability, of PVL recorded by the sensor. Surface
257 i. 20.
I
2
s 151 ._ B ! a" ; IO. z z CL 54
01
I I 1
0 0
'\ '\ '\ '\ '. '\\
0
soil moisture 150% 100% 50%
‘\
Q-_
--._ %_
‘k_ __._
---_ -_-_ -. _.._ 0 ---_ --__ ---_ --._ --._ ---_ --._ _-__ --._ -.._ ---_ ---_ -___ ----_ --__ ----__ --._--.._ -.__ -. _--_ ---__ O------______ --__ -----------------________________ ---_-__-_‘ ------------_________________ 8 5
M
10 Alldude
of sensor.
in
loo
5ca
1000
metres
Fig. 5. Per cent polarization (PVL) recorded at three altitudes The peat soil is recorded using a camera with polarizing filter. (Softrce: Curran 1978)
for three surface soil moisture
states.
Scene characteristics
The scene conditions that could potentially decrease the degree of correlation between PVL reflectance and surface soil moisture are the particle size distribution of the soil, the albedo of the soil and the range of soil moisture values. (a) Particle size distribution. The particle size distribution of the soil in large measure determines the soil’s texture and structure, both of which affect surface roughness at different scales. In laboratory studies of individual particles, grains of sand with large flat facets polarize light to a greater extent than clay aggregates (Blau et al. 1967). In field studies, smooth. poorly structured. fine-textured clay soils strongly polarize visible light while coarse-textured sandy soils produce a rough surface that is a poor polarizer (Steg and Frost 197 1). Therefore while there is a positive relationship in the laboratory between PVL reflectance and particle size, this relationship is reversed in the field. This negative relationship can be seen in Fig. 6 where particle size and PVL reflectance are plotted for a
Paul J. Curran Regressionline Cormlotion
01 01
05
IO
50 Portlcle sue in mm
100
49
Y = -3 28 ilog x I. 9.99 r .-07L sq al I%
500
1000
Fig. 6. Per cent polarization (PVL) recorded for a dry soil sieved into 13 particle size classes. The soil is recorded in the laboratory using a camera with polarizing filter. (Source: Curran 1978)
sieved soil. The particle size distribution provides the background polarization that increases with an increase in surface soil moisture. As such, it is only of importance when the background PVL reflectance is so high that low values of surface soil moisture cannot be detected by PVL, as is the case on estuarine mudbanks. (b) Soil albedo. There is an inverse relationship between soil albedo and the reflectance of PVL (Blau et al. 1967) and a positive relationship between soil albedo and atmospheric scattering of PVL (Steg and Frost 1971). Therefore the polarization of visible light from light-coloured soils is relatively low and little affected by atmospheric scatter, while the polarization of visible light from a dark-coloured soil is relatively high and greatly affected by atmospheric scatter. For measurements taken at up to aircraft altitudes these effects tend to cancel each other out, and PVL may be considered to be reasonably independent of soil albedo. (c) Range of soil moisfure
values. PVL is not sensitive to the entire range of surface soil moisture values. Soils that strongly polarize visible light when dry tend to mask small changes of PVL reflectance at low surface soil moisture values. At the other extreme the addition of extra water to a soil already at field capacity will not produce a proportional change in the intensity of PVL reflectance. Therefore, the correlation between PVL reflectance and surface soil moisture is maximized on slightly sloping and freely drained sites and minimized on very dry or wet sites. This has been confirmed in laboratory measurements of a draining slope (Fig. 7). The wet and dry proportions of the slope were outside the recording range of PVL and had a low correlation between surface soil moisture content and PVL reflectance, while the free-draining but still moist portions had a high correlation between surface soil moisture content and PVL reflectance.
