The use of polarized panchromatic and false-color infrared film in the monitoring of soil surface moisture

REMOTE SENSING OF ENVIRONMENT 8:249-266 (1979)

249

The Use of Polarized Panchromatic and False-Color Infrared Film in the Monitoring of Soft Surface Moisture PAUL J. CURRAN University of Bristo~ Department of Geography UniversityRoad, Bristo~ BS8 ISS, Eng/and Soil-moisture change was monitored using polarized visible light recorded on panchromatic film, and visible and infrared reflectance recorded on false-color film. Photographic records of a drainm"g sandy loam on a sloping apparatus in the laboratory and a peat soft in the field, were backed by detailed soft-moisture measurements and laboratory spectrophotometric recordings. There was difficulty in obtaining real-time/n situ measurements of soil surface moisture of comparable sensitivity to photographic temporal data. False-color infrared film l~ovided qualitative spatial description of soil-moisture state, especially at high water contents in the peat and low water contents in the sandy loam. This film was unable to accurately record a changing soft-moisture state. Polarized reflectance successfully recorded a fairly wide, but not unlimited, range of changing soft-moisture conditions, thus suggesting its limited suitability as a monitoring technique.

Introduction Soil moisture has great variability both spatially and temporally. The use of suitable photographic techniques with a high temporal resolution was sought to provide spatial details of soil-moisture change, data unavailable from point ground measurements, Two photographic remote sensing systems were investigated: (1) polarized visible light recorded on black and white film, and (2) visible plus near-infrared reflectance recorded on false-color film. They were used to determine the viability of obtaining a useful serial record, via densitometry, of an area's soil surface moisture pattern, initially of a sandy loam under laboratory conditions, and subsequently, on a peat soil in the field, Soft Moisture and the Polarization of Visible Light Soil reflection is primarily diffuse, whereas reflection from water is strongly ©E-Ise~er North Holland Inc., 1979

specular. The value of visible light to detect these extremes has been demonstrated by Steg and Frost (1971), Stockhoff and Frost (1971), and Curran (1978). All showed that high polarization values produced at large phase angles, in the order of 50" to 60 ° (Brewster's angle for water is 53 ° ) are highly dependent on soil moisture, with only slight dependence on atmospheric haze, surface roughness, and within certain limits, surface slope, soil type, and clouds. The use of a photographic method to record polarized visible light, in order to detect soft moisture variation was demonstrated by Curran (1978), in both the laboratory and field. The latter at ground height, mast height, and aircraft altitudes. Correlation coefficients between surface soft moisture (obtained thermogravimetricaUy) and polarization, were significant at better than the 5% level, ranging from + 0.65 to + 0.80. The advantage of the photographic polarization method is that digital data can be produced, via densitometry on the visible 0034-4257/79/030249+ 18501.75

250

light polarization of an object. The operation is simple, the cameras focal axis is set to face the light source and angled downwards. The altitude of the camera is such that a high phase angle is obtained (Fig. 1). Two exposures are made of the scene both through a polarizing filter onto a black and white film (Ilford FP4, 100 to 400 ASA). The choice of orthogonal angles used depends on the angle of surface and camera. The operator obtains a record of maximum polarization by bringing the microlines on the filter in parallel with the object viewed. The second exposure has the filter set at right angles to the first, to obtain values of maximum and then minimum polarization. Densitometry along the focal axis of both images provides the data needed to calculate percent polarization, The percent polarization formula is

PAUL J. CURRAN

pattern. The choice is between accurate reflectance measurements at a point, using a reflectometer, or relative values of reflectance via film densitometry. The first method was used to determine the form of the spectral reflectance curve and the relationship between reflectance and soil moisture, albeit in a controlled laboratory experiment. Measurements of reflectance were made on a Beckman DK 2A, twin-beam ratio spectrophotometer, over a range of 3501100 nm, using MgO as a reflectance standard. For a review of this instrument's use in the recording of natural objects, refer to Bowers and Hanks (1965) and Johannsen (1969). The problems and limitations of using this instrument (capillary action of water to the glass slide, small sample size, etc.) are summarized by Blanchard et al. (1974). Six sandy loam samples with moisture surface OD (max.) contents of 0.0%, 0.3%, 6.4%, 8.3%, standard (white) OD 17.3%, and 20.5% and four peat samples surface OD with moisture contents of 3%, 66%, 75%, - standard (white) OD (min.) and 85% were analyzed (Figs. 2 and 3). × 100. Several points are of relevance to the surface OD (max.) work that follows. The slope of the standard (white)OD spectral curve for the sandy loam and surface OD (min.) peat are of similar form, with a low + standard (white) OD blue/green, high red, and higher infrared reflectance. This is discussed filrther by where OD ---optical density (antilog) Dolgov and Vinogradova, (1973) and is typical of a soil without strong coloration. Soil Moisture and Reflectance The darker peat has a lower reflectance in Visible and Infrared Wavelengths at a given moisture content than has the sandy loam and both soil types show with The inverse relationship botween re- an increase in moisture a decrease in flectance and soil moisture is well docu- reflectance, at all wavelengths. mented (Bowers and Hanks, 1965; If the mean reflectance for the four Johannsen, 1969; Condit, 1970; Dolgov wavebands, blue, green, red, and infrared and Vinogradova, 1973; Blanchard et al., are considered in isolation, then this 1974; and Curtis, 1976). The problem is gives an insight into the appearance of the recording of the reflectance's spatial these soils on film. For reflectance is a

