BRDFs acquired by directional radiative measurements during EAGLE and AGRISAR

Radiation is the driving force for all processes and interactions between earth surface and atmosphere. The amount of measured radiation reflected by vegetation depends on its structure, the viewing angle and the solar angle. This angular dependence is usually expressed in the Bi-directional Reflectance Distribution Function (BRDF). This BRDF is not only different for different types of vegetation, but also different for different stages of the growth. The BRDF therefore has to be measured at ground level before any satellite imagery can be used the calculate surface-atmosphere interaction. The objective of this research is to acquire the BRDFs for agricultural crop types. A goniometric system is used to acquire the BRDFs. This is a mechanical device capable of a complete hemispherical rotation. The radiative directional measurements are performed with different sensors that can be attached to this system. The BRDFs are calculated from the measured radiation. In the periods 10 June - 18 June 2006 and 2 July - 10 July 2006 directional radiative measurements were performed at three sites: Speulderbos site, in the Netherlands, the Cabauw site, in the Netherlands, and an agricultural test site in Goermin, Germany. The measurements were performed over eight different crops: forest, grass, pine tree, corn, wheat, sugar beat and barley. The sensors covered the spectrum from the optical to the thermal domain. The measured radiance is used to calculate the BRDFs or directional thermal signature. This contribution describes the measurements and calculation of the BRDFs of forest, grassland, young corn, mature corn, wheat, sugar beat and barley during the EAGLE2006 and AGRISAR 2006 fieldcampaigns. Optical BRDF have been acquired for all crops except barley. Thermal angular signatures are acquired for all the crop

Kahden varpukasvin spektrien kaksisuuntaiset heijastussuhdetekijämittaukset

Recent studies have shown the benefits of multiangular remote sensing techniques for characterizing vegetation reflection properties. The study of spectral anisotropy of understory vegetation enables methods for improved plant species identification, and provides valuable input data for radiation scattering models of forests. This thesis presents the applied methods and results of a research effort carried out over the growing season of 2017 for the temporal spectral characterization of two of the economically most important wild berry species in Finland: lingonberry (Vaccinium vitis-idaea) and blueberry (Vaccinium myrtillus). The spectral bidirectional reflectance factor (BRF) data on lingonberry and blueberry shrub samples were collected in a multidirectional measurement geometry using the Finnish Geodetic Institute Goniospectrometer (FIGIFIGO) in laboratory conditions. Leaf reflectance and transmittance spectra on both species were collected with SpectroClip-TR spectral probe. The anisotropic characteristics were analysed in the spectral range from 400 to 2200 nm for view angle dependence (-40° to +40°), illumination angle dependence (+40°, +55°), seasonal dynamics over the growing season (2017), and for berry and flower detection. Both lingonberry and blueberry shrubs have strong backward and notable forward scattering characteristics on the principal plane. In the interspecies comparison, lingonberry is brighter into all view direction in the visible and near infrared wavelengths but darker in the short-wave infrared. Increasing the illumination zenith angle by 15° improves the spectral discrimination of the two dwarf shrub species by inducing a 12% ratio of the spectral responses. Vegetation indices that are commonly used in remote sensing of forests (NDVI, NDVI705, MSI, PSRI) show low sensitivity to the changes in the view- and illumination angles. The presence of lingonberries and lingonberry flowers is indicated as a spectral peak around 679 nm in the spectral ratio of samples with berries or flowers to samples without berries or flowers. It was shown that the analysis of spectral data on the reflectance anisotropy improves the spectral discrimination of the dwarf shrub species. The contribution of the berries on the obtained shrub spectra was shown to be notable enough to justify further studies by applying unmanned aerial vehicle (UAV) platforms. Future studies on the aerial spectral data are suggested to evaluate the potential of berry mapping in larger-scale.Viimeaikaiset tutkimukset ovat osoittaneet monisuunta-spektrometrian hyödyt kasvillisuuden heijastusominaisuuksien karakterisoinnissa kaukokartoituksessa. Aluskasvillisuuden spektrien anisotropian tutkiminen edesauttaa kehittämään menetelmiä kasvilajien tunnistamiseksi ja tarjoaa validointiaineistoa metsien sirontamalleihin. Tämä diplomityö esittää menetelmät ja tulokset Suomen kahden taloudellisesti tärkeimmän luonnonmarjoja tuottavan varpukasvin, mustikan (Vaccinium myrtillus) ja puolukan (Vaccinium vitis-idaea), spektrien temporaalisesta karakterisointikampanjasta kasvukauden 2017 yli. Kaksisuuntainen heijastussuhdetekijä spektriaineisto mitattiin mustikan ja puolukan varpunäytteistä monisuuntamittausgeometriassa FIGIFIGO (Finnish Geodetic Institute Goniospectrometer) goniospektrometrillä laboratorio-olosuhteissa. Lehtien heijastus- ja läpäisyspektrit mitattiin molemmista lajeista käyttäen SpectroClip-TR mittalaitetta. Anisotropiset ominaispiirteet analysointiin aallonpituuksien 400 - 2200 nm välillä katselukulmariippuvuudelle (-40° to +40°), valaistuskulmariippuvuudelle (+40°, +55°), vuodenajan aiheuttamille muutoksille (kasvukausi 2017) sekä marja ja kukintojen tunnistamiselle. Sekä puolukka että mustikka osoittavat voimakasta taaksepäin suuntautuvaa ja huomattavaa eteenpäin suuntautuvaa ominaissirontaa päätasossa. Lajien välisessä vertailussa puolukka on kirkkaampi kaikkiin mitattuihin katselukulmiin näkyvän valon ja lähi-infrapunan aallonpituuksilla, mutta tummempi lyhytaaltoisen infrapunan alueella. Valaistuskulman zeniitin kasvattaminen 15° parantaa lajien spektrien erotettavuutta aiheuttamalla 12 %:n eron lajien heijastusvasteisiin. Yleisesti metsän kaukokartoituksessa käytetyt kasvillisuusindeksit (NDVI, NDVI705, MSI, PSRI) osoittavat matalaa herkkyyttä katselu- ja valaistuskulman muutoksille. Näytteessä olevat puolukanmarjat ja -kukat erottuvat spektrissä piikkinä 679 nm:n kohdalla, kun tarkastellaan marjallisten ja kukallisten näytteiden suhdetta marjattomiin ja kukattomiin. Spektriaineiston heijastus-anisotropian analysoinnin näytettiin edesauttavan varpukasvien erotettavuutta. Marjojen vahva kontribuutio varpunäytteistä mitattuihin spektreihin osoitettiin niin selkeästi, että jatkotutkimuksia UAV (unmanned aerial vehicle) -alustalla voidaan pitää perusteltuina. Ilma-aluksilla kerättyä aineistoa ehdotetaan käytettävän marjojen laajemman kartoituksen potentiaalin selvittämiseksi