Receiving
Normal
Shedding
OF MORPHOLOGY
O. cm
30
‘ig
_
08
MAP
with
a range
+0,6 to *O-g5
contour of iso-correlation
07
OF CORRELATIONS
(Source:
Curran
1979b)
Fig. 7. The correlation between per cent polarization (PVL) and surface soil moisture (map of correlations), plotted on to a map of slope form (map of morphology). The soil, a sandy loam, on an irrigated model slope, is recorded using a camera with polarizing filter.
MAP
Paul J. Curran
51
Limitations of the technique The correlation between PVL reflectance and surface soil moisture is low under very cloudy skies or when monitoring an area with a variable surface roughness. Under such conditions PVL is not a suitable technique for estimating surface soil moisture.
(a) Cloud cover. Sunlight is unpolarized but skylight produced by Rayleigh and Mie scattering is polarized. As the sun is brighter than the adjacent sky by a factor of about 10 000 (Egan 1968), even on a moderately cloudy day the sun may be considered as the main source of light flux. This is not so when the cloud cover is so thick that the location of the sun cannot be observed from the ground. In such circumstances the relationship between PVL reflectance and surface soil moisture is low (Steg and Frost 197 1).
.
:
:
:
:P
bt
w I G
KEY -
sp Spring S w
0
25
50 Percent
15 surface
100
125
Summer W,ntcr
150
so11 moisture
8. Per cent polarization (PVL) recorded for a range of surface soil moisture values over three seasons. The peat soil is recorded from a light aircraft using a camera with polarizing filter.
Fig.
Remote sensing
52
(b) Monitoring surface soil moisture. Polarized visible light can be used to monitor changes in surface soil moisture if the soil surface roughness does not change (Curran 1979b). Polarized visible light is unsuitable for recording surface soil moisture over several seasons, as during this time the soil surface roughness will alter. Figure 8 shows a scatter plot of PVL reflectance recorded from light aircraft altitudes, plotted against stirface soil moisture for one site on 12 dates. As the soil surface roughness varied over this time the overall correlation was low Cjust significant at the 5 per cent level) decreasing to insignificant if each of the three seasons is studied separately.
Summary PVL reflectance recorded at a phase angle of between 90°-1 10’ from up to light aircraft altitudes can successfully estimate surface soil moisture. This is possible regardless of solar angle, soil albedo or soil slope. over a wide but not unlimited range of surface soil moisture states, but only if surface disturbance and cloud cover are minimized. This technique provides an economic means of remotely sensing surface soil moisture from the ground and from the air. It also has application for the recording of other natural surfaces including water and vegetation. PVL cannot rival the current range of we!l-tested techniques for the estimation of surface soil moisture. However, PVL is eminently suitable where soil moisture estimation of a small area is required at low cost. It is hoped that the current use of PVL both as a field technique and as an extra sensor, used alongside other remote sensors, will result in further evaluations of PVL for the estimation of surface soil moisture. Acknowledgement The writer wishes to thank Ian Fenwick for his comments
on the original draft.
References Beckman, P. ( 196 I) The depolarization of electromagnetic waves scattered from rough surfaces. rlclu Pcchtfica 6, 5 I l-523. Blau. H.H.. Gray. E. L. and Bourioius, G. M. (1967) Reflection and polarization properties of powder materials. Applied Opks 6, I899- 1904. Chen. H.. Rao. C. R. N. and Sekera. 2. (1967) Incesrigarions oJIhe polarization oJligh[ rejlecled bjq namral sutjaces. p. 104. Air Force Cambridge Research Laboratories. Scientific Report, 67-0089. Coulson. K. L.. Bourioius, G. M. and Gray. E. L. (1965) Optical reflection properties of natural surfaces. Jowxal ofGeophysical Research 70, 4601-46 I I. Curran, P. J. (1978) A photographic method for the recording of polarized visible light for soil surface moisture indications. Retnole Sensing oJ‘En~irotztnet~t7, 305-322. Curran. P. J. (lY79a) Preliminary investigation into the application of oblique multispectral photography for the monitoring of sewage outfalls. Journal ofEncirotztnen~a~ .\lanagetnenr 8, 249-266. Curran. P. J. (1979b) The use of polarized panchromatic and false colour infra-red film in the monitoring of soil surface moisture. Retnore Sensing of Encirontnent 8, 249-266. Curran. P. J. (1980) Relative reflectance data from preprocessed multispectral photography.