USE OF POLARIZED PANCHROMATIC AND FALSE COLOR INFRARED FILM

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sensitive indicator of moisture at high moisture levels in the peat, but at low moisture levels in the sandy loam [see also the Manual of Remote Sensing (1975) and Steiner and Guterman (1966)]. The second method of analysis involved image densitometry, for it provides qualitative spatial as well as relative reflectance measures. The film used was Kodak Ektachrome false-color infrared, recording in both the visible and near-infrared by means of a color shift, Instead of being sensitized to the three primary colors of blue, green, and red, the three emulsion layers are sensitized to green (500-600 nm), red (600-700 nm), and infrared (700-900 nm). When

using this film for soil photography, the films spectral inbalance results in a reduced sensitivity of the infrared recording layer, causing a green coloration rather than the green/red coloration expected. Densitometry of false-color film, (Sewell, 1971) shows a strong correlation of +0.72 to +0.89 between soil surface moisture and the density values of all three film layers. A similar correlation of over + 0.80 between digitized false-color film and soil moisture was found by Werner et al. (1973), during field measurements in July. The film was 35 mm, rated 125 ASA with a Wratten 12 (yellow) filter. The

USE OF POI_ABIZED PANCHROMATIC AND FALSE COLOR INFRARED FILM

handling problems of this film are well known and are reviewed by Kodak (1972). Laboratory Studies A model slope was used to simulate the rapid soft moisture changes needed to evaluate the two forms of photography, While it is not ideal, a hardware model is employed "to reformulate some features of the real world into a more familiar, simplified, accessible, observable, easily formulated, or controllable form, from which conclusions can be deduced, which in turn can be reapplied to the real world" (Chorley, 1964). Within the earth sciences, such draining slopes have been used, for example, by Hewlett and Hibbert (1963) and Anderson and Burt (1977). The slope used was a simple hollow, 1.1 by 1.1 m, formed in a 12 cm deep sandy loam, within a plastic-lined metal frame (Bull, 1978). The slope was tilted in order to allow drainage from the base of the hollow. Fifteen water-filled tensiometers (Curtis and Trudgill, 1975),

253

placed at a depth of 5 cm below the soft surface, provided measures of nearsurface soft moisture. Changing surfacemoisture state was not immediately recorded by the subsurface tensiometers. The lag time of tensiometer response was such that considerable moisture change at the surface had taken place before such a change was recorded. This demonstrates the difficulty of obtaining realtime in situ measurements of surface moisture of comparable sensitivity to photographic temporal data. Prior to photography the slope was fully saturated and then allowed to drain freely for 5 hr. During this time 18 tension readings were made for each tensiometer, at time intervals of 5 mins at the beginning, and 25 min towards the end of the experiment. The slope drainage pattern is one of rapid throughflow from the margins into the base of the hollow, while drainage for the whole slope decreases with time. The graph of mean soil moisture for the slope plotted against time shows this effect well (Fig. 4).

