DROSMON workshop, 29th-30th of November 2007, Vienna, Austria
EXPLOITING CANOPY BRDF; BETWEEN THEORETICAL CONCEPT AND PRACTICAL IMPLICATIONS Schneider, Th.(1,2), Dorigo, W.A. (1,3), Schneider, W.(3) 1)
Limnological Station, Technischen Universität München, Hofmark 1-3, D-82393 Iffeldorf, Germany, Email:
[email protected], (2) Institute of Forest Management, Technischen Universität München, Am Hochanger 13, D-85354 Freising, Germany, Email:
[email protected] (3) Imaging Spectroscopy WG, DFD, German Aerospace Center (DLR), P.O. Wessling, Germany, Email:
[email protected] ABSTRACT Starting from the theoretical concept of the Bidirectional Reflectance Distribution Function (BRDF) the implication of the directionality effect on RS data analysis is discussed under the aspect of precision farming requirements. In the larger context of a RS based expert system for precision farming applications BRDF data bases should allowe to calibrate air- and spaceborne RS data taken under any illumination and view geometries at any time within the vegetation period. In the frame of the ground segment of such an expert system (“laboratory loop”) field goniometric measurements are used to approximate the BRDF of crops at the respective development stage. Ancilliary data about the plot, its past and and present management, climate and weather data of the ongoing vegetation period as well as destructive vegetation samples serve to adapt physical models to specific site, crop and managment conditions. Within the “operational loop” air and satellite data are first calibrated by the help of the BRDF data base and agro-climatic data. Subsequently bio-geo-chemo-physical parameter are extracted by physical model inversion techniques. These information are used to develop application maps for fertilizer, pesticid or water supply distribution. Growth models are integrated to close the gap between data take and application. A real time sensor system may fine tune the distribution by crossvalidation of on the run data with the recommendations of the application map. On basis of results of a campaign in summer 2005 in support of research in the frame of the DROSMON project of the University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria, the evaluation chain is presented, limitations discussed and an outlook is given. Measurements of the hemispherical directional reflectance factor (HDRF) were taken with the Mobile Goniometer Device (MGS). The “inscribed-“ or “contact angle” principle of the MGS design minimize the error source of a changing surface area with changing view zenith angles. The measured surface section of about 1.2m ² at an IFOV of 10° asure a signal, fairly repesenting the plot surface, theoretically even in case of maize. A special focus of the study was to investigate whether biophysical and biochemical parameter retrieval by inversion with the „Canopy variable Retrieval Algorithm based on Prospect and Sailh“ (CRASh) perform stable for changing sun and view geometry and to see which combination of view/sun geometries performs best. For this purpose, MGS measurements of a maize field near Raasdorf were taken in the morning, at midday and in the afternoon. The reflectance data were resampled to the spectral bands and view angles of the Proba/CHRIS sensor. Evaluation of the CRASh radiative transfer model inversion consists of two parts: (i) comparison of the retrieval performances for different view configurations, and (ii) intercomparison of the different time steps. The results revealed that in general both, more spectral bands as well as more observation directions improve the retrieval accurracy. Surprising was that especially excluding the hotspot observations and the large view zenith angles contributed to stabilize the results of the of the inversion algorithm. When the retrieved canopy variables are compared to the ground validation measurements it gets clear that for all stable view/azimuth combinations LAI is underestimated whereas chlorophyll content significantly is overestimated. Keywords: field spectroscopy, field goniometer, MGS, BRDF, physical modeling, PROSPECT, SAILh, CRASh 1.
BACKGROUND AND MOTIVATION
1.1 Application background According to recent ESA publications at the present more than 50 Earth observation (EO) systems regularly provide data of the Earth surface. Because these satellites often use different methodologies, using data for trend analysis and environmental monitoring can be difficult, making it essential to establish globally recognised guidelines for Calibration and Validation processes. Calibration is the process of quantitatively defining the system responses to known, controlled signal inputs. Validation is the process of assessing, by independent means, the quality of the data products derived from the system outputs.