Inrertla~iut~alJournal of Retvole Sensiq
I, 77-83.
P. J. (198 I) The relationship between polarized visible light and vegetation amount. Retnote Sensing 0j‘Enc~irot~tnenr10 (in press). Egan. W. G. and Hallock. H. B. (1966) Polarimetry signature of terrestrial and planetary materials. Proceedings of rhe 4th inrernarional svnposium on remote sensing of encironment, pp. 67 I-689. Ann Arbor: University of Michigan.
Curran.
Paul J. Curran
53
Egan. W. G. (1968) Aircraft polarimetric and photometric observations. Proceedings of the Sfh international symposium on remote sensing of environment, pp. 169-189. Ann Arbor: University of Mlchigan. Egan. W. G.. Grusauskas. J. and Hallock. H. B. (1968) C ptica’i depolarization properties ofsurfaces illuminated by coherent light. Applied Optics 7. I S29- 1534. Egan. W. G. and Hallock, H. B. (1969) Coherence-polarization phenomena in remote sensing. Proceedings of the Institute of Electrical and Electronic Engineers 57.62 l-628. Secchi. A. (1860) Polarization oflunar light. Astronomische Nachrichten 52.93-97. Steg, L. and Frost. R. T. (1971) Visible polarization signature for remote sensing of soil surface moisture. COSPAR Plenary Meeting. Leningrad. USSR (I 970). pp. 7 17-720. Stockhoff. E. H. and Frost, R. T. (1971) Polarization of light scattered from moist soils. Proceedings of the 7th international symposium on remote sensing of environment, pp. 345-363. Ann Arbor: University of Michtgan. (Received
29 Ju/y 1980)
41
Remote sensing: the use of polarized visible light (PVL) to estimate surface soil moisture Paul J. Curran
Department of Geography, Unicersity of Reading, England
Abstract Sunlight is unpolarized: if it is reflected from a soil surface it will become partially polarized. The degree of polarization will be dependent upon the soil surface roughness and moisture content. The study of polarized visible light (PVL) reflectance has developed in the past two decades, from the use of a non-imaging photometer-polarimeter to record the surface roughness of laboratory samples, to the use of a camera and polarizing filter to estimate surface soil moisture in the field. It is demonstrated that PVL recorded at a high phase angle, from up to light aircraft altitudes, can successfully estimate surface soil moisture. This is possible regardless of solar angle. soil albedo or soil slope. over a wide but not umlimited range of surface soil moisture states, but only if surface disturbance and cloud cover are minimired. While PVL cannot, in general. rival currently available techniques for the remote sensing of surface soil moisture it is eminently suitable where an estimate of surface soil moisture is required at low cost for a small area,
Introduction
To sense remotely the surface soil moisture content of an area the size of a field, the well-established non-visible thermal or radar sensors are, in most cases, accurate but very expensive. Photography in visible and near visible wavelengths is a suitable alternative and is less expensive. However, although photographic multispectral sensing for surface soil moisture assessment is well documented, the use of polarized visible light (PVL) has received little attention. This review assesses the value of PVL for the estimation of surface soil moisture. The basis of the relarionship
between
PVL and surface soil moisture
Light waves from the sun are unpolarized and vibrate in a number of planes. When these waves are reflected from a soil surface the degree of polarization at a high phase angle (Fig. I) will be dependent upon several factors (Beckman 1961); one of the more important of these is surface soil moisture. For completely saturated soils where free water appears on the surface, light will not be scattered but will be reflected specularly in one vibrational plane and will therefore be completely polarized. For rough surfaced soils, below saturation, reflectance is not specular but diffuse and polarization is low but will increase as saturation is approached. The intensity of PVL reflected from a soil surface is therefore related to the moisture content of that soil surface; to the depth of a few microns, in theory, but found to be to a depth of about a centimetre in practice.