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moisture tension

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254

PAUL J. CURRAN

Photography During slope drainage, polarized panchromatic and false-color photographs were taken at similar time intervals for 110 and 290 min, respectively, using the instrument geometry shown in Fig. 1. In order to remove variable effects of film exposure and processing, reflectance panels of known spectral properties were positioned to appear on each image. The films were digitized using a Photolog Spot Densitometer,1 measuring transmitted light by a simple photomultiplier. Density values were checked to be on the straight line portion of the D log E curve for the film used and were then exposure standardized by the white reflectance standard. In this instance, 26 sampling points were placed within a stratified sampling grid avoiding either tensiometers or shadow, The polarized imagery was of sufficient quality to enable the plotting of contours of isocorrelation. The falsecolor imagery was subject to many specular and shadow effects, a result of highcontrast film and point light sources. The imagery was therefore double standardized, with values for relative visible and

where OD = optical density (antilog). The infrared reflectance formula is infrared ref" from surface OD infrared ref n from standard OD white ref n from surface OD white ref n from standard OD

-1

The Relationship Between Slope Surface Moisture and Polarized Imagery Soil moisture tension (SMT) and polarization were mapped on a slope plan. A sample of 208 paired data points, 26 per slope map, were used to construct Fig. 5. This shows the high correlation (+ 0.87, significant at better than 1%), between SMT and polarization. Despite such a strong correlation, closer 3crutiny of the data reveals a pattern in the distribution of correlation strength both temporally and spatially. Temporal distribution of o~rrelations

Both polarization and SMT were negatively correlated to time since the termination of irrigation, with correlation coefficients of -0.89 and -0.86 respecinfrared reflectance expressed as a mean tively (significant at better than 1%). The for the whole slope. correlations between polarization and Double standardization SMT varied during drainage, as the of false color imagery tensiometer response rate was below the The visible reflectance formula is rate of minimum drainage. For the initial period, when the slope was near satura-1 tion, the mean correlation coefficient was red rof a from surface OD ] + 0.80 (significant at better than 1 £). red rei~ from standard OD

g~n ~t~ fromsurfaceon I + greenreVfrom~ d ~ o D 9. white r e ~ from surface OD whitereP fromstandardOO 1 Trade mark.

During the period of rapid slope drainage a lower mean correlation coefficient of + 0.55 (significant at 1%) was obtained, rising to a mean correlation coefficient of + 0.65 (significant at better than 1%), for the latter stage of drainage rate stabiliza-

USE OF POLARIZED PANCHROMATIC AND FALSE COLOR INFRARED FILM

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tion. Figure 4 shows that the lower correlations during times of rapid slope drainage are due not to a lack of sensitivit), in the photographic system, rather they are due to the lag inherent in SMT measurements, Spatial distribution of correlations For each slope, 26 sampling points the correlation coefficient of polarization and SMT are mapped. For a discussion of this technique, see Robinson (1968). The areas of low correlation (Fig. 6) are all positioned on the shedding or receiving portions of the slope, areas showing extremes in soil moisture. The area with a high correlation between polarization

and SMT is in the draining area of slope, displaying the smallest moisture range. The extreme values of SMT in the shedding and receiving slope regions are simply out of the recording range of polarized light. Any SMT below 0.9 cm H20 (1/100 atm) is unrecordable, for the sandy loam has the ability to polarize 1.8% of visible light when dry. This is confirmed by the sharp cut-off in Fig. 5, and is discussed further by Steiner and Gutermann (1966). At the other extreme, when the grains have a heavy surface film of water, the addition of extra water, while decreasing the tension with which the water is held, cannot be expected to produce a proportional increase in light

256

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polarization. This is further substantiated within the receiving portions of the slope, the region of greatest change. A correlation coefficient of + 0.89 (significant at better than 1%) was found between the rate of change in SMT, (measured as standard deviations of SMT), and the correlation map of polarization and SMT.

Temporal, spatial autocorrelation Maps of spatial autocorrelation produced by plotting the errors of the regression estimate for each time interval, told a similar story. The pattern of lower soil moisture, higher SMT, than would be expected, for the shedding areas and the

reverse for the receiving areas. This was remarkably constant over time, therefore, one example, 105 min after irrigation had ceased, is presented in Fig. 7. The Relationship Between Slope Surface Moisture and Visible and Near.Infrared Reflectance. For reasons discussed earlier, no spatial analysis was possible, therefore only time, SMT, visible and infrared reflectance were investigated. The two independant variables of time and SMT are negatively correlated (-0.86, significant at better than 1%). The two dependent variables, visible and near-infrared reflec-

257

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tance were slightly correlated (+0.52, significant at 5%). This low correlation is to be expected for visible light, and is uncorrelated to both time (+0.47, not significant), and tension (-0.21, not significant); While infrared reflectance is correlated with time (+0.73, significant at better than 1%), and SMT, (+0.79, significant at better than 1%). In the spectrophotometric study, reflectance of visible light was related to soft-moisture change, the photographic system used is evidently not sensitive enough to record temporal variability of such a small magnitude. The relationship between infrared reflectance and SMT was predicted to be strong, yet suffered

an information loss during digitization and data handling, plus problems due to the tensiometer lag times. Nevertheless, a temporal trend was observed, (Fig. 8). The distribution of points from the regression line shows that for the first onesixth of the drainage time, the slope was wetter than recorded, the reverse being true for the latter five-sixths of the drainage time. Field Observations Shapwick Heath is a nature reserve on the Somerset levels, adjoining areas which are being actively exploited for commercial peat extraction. Much of this