A new lab facility for measuring bidirectional reflectance/emittance distribution functions of soils and canopies

Recently, a laboratory measurement facility has been realized for assessing the anisotropic reflectance and emittance behaviour of soils, leaves and small canopies under controlled illumination conditions. The facility consists of an ASD FieldSpec 3 spectroradiometer covering the spectral range from 350 – 2500 nm at 1 nm spectral sampling interval. The spectroradiometer is deployed using a fiber optic cable with either a 1°, 8° or 25° instantaneous field of view (IFOV). These measurements can be used to assess the plant pigment (chlorophyll, xanthophyll, etc.) and non-pigment system (water, cellulose, lignin, nitrogen, etc.). The thermal emittance is measured using a NEC TH9100 Infrared Thermal Imager. It operates in a single band covering the spectral range from 8 – 14 mm with a resolution of 0.02 K. Images are 320 (H) by 240 (V) pixels with an IFOV of 1.2 mrad. A 1000 W Quartz Tungsten Halogen (QTH) lamp is used as illumination source, approximating the radiance distribution of the sun. This one is put at a fixed position during a measurement session. Multi-angular measurements are achieved by using a robotic positioning system allowing to perform either reflectance or emittance measurements over almost a complete hemisphere. The hemisphere can be sampled continuously between 0° and 80° from nadir and up to a few degrees from the hot-spot configuration (depending on the IFOV of the measurement device) for a backscattering target. Measurement distance to targets can be varied between 0.25 and 1 m, although with a distance of more than 0.6 m it is not possible to cover the full hemisphere. The goal is to infer the BRDF (bidirectional reflectance distribution function) and BTDF (bidirectional thermal distribution function) from these multi-angular measurements for various surface types (like soils, agricultural crops, small tree canopies and artificial objects) and surface roughness. The steering of the robotic arm and the reading of the spectroradiometer and the thermal camera are all fully automated