42
Remote sensing SENSOR SUN
ALTITUDE PHASE
SOLAR
SURFACE
ANGLE 1
Fig. I. Geometrical Developments
ANGLE
relationships
and terms.
in the use of PVL for measuring surface conditions
The polarization of visible light has many practical applications in fields as diverse as astronomy, optics, engineering and zoology. Aside from several 19th century lunar studies using PVL (Secchi 1860), research into the relationship between PVL reflectance and surface condition is a product of the last two decades and has followed four distinct development stages (Table 1). Tnhle I. Development Development stage
Stage I
stages in the use of PVL Era
for recording
surface
conditions
Objects of study
Equipment
Location
Non-imaging Non-imaging
Imaging
Laboratory Laboratory and field Laboratory and field Laboratory and field
Stage 2
Mid- 1960s Late 1960s
stage 3
Early 1970s
Artificial surfaces Artificial and natural surfaces Soils
Stage 4
Late 1970s
Soils
Non-imaging
Sfage one Laboratory-based experiments were first used to determine the relationships between surface texture and PVL reflectance; for example Coulson el al. (1965), Egan and Hallock (1966), Blau et al. (1967) and Chen et al. (1967). These studies concluded that while PVL could be used to distinguish objects with different surface roughness, further experiments were required before this technique could be used for the remote sensing of absolute surface roughness. Srage two Egan and his co-workers used PVL to study a number of natural surfaces, in both the laboratory and the field (Egan 1968; Egan et al. 1968; Egan and Hallock 1969). They
Paul J. Curran
43
demonstrated, first, that PVL reflectance was maximized by smooth and wet surfaces and minimized by rough and dry surfaces and, second, that PVL reflectance was so highly correlated with moisture that it could be used to estimate surface moisture content. Stage three
PVL was first used to record surface soil moisture by Steg and Frost (197 1) and Stockhoff and Frost (I97 1). They demonstrated that high values of PVL reflectance, produced at phase angles above 80*, were highly dependent upon soil moisture (Fig. 2), with only slight dependence on clouds, atmospheric haze, soil type and surface slope. Percent 2
10
20
Polarisation 30
40
50
60
70
20-
Phase
angle, in degrees
Fig. 2. The degree of polarization (PVL) recorded using a photometer-polarimeter for a range of soil moisture contents and phase angles. (Source: Steg and Frost 197 I)
in the laboratory
By the end of the 1970s a camera and polarizing filter were replacing the non-imaging techniques. Using this method Curran (1978, 1979b) estimated a wide range of soil moisture conditions from ground, mast and aircraft altitudes. An example of the near-linear re!ationship between the surface moisture content of a peat soil and the intensity of PVL reflectance recorded from a light aircraft is given in Fig. 3. Future development stages are seen to be: first, the use of PVL to record the roughness of a range of natural surfaces, for example sea surface roughness (Curran 1979a) and vegetation cover (Curran 1981); and second, the use of PVL to estimate surface soil moisture in conjunction with other remote sensing techniques.
Remote sensing
44
L
l 0
C .o 5
0
3
.Y, b
l
0
0
B l
E u” 2 t a
0
1
l
0 100
200
300
Percent surface soil moisture Fig. 3. Per cent polarization (PVL) recorded for a range of surface soil moisture contents. The peat soil is recorded from a light aircraft using a camera with polarizing filter. (Source: Curran I9 78)
Equipment used to measure PVL in the field There are two types of equipment suitable for measuring PVL in the field. The first is a non-imaging photometer-polarimeter that records the intensity of PVL reflectance for a point on the soil surface; the second is a camera and polarizing filter that records the spatial distribution of PVL reflected from the soil surface.