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DROSMON workshop, 29th-30th of November 2007, Vienna, Austria
Vegetation focused RS data analysis has to face the situation of largely unspecific, spectral broad band diagnostic reflectance features. The signal is allways a mixture of the targeted vegetation signal superimposed by spectral mostly neutral specular and shade fractions and by fractions of differring spectral response like soil and untargeted vegetation fraction from spadices, stalk, dried up leves or even weed. 1.2 BRDF - theoretical concepts Reflectance anisotropy is an intrinsic surface property of practical all Earth’s surfaces. The roots of the anisotropic scattering behaviour of vegetation canopies may be found in factors such as leaf angle distribution and orientation, thickness and size of leaves, contributions of ears, spadices, etc. and their spatial distribution [3-5], multiple scattering processes within the canopy layers as well as in soil properties such as roughness, color, and organic matter content [6]. The BRDF is defined as the ratio of the radiance Lr (W m-2 sr-1 nm-1), reflected in an outgoing direction (θr, φr) to the incident irradiance Ei (W m-2 nm-1) from a specific direction (θi, φi). BRDF is an intrinsic property of materials and in reality it can only be approximated by dividing measured radiances, Lr from small aperture solid angles by the hemispherical irradiance, Ei since an infinitesimally small sensor field of view is impossible to obtain [2]. Physically the phenomenon is described in terms of the Bidirectional Reflectance Distribution Function (BRDF).
BRDF = fr (θi,φi; θr,φr;λ) = dLr (θi,φi; θr,φr;λ) / dE (θi,φi;λ) [sr-1] where: Lr = sensor radiance [W m-2 sr-1 nm-1], Ei = hemispherical irradiance [W m-2 nm-1],, λ = wavelength [nm], θi, φi = source zenith and azimuth angles θr, φr = view zenith and azimuth angles. fr= function value range {0;1}
1.3 Motivation The general frame conditions of our research may be headlined by the keywords: “close to application”, “local” and “precision”. An outline of the general concept, the advantage of BRDF for remote sensing data normalisation but identification and status assessment, the need of HDRF measurements for BRDF approximations, an overview on field goniometer solutions and their specific constraints, as well as the special limitations of the measurement system “Mobile Unit for Field Goniometer Measurements” (MUFSPEM) as used in a four year experiment in the frame of the IKB Dürnast “precision farming” joint project have already been presented at the 2nd Proba/CHRIS workshop [1]. For MUFSPEM data inversion an adaptation of the coupled Prospect [2] and Sail [3] models by [4] further referred to as “ProSail” was used. The full story of the MUFSPEM IKB Dürnast [5] experiment may be found by [6], a summarizing version by [7]. Balancing the results of the MUFSPEM experiment the following success stories could be reported: • The measurement arrangement proved to be high, mobile and fast enough to measure from 10 m above the maize canopy and to change position as fast that two subplots with different useful water storage capacity (UWSC) could be measured within one hour. • Differences in the anisotropy behaviour of the canopy reflectance of high and low UWSC sites were imaged by the HDRF data sets at each time of the day and at each measurement date. • The vegetation development across the vegetation period was displayed in data sets taken at comparable illumination conditions. • The inversion results showed a logical trend. The values for the low UWSC sub plot indicated a reduced vitality compared to the high UWSC sample plot. For comparable illumination conditions the trends of parameter changes across the vegetation period are constant either increasing or decreasing and similar for both subplots. The point where MUFSPEM experiment failed was the parameter retrieval by ProSail