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area now consists of bare peat ridges interspaced with cuttings, A simple syphon and sprinkler was installed to further saturate a ridge after a week of heavy rainfall (47 ram). The bare peat surface was 5.5-m wide, and was monitored for a length of 8.5 m. Soil moisture is presented, not in tensions, but in soil water per unit volume of peat, obtained by two techniques. The first involved ten mercury tensiometers, as the moisture/suction curve for the peat is known (Bonniger, 1977), conversion to soil moisture values is possible. The second was the use of twenty-one g0-g thermogravimetric sampling points (Curtis and Trudgull, 1975). The imagery, polarized and false color, was taken as before. The sun's altitude was at 45 ° necessitating a camera "lookangle" of 40 ° to 50 ° to ensure an angle suitable for recording visible light polarization. Photographs were taken from a 10-m hydraulic mast on the rear of a Land Rover, 2 with shutter release, rewind, and camera angle controlled automatically from within the vehicle. For fuller discription of instrumentation, see Williams and Curtis (1977).

259

each hour of recording made up of a balanced mix of locations and minutes between readings. The sampling order of the locations (thermogravimetric and tensiometer) is random, with the time interval between samples dependent on the agility of the operator. The experiment was run for 300 rain, with photography taken every 15 rain for the false color and 10 min for the polarized iraagery. The resultant time/moisture plot, enabled the production of moisture maps. These showed the expected pattern of increased drainage towards the subsurface soil fissures and towards the edge of the ridge. The photography was handled as in the laboratory study. The higher qt, ality of the imagery enabled single rather than double standardization. Standardization of f ~ l o r

imagery

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Sampling

The real world possesses many time/ space sampling problems. An ideal situation would be one where every point in a sampling grid could be recorded simultaniously and sequentially. As this is impractical, a latin-square technique was used, "by employing a mixed strategy, with some fixed and some mobile recording stations" (Haggett, 1972). These are recorded at varying time periods with ~Trade mark.

( in~--ff-~-~infraredre-~ refn~fr°mst---~--d~dsurfaceODo__D]~-1 where OD=optieal density (antflog). Results The correlation coefficients for soil moisture, visible, infrared, and polarized reflectance collected from 31 sampling points, over 300 min are tabluated in Table 1. All are positively correlated at a significance level of 5%. Polarized visible

260

PAUL J. CUI{BAN

TABLE 1 ~aapwick Heath Field Study, Table of Correlationsa I ~ D

VISIBLE

~ O N Time Moisture (%) Polarization (%) Visiblereflection Infrared Reflection

POLABIZATION

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+ 0.42 +0.55 + 0.34 + 0.64 1.00

+ 0.33 +0.43 + 0.22 1.00

MomTm~

(%)

(%)

TIME

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+ 0.83 1.00

1.00

°Significancelevel: 5%, r=0.11; 1%, r=0.15

light, infrared reflectance, and visible reflectance provide, respectively, the highest to lowest correlations (see Figs. 9

suits had a pattern in their variance both spatially and temporally.

and 10). The correlation coefficients were not the same as those found in the laboratory due to experimental variation, and the peat's lower hydraulic conductivity, reflectance, and ability to polarize visible light, (Coulson, 1966). As with the laboratory study the re-

Spatial distribution of correlations between soil moisture and polarized, infrsazd, and vlsllde rdleetanee C o r r e l a t i o n coefficients, w h e n mapped, showed that variance is due to the pattern and rate of drainage (see Figs. 11 and 19.).