Non-imaging:
Portable photometer-polarimeter
The portabie photometer-polarimeter is a battery-powered optical sensor; it consists of a photo-diode, a polarizer, a polarization analyser, spectral filters and optics. These are all enclosed in a !ight-weight housing suitable for mounting on to a tripod, a vehicle or an Piate 1. An examp!e of panchromatic, at minimum
and maximum.
light aircraft photography taken using a polarizing filter set The site is an area of peat extraction in central Somerset: note the
lighter image tone of the trees (central) and saturated polarization (PVL) imagery. (Source: Curran 1978)
zone (bottom left) in the maximum
MINIMUM
POLARISATION
SHAPWICK
HEATH,
SOMERSET
1977
MAXIMUM
22 “(Ii JUNE
POLARISATION
Remote sensing
46
aircraft. Incoming radiation passes through a quartz lens, a spectral filter, a rotating polarizer and then as an electric signai through a photomultipler and an alternator. The maximum and minimum light intensity (I) is recorded during each revolution of the polarizer; this occurs when the polarizer is respectively parallel and then at right angles to the ground surface. These values of maximum and then minimum intensity are first standardized against an internal reflectance standard and then recorded on magnetic tape or strip chart recorder (Egan 1968). The standardized values of maximum and minimum intensity (~1) are used to calculate per cent PVL using formula 1. Per
cent
pvL
=
ts1hax) - sI Cm31x
1oo
(Formula
1)
Is1 (max) + s.1 (min)l Imaging: Camera andpolarizingfilter The camera’s focal axis is set to face the sun and is angled towards the ground surface. Two exposures are made of the scene, both through a polarizing filter on to a panchromatic film. The operator obtains the record of maximum polarization by bringing the fine parallel lines on the polarizing filter in parallel with the ground surface. The second exposure has the filter set at right angles to the first, to obtain values of the maximum and then the minimum polarization (Curran 1978). An example of this type of photography is given in Plate 1. The opacity (tone) of the film negative is recorded quantitatively using a densitometer (Curran 1980). To suppress the variable effect of film exposure and processing, the calculation of per cent PVL (formula 2) includes standardization of the surface opacity (SuO) against the opacity of a scene standard (StO), for example a marker target or vehicle roof positioned within the scene. SuO (max)
Per cent PVL =
(
SuO (min)
St0 (max)
- St0 (min) i x 100 SuO (max) SuO (min) + St0 (min) 1 1 St0 (max)
(Formula
2)
The relationship between PVL and surface soil moisture The degree of correlation between PVL reflectance and surface soil moisture is known to be dependent upon the user’s observation of, and allowance for, (i) sensor characteristics, (ii) scene characteristics and (iii) inherent limitations of the technique. Unless adequate attention is paid to these three factors the degree of correlation between PVL reflectance and surface soil moisture is likely to be low. For example, using PVL, Curran (1978, 1979b) obtained an explained variance (13) of only 42 per cent when flying in suboptimal conditions. This increased to 84 per cent under more favourable sensor and scene conditions. Sensor characteristics The user-controlled characteristics of the sensor that affect the degree of correlation between PVL reflectance and surface soil moisture are, first, the wavelengths recorded by the sensor and second the geometry of the sensor; its phase, look and solar angle and its altitude (Fig. I).
Paul J. Curran
47
(a) Wavelengths. Polarized short visible wavelengths, although more sensitive to surface conditions than long visible wavelengths, are greatly reduced in intensity by atmospheric scatter. Therefore, the polarization of the whole visible spectrum from 400 nm to 700 nm is usually recorded (Chen ef al. 1967). An example of the relationship between PVL reflectance, recorded in five wavebands at a height of 10 metres, and the surface moisture of a peat soil is given in Fig. 4; the correlation between PVL reflectance and surface soil moisture is noted to be high when using the whole of the visible spectrum.
