Atmopheric correction of high reolution multi-pectral atellite image uing a implified method baed on the 6S code A L Nune 1,2 and A R S Marçal 1 1 Faculdade de Ciência, DMA Unieridade do Porto Rua do Campo Alegre 687 4169-007 Porto, Portugal 2 Intituto Superior de Engenharia do Porto Rua Dr. António ernardino de Almeida 431 4200-072 Porto, Portugal Email: anune@fc.up.pt; andre.marcal@fc.up.pt Abtract A et of high-reolution multi-pectral atellite image were collected and proceed to etimate the ediment concentration in the ea-breaking zone. he alue of otal Supended Matter (SM) i empirically related to the ea urface reflectance. Howeer, in order to obtain accurate reflectance meaurement from the atellite image, the effect of the atmophere need to be accounted for. A et of atellite image from the SPO/HRVIR and erra/aser enor were collected for the tudy area, a ection of the northwet coat of Portugal around Aeiro. he image were atmopherically corrected uing a method combining 6S imulation and ground horizontal iibility meaurement. he method ue a reference cenario baed on the typical alue for each image of the following parameter: olar zenith angle, iewing zenith angle, iewing azimuth angle, ground height, and ground horizontal iibility. Some of thee parameter are nearly contant for the whole cene. Each of the releant parameter i allowed to ary within a reaonable range around it reference alue. he 6S radiatie tranfer code i ued to generate bottom of the atmophere reflectance alue a a function of the top of the atmophere recorded reflectance, for each of the parameter. Suitable atmopheric and aerool model are elected according to the meteorological auxiliary data aailable. For each pixel in the image, an atmopheric corrected reflectance i obtained a a um of the indiidual correction due to each of the parameter. Sea and location were ued a tet ite for an ealuation of the atmopherically corrected reflectance accuracy. 1 Introduction Earth oberation atellite are increaingly important for enironmental monitoring. In order to make quantitatie analyi of the Earth urface, the effect of the atmophere on the recorded ignal need to be conidered. he atmopheric correction of the data allow for reflectance alue of the obered target at the Earth urface to be obtained from the recorded radiance alue at the atellite enor. he iible and near-infrared radiation i affected by the atmophere through gaeou aborption and cattering by molecule and aerool and the un-target-enor geometry and the urface characteritic need alo to be accounted for in order to obtain accurate urface reflectance alue (eillet 1992, anré et al. 1992).
he effectie application of atmopheric correction i howeer a problematic tak. he atmophere compoition i highly ariable, both temporally and patially, and the information aailable about the atmophere i uually too pare. Although the topic of radiatie tranfer in the atmophere i reaonably well undertood, the actual implementation of atmopheric correction i till complex. Seeral methodologie hae been deeloped to remoe the effect of the atmophere on the recorded atellite ignal. Radiatie ranfer Code (RC) hae been widely ued by the remote ening community for thi purpoe, uch a the 6S code (Second Simulation of the Satellite Signal in the Solar Spectrum, Vermote et al. 1997a) and MODRAN (erk et al. 1989). Een if the exact atmopheric profile wa known, the computational effort inoled in a pixel-by-pixel correction would be too large - epecially for large amount of data, a i uual the cae with atellite image. he lack of input atmopheric data aailable, together with computational burden explain why for many application atmopheric correction are till not ued (Song et al. 2001). he deelopment of imple, eay to implement, atmopheric correction trategie i an important iue. Such method hould rely on a limited amount of eaily obtainable atmopheric input data and be computationally efficient. An abolute accuracy i not expected from a imple atmopheric correction method, but it hould neerthele proide a more realitic reult than uing uncorrected data. High patial reolution enor uch a SPO/HRVIR (Satéllite Probatoire d Obération de la erre / High Reolution Viible and Infrared) or Landat/M (hematic Mapper) are till a ery commonly ued ource of data, which need to be atmopherically corrected for mot application. Although ome of the mot recently deeloped enor (e.g. ASER Adanced Spaceborne hermal Emiion and Reflection Radiometer) already proide atmopherically corrected data, thee dataet are often not aailable for near real-time application. Furthermore, for long-term monitoring application uing hitoric data, enor like AVHRR, M and HRV(IR) might be the only aailable ource, and contemporary atmopheric information might be limited to meteorological data. hi paper decribe the application of a imple and fat pixel-by-pixel atmopheric correction method to high patial reolution image. hi application reult from the preliminary ongoing attempt for the monitoring of the ea breaking