Puiden runkojen monikulmamittaus kannettavalla hyperspektrikameralla

Laboratory measurement settings that can acquire spectral and multi-angular information on canopy elements (e.g. leaves and woody tree structures) provide invaluable data for the interpretation and development of forest reflectance models and other optical remote sensing techniques. Previous studies have pointed out that the spectral properties of woody tree structures of boreal tree species have been studied little in comparison to leaves, and that there is a need to fill this gap in knowledge. This thesis presents a custom-built multi-angular measurement system with imaging capabilities that was used to acquire a hyperspectral dataset of boreal woody tree structures of the three most common tree species found in Finland. A total of six trees, two trees per species of Norway spruce (Picea abies (L.) Karst), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth) stems were sampled at different heights (at every meter of height between 1–10 m) and sides (northward and southward facing sides of the stem), and the stem surface (bark) was measured with a novel mobile hyperspectral camera called Specim IQ. The camera operates in the wavelength range of 400–1000 nm. The acquired dataset contains hyperspectral images of 120 stem samples, each imaged from six different view angles. A designed pixel-by-pixel data processing chain is described. It can calculate and extract accurate pixel specific reflectance information that is invariant to uneven spatial distribution of incident irradiance from the lamp. Finally, the processed data was analyzed to reveal within- and between-species, angular, and spatial variations in stem bark reflectance for the three species. In concordance to previous studies, this thesis found that the species varied highly in their mean spectra and were distinguishable from one another. In addition, the within-species variation and standard deviation between mean spectra of samples was surprisingly low with very similar spectral signatures between samples of the same species. Investigating angular variation revealed that both pine and birch present strong specular reflections in the forward-scattering angles, in comparison to spruce, which presented a hot spot effect in the backward-scattering angles when measured near the lamp. Birch and spruce showed weak trends when looking at the spatial variations occurring in reflectance due to sampling height or side of the stem. However, pine displayed a clear increase in reflectance from 1 m to 4 m height at 663.81 nm (red band) and from 1 m to 5 m height at 865.5 nm (near-infrared band). The data obtained in this study show potential for future tasks such as tree species classification and the further development of forest reflectance models. The methods and materials presented in this study can give ideas for developing imaging goniometer systems that can acquire even more information on various vegetation canopy elements than what were presented in this thesis.Laboratorio-olosuhteissa käytettävät mittausjärjestelmät, jotka pystyvät keräämään spektriaineistoa eri mittauskulmista, tuottavat arvokasta tietoa metsien heijastusmallien ja muiden kaukokartoitustekniikoiden tulkintaa ja kehittämistä varten. Aikaisemmat tutkimukset ovat osoittaneet, että boreaalisen vyöhykkeen puulajien puumaisten osien spektriominaisuuksia ei ole tutkittu yhtä paljon kuin lehtien, ja tämän takia kyseiselle tiedolle on selkeä tarve. Tässä diplomityössä rakennettiin monikulmainen kuvantava mittausasetelma, jolla mitattiin hyperspektriaineisto kolmesta Suomen yleisimmästä puulajista: kuusesta (Picea abies (L.) Karst), männystä (Pinus sylvestris L.) ja rauduskoivusta (Betula pendula Roth). Mitattavat näytteet kerättiin yhteensä kuudesta puusta, kahdesta puusta per puulaji. Näytteitä otettiin rungon eri korkeuksilta (metrin välein kymmeneen metriin asti) ja ilmansuunnista (pohjoinen ja eteläinen puoli runkoa). Rungon pintaosan (kuoren) heijastusspektri mitattiin uudella kannettavalla Specim IQ -hyperspetrikameralla, joka pystyy keräämään tietoa 400–1000 nanometrin aallonpituuksilta. Kerätty spektriaineisto koostuu Specim IQ:lla mitatuista hyperspektrikuvista, joita otettiin 120 näytteestä. Jokainen näyte kuvattiin kuudesta eri kulmasta. Mittausasetelman lisäksi tässä diplomityössä kehitettiin pikselikohtainen prosessointimenetelmä, jonka avulla voi laskea näytteen heijastusspektrin siten, että siihen ei vaikuta lampusta epätasaisesti jakautuva valo. Prosessoitujen hyperspektrikuvien avulla tutkittiin, kuinka heijastusspektri vaihtelee puulajien välillä sekä sisäisesti puulajin näytteiden välillä. Lopuksi tutkittiin, kuinka eri mittauskulmat ja näytteenottokorkeus vaikuttavat kuusen, männyn ja koivun heijastusspektreihin. Yhtenevästi aikaisempien tutkimusten kanssa tämän diplomityön tulokset osoittavat, että puulajien väliset erot heijastusspektreissä olivat suuria ja puulajit toisistaan erotettavissa. Toisaalta puulajin sisäinen vaihtelu oli yllättävän pientä ja saman puulajin näytteiden spektrit olivat samanlaisia toisiinsa nähden. Mittauskulman vaikutuksen selvittäminen osoitti, että männyllä ja rauduskoivulla heijastus suuntautuu voimakkaasti eteenpäin. Toisaalta kuusi osoitti voimakkaampaa taaksepäin valonlähdettä kohti suuntautuvaa heijastusta. Kuusella ja rauduskoivulla oli havaittavissa vähäistä heijastuksen spatiaalista vaihtelua näytteenottokorkeuksien ja ilmansuuntien välillä. Toisin kuin kuusi ja rauduskoivu, mänty osoitti selkeää nousevaa heijastusta 663.81 nm aallonpituudella (punainen kanava) 1 m korkeudelta 4 m korkeuteen. Lähi-infrapunakanavalla (865.5 nm) vastaava kasvu havaittiin välillä 1–5 m. Tässä diplomityössä kerättyä tietoa on mahdollista käyttää puulajien tunnistusmenetelmien sekä metsien heijastusmallien kehittämisessä. Tämän lisäksi esitetyt koetekniikat ja tutkimusmenetelmät voivat auttaa kehittämään kuvantavia goniometrijärjestelmiä, joilla voisi kerätä vieläkin laajempaa tietoa kasvillisuudesta ja sen eri heijastusominaisuuksista