LIGHT . . . . . . . . .. . . . --_______
white b,“e green orange yellow red
a-
2-
__--__-___ _______
__--Ot
0
I
I
I
-_________ I
,
50
,
__---
-----_________---1
I
1
,
,
,
100
,
1
,
I50
,
,
1
,
-I
200
Percent Surface Soil Masture
Fig. 4. Per cent polarization (PVL) recorded for a range of surface soil moisture contents in the field, from an altitude of 10 metres, using a camera with polarizing filter. (Source: Curran 1978)
(b) Phase angle. The term phase angle is used to define the angle between the sun and the sensor in a vertical plane (Fig. 1). Each surface type has a polarization maximum over a specific range of phase angles. The polarization maxima for soils is recorded over a wide range of phase angles, from around 90” for smooth surfaced soils to around 1 IO0 for rough surfaced soils. This wide phase angle range ensures that the relationship between PVL reflectance and surface soil moisture changes little with the small changes in imaging geometry (Fig. 2) that occur during every photographic mission, thus making PVL a suitable technique for remotely sensing undulating terrain from an unstable light aircraft platform. (c) Solar and look angle. The solar angle varies diurnally and seasonally; to maintain a phase angle of between 90’ and 110“ the look angle will have to move from very high on winter mornings to very low at midday during the summer (Fig. 1). Steg and Frost (1971) have shown that the actual solar and look angles are unimportant providing the phase
Remote sensing
48
angle remains within the phase angle observation that only the illuminated and therefore the increase in shadow polarization once the measurement is
range of soils (90°-1 loo). They attribute this to the portion of the soil contributes to the measured signal at low solar angles has little effect on the degree of standardized against a scene standard.
(6) Altitude of rhe sensor. The relationship between PVL reflectance and surface soil moisture is unique for each sensor altitude. This is a result of first, atmospheric scattering that decreases the intensity of PVL reflectance recorded with altitude (Egan 1968), as in Fig. 5. and second the problem of sampling, as an increase in sensor altitude results in the integration of a progressively larger and more variable area of soil within the area sampled by PVL (Curran 1978). An increase in sensor altitude therefore decreases the amount, while increasing the variability, of PVL recorded by the sensor. Surface
257 i. 20.
I
2
s 151 ._ B ! a" ; IO. z z CL 54
01
I I 1
0 0
'\ '\ '\ '\ '. '\\
0
soil moisture 150% 100% 50%
‘\
Q-_
--._ %_
‘k_ __._
---_ -_-_ -. _.._ 0 ---_ --__ ---_ --._ --._ ---_ --._ _-__ --._ -.._ ---_ ---_ -___ ----_ --__ ----__ --._--.._ -.__ -. _--_ ---__ O------______ --__ -----------------________________ ---_-__-_‘ ------------_________________ 8 5
M
10 Alldude
of sensor.
in
loo
5ca
1000
metres
Fig. 5. Per cent polarization (PVL) recorded at three altitudes The peat soil is recorded using a camera with polarizing filter. (Softrce: Curran 1978)
for three surface soil moisture
states.