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The fast draining areas near the ditches and over the soil structural fissures were highly correlated with polarized and poorly correlated with visible and infrared reflectance. The areas with standing water during the early stages of

and infrared, yet poorly correlated with polarized reflectance.

photography showed a reversed trend, they were highly correlated with visible

The drainage/time relationship was slower but similar in form to that ob-

Temporal distribution of correlations between soft moisture and polarized, infrared, and visible reflectance

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tained in the model slope, Fig. 4. During the period of initial drainage when the slope was still partially saturated, eorrelations were low between soil moisture and polarization (+0.50), yet high (+0.82 and + 0.74) between soil moisture, and infrared and visible reflectance, respectively. Polarized imagery is clearly not sensitive at this high level of soil mois-

ture, unlike false-color film which could delineate the saturated area with clarity. This latter observation was expected, for speetrophotometer studies, displayed peat's sensitivity to high moisture levels. As the standing water drained, the correlations between soil moisture and polarization rose (to +0.80)while correlations between soil moisture and in-

264

PAUL J. CURRAN

frared and visible reflectance decreased dependance of film layers. Fortunately, markedly (to +0.46 and +0.38, respec- the use of one film and the imaging of tively). This surface had a moisture state one object (soil), overcomes to a certain within the range of polarized reflectance, extent these two objections. Although it but is in the less sensitive region of in- should still be borne in mind that to date, frared and visible reflectance. "knowing the scene spectral reflectance and illumination one can predict the reDiscussion sultant film densities, but the reverse procedure has not been possible" Photography has proved useful in (McDowell and Specht, 1974). monitoring the short term patterns of Nevertheless, in all situations this film surface soil moisture. It is clear that the provided excellent qualitative spatial deability to estimate surface soil moisture tail of soil moisture. This is as a result of change is dependent on choosing the a color photograph having greater interfilm/filter combinations with the ability pretability than two polarized black and to record the soil moisture state of inter- white images. est, in a given soil type at a given level of illumination. The recording of polarized Conclusions reflectance provides accurate spatial and temporal detail even on the dark peat False-color infrared film provided an soil. Its disadvantages are the need for excellent qualitative description of soiloptimum illumination and the inability to moisture state. Especially at high water record extremes of soil moisture, contents in the peat and low water conStudies using the Beckman Spectro- tents in the sandy loam. Its success is due phometer in the laboratory suggest that to the combination of visible and infrared longer wavelengths would be of greatest wavelengths and their presentation in value in the determination of soil mois- color. This film was unable to record ture, and that the sensitive region for accurately a changing soil-moisture state. soil-moisture estimation was at low mois- Polarized reflectance, recorded phototure levels in the sandy loam and high graphically, successfully monitored a moisture levels in the peat. fairly wide, but not unlimited, range of There has been much work on the soil moisture conditions. densitometry of false color film, RichardCurrent monitoring of test sites using son et al. (1970) and Duggin et al. (1975) aerial false color and polarized imagery are examples. This is based on the has lead to the appreciation of false-color assumption of a known or at least know- films ability to locate patterns of soil able relationship between target reflec- moisture, although quantification of these tance and image density. This assumption patterns is more successful with is perfectly acceptable when the film polarized imagery. characteristics are available. Unfor- Acknowledgments tunately, this is not the case with falsecolor film due to its large inter- and The author wishes to acknowledge Sue intrabatch film variation and the nonin- Bull for the construction and recording of

USE OF POLARIZED PANCHROMATIC AND FAI~E COLOR INFRARED FILM

the laboratory slope, Dr. K. Dunning for instruction in the use of the spectrophotometer and Dr. L. F. Curtis for reading the original draft. The Natural Environment Research Council providedfunding for both photography and equipment, under a grant to Dr. L. F. Curtis, GR3/1481. References Anderson, M. G. and Burt, T. P. (1977), A laboratory model to investigate the soil moisture conditions on a draining slope. 1. Hydrol. 33:383-390. Blanehard, M. B., Greeley, R., and Gottelman, R., (1974), The use of visible, near-infrared and thermal infrared remote sensing to study soft moisture. Proceedings of the

Ninth International Symposium on Remote Sensing of Environment, Univ. of Mich., Ann Arbor., Mich., pp. 693-700.

265

Curran, P. J. (1978), A photographic method for the recording of polarized visible light for soil surface moisture indications. Remote Sens. Environ. 7:305-322. Curtis, L. F. and Trndgfll, S. (1974), Measurement of soft mositure. British Geo-

morphologicalResearch Group, Tech. Bull. No. 13. Curtis, L. F. (1976), Remote sensing of soil moisture: user requirements and present prospects. Remote Sensing of the TerrestrialEnvironment. (R. F. Peel, L.F. Curtis, and E. C. Barrett, Eds.) Butterworth, London. Dolgov, S. I. and Vinogradova, G. B. (1973), Reflection coefficient of moist soils. Pochvovedenie 11:143 Duggin, M. J., Roberts, R. J., and George, J. M. (1975), The reflectance properties of grazing pastures as determined in the LANDSAT satellite bandpasses and from oblique color infrared aerial photography.

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