inversion, more precisely the parameter retrieval across the day. The expectations of the coupled growth model/physical backscatter model concept as described by [1] are that all measurements representing one phenologic development stage should deliver similar values regardless the view and illumination geometries. The theory accepts some slight differences which can be explained by diurnal physiological cycles of a vital canopy in the range of 5% [9]. The ProSail retrieval results of MUFSPEM HDRF data sets reported by [1] for the diurnal cycle of maize at the sample plots both with high and with low useful water
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DROSMON workshop, 29th-30th of November 2007, Vienna, Austria
storage capacity (UWSC) surprised in magnitude and are not acceptable: LAI ranged from 1-6, LAD ranged from 43° to 77° with increasing values from morning to the afternoon, Chlorophyll (a+b) content (Cab), Canopy dry matter (Cm), Canopy water content (Cw) decrease from the morning to the afternoon calculations. At the low UWSC site Chlorophyll (a+b) content (Cab) from 97 to 23 [µg/cm²], Canopy dry matter (Cm) from 0,012 to 0,002 [g/cm²] and Canopy water content (Cw) from 0,037 to 0,008 [g/cm²] and at the high UWSC site Chlorophyll (a+b) content (Cab) from 101 to 47 [µg/cm²], Canopy dry matter (Cm) from 0,03 to 0,003 [g/cm²] and Canopy water content (Cw) from 0,045to 0,017 [g/cm²]) have been retrieved. Similar results are reported by [7][8] obtained by analyzing the behaviors of two wheat subplots of high and low UWSC over the vegetation periods 2000 and 2001 with MUFSPEM. Analysing and trying to trace back the reasons for these controversial results we identified three main groups of potential error sources: The first group is related to the assumption of a homogeneous sample area which in case of the MUFSPEM design is not necessarily true. Management lanes, row structure of the plot, wind aligned ears, leaves, spadices, field capacity, resp. growth differences within the appr. 400m² in question, etc.. A second group is related to the atmospheric conditions, which may change within a measurement series which takes about 20 minutes. Rayleigh, MIE and especially the unspecific scattering components of differing aerosol types shows an anisotropic scattering behaviour and are wavelength dependent. These contributions have to be accounted as well. The last and maybe the most severe error source is provided by the physical model itself. The question to be answered is whether ProSail as used for MUFSPEM data inversion is sufficiently adapted to the crop and site conditions investigated. After all we found there are too many error sources to be verified and back-traced further working with MUFSPEM. Our conclusions resulted in the decisions: • first to develop a new measurement concept by keeping proven design features and changing the ones which introduced doubts, e.g. the assumption of a homogeneous measurement area, but the different period between reference and object measurement, resulting in the Mobile Goniometric System (MGS). • second to construct an improved inversion algorithm for parameter retrieval based on the coupled Prospect / Sailh radiative transfer model which from now on will be referred to as “ProSailh” In the following we briefly present the MGS concept and measurement strategy as well as the results of the variable retrieval with ProSailh. To assure the direct comparability with the results shown by at the 2nd Proba/CHRIS workshop in 2004 [1], the current paper focuses again on a diurnal cycle of maize, in this case retrieved with the new MGS. The only difference is the development stage of maize which according to the Eucarpia scale (EC) used by the German Biological State Foundation for Agriculture and Forestry (Biologische Bundesanstalt für Land- und Forstwirtschaft [10]) is in EC stage 75 compared to EC 67 in the former study. 2.