Scene characteristics
The scene conditions that could potentially decrease the degree of correlation between PVL reflectance and surface soil moisture are the particle size distribution of the soil, the albedo of the soil and the range of soil moisture values. (a) Particle size distribution. The particle size distribution of the soil in large measure determines the soil’s texture and structure, both of which affect surface roughness at different scales. In laboratory studies of individual particles, grains of sand with large flat facets polarize light to a greater extent than clay aggregates (Blau et al. 1967). In field studies, smooth. poorly structured. fine-textured clay soils strongly polarize visible light while coarse-textured sandy soils produce a rough surface that is a poor polarizer (Steg and Frost 197 1). Therefore while there is a positive relationship in the laboratory between PVL reflectance and particle size, this relationship is reversed in the field. This negative relationship can be seen in Fig. 6 where particle size and PVL reflectance are plotted for a
Paul J. Curran Regressionline Cormlotion
01 01
05
IO
50 Portlcle sue in mm
100
49
Y = -3 28 ilog x I. 9.99 r .-07L sq al I%
500
1000
Fig. 6. Per cent polarization (PVL) recorded for a dry soil sieved into 13 particle size classes. The soil is recorded in the laboratory using a camera with polarizing filter. (Source: Curran 1978)
sieved soil. The particle size distribution provides the background polarization that increases with an increase in surface soil moisture. As such, it is only of importance when the background PVL reflectance is so high that low values of surface soil moisture cannot be detected by PVL, as is the case on estuarine mudbanks. (b) Soil albedo. There is an inverse relationship between soil albedo and the reflectance of PVL (Blau et al. 1967) and a positive relationship between soil albedo and atmospheric scattering of PVL (Steg and Frost 1971). Therefore the polarization of visible light from light-coloured soils is relatively low and little affected by atmospheric scatter, while the polarization of visible light from a dark-coloured soil is relatively high and greatly affected by atmospheric scatter. For measurements taken at up to aircraft altitudes these effects tend to cancel each other out, and PVL may be considered to be reasonably independent of soil albedo. (c) Range of soil moisfure
values. PVL is not sensitive to the entire range of surface soil moisture values. Soils that strongly polarize visible light when dry tend to mask small changes of PVL reflectance at low surface soil moisture values. At the other extreme the addition of extra water to a soil already at field capacity will not produce a proportional change in the intensity of PVL reflectance. Therefore, the correlation between PVL reflectance and surface soil moisture is maximized on slightly sloping and freely drained sites and minimized on very dry or wet sites. This has been confirmed in laboratory measurements of a draining slope (Fig. 7). The wet and dry proportions of the slope were outside the recording range of PVL and had a low correlation between surface soil moisture content and PVL reflectance, while the free-draining but still moist portions had a high correlation between surface soil moisture content and PVL reflectance.
Receiving
Normal
Shedding
OF MORPHOLOGY
O. cm
30
‘ig
_
08
MAP
with
a range
+0,6 to *O-g5
contour of iso-correlation
07
OF CORRELATIONS
(Source:
Curran
1979b)
Fig. 7. The correlation between per cent polarization (PVL) and surface soil moisture (map of correlations), plotted on to a map of slope form (map of morphology). The soil, a sandy loam, on an irrigated model slope, is recorded using a camera with polarizing filter.
MAP
Paul J. Curran
51
Limitations of the technique The correlation between PVL reflectance and surface soil moisture is low under very cloudy skies or when monitoring an area with a variable surface roughness. Under such conditions PVL is not a suitable technique for estimating surface soil moisture.
(a) Cloud cover. Sunlight is unpolarized but skylight produced by Rayleigh and Mie scattering is polarized. As the sun is brighter than the adjacent sky by a factor of about 10 000 (Egan 1968), even on a moderately cloudy day the sun may be considered as the main source of light flux. This is not so when the cloud cover is so thick that the location of the sun cannot be observed from the ground. In such circumstances the relationship between PVL reflectance and surface soil moisture is low (Steg and Frost 197 1).
.
:
:
:
:P
bt
w I G
KEY -
sp Spring S w
0
25
50 Percent
15 surface
100
125
Summer W,ntcr
150
so11 moisture
8. Per cent polarization (PVL) recorded for a range of surface soil moisture values over three seasons. The peat soil is recorded from a light aircraft using a camera with polarizing filter.
Fig.
Remote sensing
52
(b) Monitoring surface soil moisture. Polarized visible light can be used to monitor changes in surface soil moisture if the soil surface roughness does not change (Curran 1979b). Polarized visible light is unsuitable for recording surface soil moisture over several seasons, as during this time the soil surface roughness will alter. Figure 8 shows a scatter plot of PVL reflectance recorded from light aircraft altitudes, plotted against stirface soil moisture for one site on 12 dates. As the soil surface roughness varied over this time the overall correlation was low Cjust significant at the 5 per cent level) decreasing to insignificant if each of the three seasons is studied separately.