Towards a comparison of spaceborne and ground-based spectrodirectional reflectance data

Almost all natural surfaces exhibit an individual anisotropic reflectance behaviour due to the contrast between the optical properties of surface elements and background and the geometric surface properties of the observed scene. The resulting bidirectional effects are present in all reflectance data and thus occur as well in various vegetation indices (VI’s) retrieved from multiangular data. No matter whether these effects are considered as noise or as a source of additional information, accurate knowledge about their magnitude is important. This preliminary study is based on data of the spaceborne ESA-mission CHRIS (Compact High Resolution Imaging Spectrometer) onboard PROBA-1 and on ground-based spectrodirectional measurements performed with the dual view field goniometer system FIGOS. The objectives of this study are focused on directional effects in CHRIS and FIGOS reflectance data of a Triticale field as well as on the variability of retrieved vegetation indices for selected view angles in both multiangular datasets

Measuring the Effects of Soil Parameters on Bidirectional Reflectance Distribution Function

Remote sensing data acquisition often requires a revisit to the same target. Therefore, it is not always possible to have the same illumination and viewing conditions. Bidirectional Reflectance Distribution Function (BRDF) is an attempt to predict the reflectance of an object for any given viewing and illumination geometry by explaining the interaction of the incident energy with the target object, the medium lying between the source and the target, and the interaction of the reflected energy with the medium between the target and the sensor. In this study various factors affecting BRDF were explored. Various factors contribute to this characteristic of the surface to reflect unequally in different directions like its structure, shape, degree of absorption and transmittance. Bidirectional Reflectance Factor, Anisotropic Factor, and Anisotropic Index were used in the research. Radiances were recorded using the Sandmeier Field Goniometer of target areas at the agricultural farms of Mississippi State University

A STUDY ON THE EFFECTS OF VIEWING ANGLE VARIATION IN SUGARCANE RADIOMETRIC MEASURES

Remote Sensing techniques, such as field spectroscopy provide information with a large level of detail about spectral characteristics of plants enabling the monitoring of crops. The aim of this study is to analyze the influence of viewing angle in estimating the Bidirectional Reflectance Distribution Function (BRDF) for the case of sugarcane. The study on the variation of the spectral reflectance profile can help the improvement of algorithms for correction of BRDF in remote sensing images. Therefore, spectral measurements acquired on nadir and different off-nadir view angle directions were considered in the experiments. Change both anisotropy factor and anisotropy index was determined in order to evaluate the BRDF variability in the spectral data of sugarcane. BRDF correction was applied using the Walthall model, thus reducing the BRDF effects. From the results obtained in the experiments, the spectral signatures showed a similar spectral pattern varying mainly in intensity. The anisotropy factor which showed a similar pattern in all wavelengths. The visual analysis of the spectral reflectance profile of sugarcane showed variation mainly in intensity at different angles. The use of Walthall model reduced the BRDF effects and brought the spectral reflectance profiles acquired on different viewing geometry close to nadir viewing. Therefore, BRDF effects on remote sensing data of vegetation cover can be minimized by applying this model. This conclusion contributes to developing suitable algorithms to produce radiometrically calibrated mosaics with remote sensing images taken by aerial platforms