MATERIAL AND METHODS
2.1 Testsite The results presented are achieved with measurements at the testsite Raasdorf, a an agricultural farm managed by BOKU 30 km North East of Vienna in the Danube alluvial plains (N 48.2355°, E 16.5922°). Common soils in this area are sandy loams and loam with soil ranking points from 60 to 75 on a scale from 0 (for an unproductive soil) to 100 (excellent productive soil). 2.2 The Mobile Goniometric System (MGS) The basic design feature of the Mobile Goniometric System (MGS) is the application of the “inscribed–” or “contact angle” principle. The same section of the surface is observed, changes in area at increasing view zenith angle, which are common for “pure” goniometer systems, are compensated by the pivot mounted arm with the entrance object optic and are of less than 10%. At view zenith angle (VZA) of 0° (nadir position) the foot print is of about 1.2 m². MGS is mounted on the top of a 4WD vehicle (Fig. 1), the change of position is very fast, the nadir measurement height is adjusted at 10m above the canopy and the 24 directional measurements needed for a BRDF approximation are performed within 20 minutes. The set up of the whole construction lasts about 90 minutes for a two men team. Fig. 1 shows the system at the 2nd assembly test in Iffeldorf, on the 1st of October 2003. The MGS works with a Field Spec FR field spectroradiometer from ASD company covering the wavelength region from 400 to 2500 nm. A bifurcated fiber cable with two fibers of 5 m in length (ASD accessory) is attached at the standard ASD
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DROSMON workshop, 29th-30th of November 2007, Vienna, Austria
Fig. 1: The “Mobile Goniometric System” (MGS) at the second assembly test. The ASD Field Spec FR is operated via a bifurcated fiber cable with 10° aperture for nearby simultaneous object and Spectralon™ reference measurements. The slides show five positions from 60° view zenith angle to nadir view. The subsection of the canopy to be measured can be additionally changed by rotating the device on the roof of the SUV. Field Spec Pro FR instrument fiber cable of about 1.5 m. The negative consequence of this compromise is a signal reduction of about 50% for the summed up signal of both fiber cables due to the length of the fiber and the loss of signal at wavelengths higher than 2380 nm (Fig. 2). The advantage of this solution is the fact that both the reference and the object are measured with the same detector set, facilitating the radiometric calibration of the system. Two operation modes are foreseen: 1.
Both cable ends are fixed parallel in the sense of a prolongation of the standard 1.5 m ASD fiber optic. In this case reference is taken at the beginning and the end of a series as known from the MUFSPEM set-up, or
2.
One fiber cable end is measuring the reference panel and the other end the object. The drawback is a signal reduced to approximately 25% of the standard 1.5 m ASD fiber optic, the advantage is that the time span between reference and object measurement is similar to the one known from the common case ASD measurements of a few seconds. Tab. 1: Technical details of the “Mobile Goniometric System” (MGS) „goniometer“ device: • height above ground: 10-12,5m, • height above canopy: 10m • azimuth positioning accurracy +/- 3° (worst case) • zenith angle positioning accurracy +/- 3° • azimuth angle range: 270° • zenith angle range: 0° to 70° • rotating assembly on the roof rack: 360° • mounting time 2 persons: appr. 1,5h Photogrammetric stereo device: • 2 Canon G2 digital cameras, remote control • base distance: 1m • height above canopy: 5m Detectors: • Field Spec FR (350 – 2500nm) ASD company • aperture: 10° • bifurcated fiber cable for alternate object/ reference measurement
2.3 Measurement strategy HDRF measurements with the intention of BRDF approximation are time sensitive and time consuming. The measurement sequence begins by placing MGS’s vehicle long axis parallel to the sun principal plane which is the origin of the positioning system (0° azimuth angle (AZ) backward scattering directions, 180° AZ forward scattering). The measurement sequence suggested and adopted for the multiangular mode is described by Fig. 2. The radiation field is assumed to be symmetric right and left of the principal plane, thus solely one hemisphere is registered. For time saving reasons and accounting for the results of the LISPM/Prism study [12] the backward scattering field is measured at 30°
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DROSMON workshop, 29th-30th of November 2007, Vienna, Austria
azimuth differences, the forward scattering one solely at 45°. The “hot spot” position is imaged by four additional measurements at view azimuth, respectively view zenith angle differences of 5° from the “hot spot” position. During the campaign at the BOKU test sites a high absolute backscatter signal was decided to have a higher priority in front of the possible inaccuracies due to changing atmospheric conditions during one series. Similar to the MUFSPEM solution reference measurements are registered at the beginning and the end of a measurement sequence. Thus the measurement series are quite similar to the MUFSPEM series and the only difference is the measured footprint which does not change in case of MGS.
Fig. 2: Measurement scheme of the MGS series for BRDF approximation. The number of data takes is minimized according to the recommendations of the LISPM/Prism study [12] The hot spot position is surrounded by four additional measurements by changing the view zenith angle (VZA) and the view azimuth angle (VAA) for +/- 5° Due to the BRDF database requirements [12] the reflectance of the same plot site was registered at three different times in a day. The first measurement series was taken as early in the morning as sun position allows (