Summary PVL reflectance recorded at a phase angle of between 90°-1 10’ from up to light aircraft altitudes can successfully estimate surface soil moisture. This is possible regardless of solar angle, soil albedo or soil slope. over a wide but not unlimited range of surface soil moisture states, but only if surface disturbance and cloud cover are minimized. This technique provides an economic means of remotely sensing surface soil moisture from the ground and from the air. It also has application for the recording of other natural surfaces including water and vegetation. PVL cannot rival the current range of we!l-tested techniques for the estimation of surface soil moisture. However, PVL is eminently suitable where soil moisture estimation of a small area is required at low cost. It is hoped that the current use of PVL both as a field technique and as an extra sensor, used alongside other remote sensors, will result in further evaluations of PVL for the estimation of surface soil moisture. Acknowledgement The writer wishes to thank Ian Fenwick for his comments
on the original draft.
References Beckman, P. ( 196 I) The depolarization of electromagnetic waves scattered from rough surfaces. rlclu Pcchtfica 6, 5 I l-523. Blau. H.H.. Gray. E. L. and Bourioius, G. M. (1967) Reflection and polarization properties of powder materials. Applied Opks 6, I899- 1904. Chen. H.. Rao. C. R. N. and Sekera. 2. (1967) Incesrigarions oJIhe polarization oJligh[ rejlecled bjq namral sutjaces. p. 104. Air Force Cambridge Research Laboratories. Scientific Report, 67-0089. Coulson. K. L.. Bourioius, G. M. and Gray. E. L. (1965) Optical reflection properties of natural surfaces. Jowxal ofGeophysical Research 70, 4601-46 I I. Curran, P. J. (1978) A photographic method for the recording of polarized visible light for soil surface moisture indications. Retnole Sensing oJ‘En~irotztnet~t7, 305-322. Curran. P. J. (lY79a) Preliminary investigation into the application of oblique multispectral photography for the monitoring of sewage outfalls. Journal ofEncirotztnen~a~ .\lanagetnenr 8, 249-266. Curran. P. J. (1979b) The use of polarized panchromatic and false colour infra-red film in the monitoring of soil surface moisture. Retnore Sensing of Encirontnent 8, 249-266. Curran. P. J. (1980) Relative reflectance data from preprocessed multispectral photography.
Inrertla~iut~alJournal of Retvole Sensiq
I, 77-83.
P. J. (198 I) The relationship between polarized visible light and vegetation amount. Retnote Sensing 0j‘Enc~irot~tnenr10 (in press). Egan. W. G. and Hallock. H. B. (1966) Polarimetry signature of terrestrial and planetary materials. Proceedings of rhe 4th inrernarional svnposium on remote sensing of encironment, pp. 67 I-689. Ann Arbor: University of Michigan.
Curran.
Paul J. Curran
53
Egan. W. G. (1968) Aircraft polarimetric and photometric observations. Proceedings of the Sfh international symposium on remote sensing of environment, pp. 169-189. Ann Arbor: University of Mlchigan. Egan. W. G.. Grusauskas. J. and Hallock. H. B. (1968) C ptica’i depolarization properties ofsurfaces illuminated by coherent light. Applied Optics 7. I S29- 1534. Egan. W. G. and Hallock, H. B. (1969) Coherence-polarization phenomena in remote sensing. Proceedings of the Institute of Electrical and Electronic Engineers 57.62 l-628. Secchi. A. (1860) Polarization oflunar light. Astronomische Nachrichten 52.93-97. Steg, L. and Frost. R. T. (1971) Visible polarization signature for remote sensing of soil surface moisture. COSPAR Plenary Meeting. Leningrad. USSR (I 970). pp. 7 17-720. Stockhoff. E. H. and Frost, R. T. (1971) Polarization of light scattered from moist soils. Proceedings of the 7th international symposium on remote sensing of environment, pp. 345-363. Ann Arbor: University of Michtgan. (Received
29 Ju/y 1980)