Multiangular crop differentiation and LAI estimation using PROSAIL model inversion

xiii, 161 leaves : ill., map ; 29 cmUnderstanding variations in remote sensing data with illumination and sensor angle changes is important in agricultural crop monitoring. This research investigated field bidirectional reflectance factor (BRF) in crop differentiation and PROSAIL leaf area index (LAI) estimation. BRF and LAI data were collected for planophile and erectophile crops at three growth stages. In the solar principal plane, BRF differed optimally at 860 nm 60 days after planting (DAP) for canola and pea, at 860 nm 45 and 60 DAP for wheat and barley, and at 860 nm and 670 nm 45 and 60 DAP for planophiles versus erectophiles. The field BRF data helped better understand PROSAIL LAI estimation. NDVI was preferred for estimating LAI, however the MTVI2 vegetation index showed high sensitivity to view angles, particularly for erectophiles. The hotspot was important for crop differentiation and LAI. Availability of more along-track, off-nadir looking spaceborne sensors was recommended for agricultural crop monitoring

Polarised Multiangular Reflectance Measurements Using the Finnish Geodetic Institute Field Goniospectrometer

The design, operation, and properties of the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) are presented. FIGIFIGO is a portable instrument for the measurement of surface Bidirectional Reflectance Factor (BRF) for samples with diameters of 10 – 50 cm. A set of polarising optics enable the measurement of linearly polarised BRF over the full solar spectrum (350 – 2,500 nm). FIGIFIGO is designed mainly for field operation using sunlight, but operation in a laboratory environment is also possible. The acquired BRF have an accuracy of 1 – 5% depending on wavelength, sample properties, and measurement conditions. The angles are registered at accuracies better than 2°. During 2004 – 2008, FIGIFIGO has been used in the measurement of over 150 samples, all around northern Europe. The samples concentrate mostly on boreal forest understorey, snow, urban surfaces, and reflectance calibration surfaces

Bidirectional Distribution Reflection Function of Algodones Dunes

The primary objective of this project was to develop the Algodones Dunes as a pseudo-invariant calibration site (PICS) suitable for absolute calibration of satellite sensors based on a surface reflectance model. Two approaches were taken during this research: a field campaign and laboratory measurements. The first approach, the field campaign, was accomplished with a trip to the Algodones Dunes from the 8th to the 13th of March, 2015. During the field campaign, several test points from spatially different regions of the Algodones Dunes were studied. Reflectance of the sand at each test point was observed from different view angles. The second approach, laboratory testing, involved bringing several sand samples from different regions of the Algodones Dunes back to SDSU for further analysis. The laboratory setup was built in the SDSU optics laboratory and included the use of a light source, digital power supply, and mechanical arm to study the spectral responses of the sand samples from the field. During the laboratory measurements, the reflectance of each of the sand sample, was observed from different view angles to replicate field measurement techniques. Through both approaches it was found that the reflectance of sand samples from the Algodones Dunes changes quadratically with respect to view zenith angle. To correlate field and laboratory measurements, two solar zenith angles were chosen for laboratory simulation, i.e. 45and 54.4. Since the solar zenith angle varies from 20to 60 over a year in the Algodones Dunes, angles within that range were chosen for the solar zenith angles used in the laboratory measurements. The spectral response of different sand samples were only observed under those two chosen solar zenith angles. Since different equipment was used in the laboratory than in the field, there was some degree of uncertainty due to each of the differing instruments which influenced the data. A Linear Mixed Model was therefore developed in order to incorporate the laboratory uncertainties and predict a more accurate model using the raw data acquired in the laboratory. The data modeled by the Linear Mixed Model approach for different BRDF runs of the same sample, and for different sand samples, were compared to determine whether the spectral response of sand samples from the Algodones Dunes is the same or not. Based on the data modeled by the Linear Mixed Model, it was found that the spectral responses of sand samples brought from the Algodones Dunes are the same. A simple BRDF model was then developed for those angles that are perpendicular to the principal plane of the solar illumination