zone in the northwet coat of Portugal uing high patial reolution atellite image. he low reflectance alue of ea water in the iible and near infrared pectral region make atmopheric correction an eential proceing tak, a mot of the ignal recorded by the enor i due to the atmophere. he method relie on a implified ue of the 6S RC without recoure to multidimenional look-up table (LU). It can be ued for both preent and pat data and i alo uited for near real time application. An etimation of the urface (or ottom Of Atmophere, OA) reflectance i made from the ignal recorded by the atellite enor at the op Of Atmophere (OA). he input information required include a et of ground horizontal iibility alue at 0.550µm, and the oberation / illumination geometry and target height for each pixel. he ground horizontal iibility alue are ued by the RC to etimate the aerool loading, for a gien atmopheric cenario. 2 he 6S radiatie tranfer code he Second Simulation of the Satellite Signal in the Solar Spectrum (6S) i a radiatie tranfer code deeloped by Vermote et al. (1997a), following earlier erion deeloped by anré et al. (1990). he 6S RC imulate the effect on the radiation tranferred through the Earth atmophere in the pectral range 0.25-4.00µm, and alo account for combined urface-atmophere effect. he 6S code allow for the imulation of the ignal meaured by a atellite enor. Gien the target reflectance of a pixel and the enor characteritic, the code imulate the effect of the atmophere in the ignal due to cattering by molecule and aerool, and aborption - mainly by
H 2 O, CO 2, O 2, O 3, CH 4, N 2 O and CO (Vermote et al. 1997b). he input parameter include the iewing and illumination geometry, atmopheric model for the gaeou component, aerool model, enor/band information, and OA reflectance. he ground horizontal iibility i alo an input parameter, ued to etimate the optical depth at 0.550 µm due to aerool loading. he 6S code compute the OA reflectance amongt other output. he 6S RC can alo be run in the atmopheric correction mode, computing in thi cae the OA reflectance, gien the at-enor meaured alue. he input information on atmopheric condition i the ame a in the ituation decribed aboe, but no RDF are conidered apart from a Lambertian target aumption. he urface i alway aumed homogeneou. he input on iewing and illumination geometry, atmopheric condition and ground height, are ued to etimate the atmopherically corrected reflectance ( ac ): ac ( θ, θ, φ φ ) = 1 ' ac ' ac (1) where tand for the pherical albedo of the atmophere, θ and θ are the un illumination and iewing zenith angle, and φ - φ i the relatie azimuth angle. ac i gien by equation 2, where * i i the input apparent reflectance, a i the atmopheric reflectance, g i the total two-way gaeou tranmittance, and (θ ) and (θ ) are the total cattering tranmittance on the downward and upward path (Vermote et al., 1997b): ' ac = * i ( θ, θ, φ φ ) g ( θ ) ( θ ) a ( θ, θ, φ φ ) (2) he main limitation to the operational ue of the 6S code i the difficulty in getting the required atmopheric parameter and the computational time inoled in running the code on a pixel-by-pixel bai (Zhao et al. 2000). 3 he atmopheric correction implified method he approach ued here to atmopherically correct the high patial reolution image i briefly decribed in thi ection. It eentially conit of a light modification of the method preented in Nune et al (2004) where a detailed decription i made. 3.1 Decription of the method he method perform a implified atmopheric correction on the remotely ened Earth urface reflectance image, uing a reduced et of input. he enitiity analyi performed on the 6S RC (Nune et al. 2004) upport the aumption of computing the OA reflectance ( ) for each image pixel a a function of only ix ariable (equation 3), being the ground horizontal iibility one of the mot releant parameter: (, θ, θ, φ, h ) = (3) S V V, i the OA reflectance, θ the un zenith angle, θ the iewing zenith angle, φ the iewing azimuth angle, h the ground height and the ground horizontal iibility at 0.550µm. he approach propoed i baed on two main aumption: (1) the effect on OA reflectance alue of arying each ariable ha reduced influence on the remainder, i.e. that the ariable are nearly independent on a firt approach; (2) the et of alue correponding to each image pixel may be
conidered a a light deiation around a reference et of alue. hee reference alue, ( 0, θ 0, θ 0, φ 0, h 0, 0 ), correpond to the mot repreentatie cenario for a gien location and period of the year. Adequate atmopheric and aerool model are et and an appropriate range and ariation tep increment i etablihed for each ariable. he 6S RC i run in adance, with each ariable being aried eparately around the reference alue. he partial ariation induced on the OA reflectance are ealuated and tored on one-dimenional LU. For any et of input alue, the OA reflectance i etimated by adding to the reference alue 0 all the partially induced ariation, uing a imple finite difference firt order approximation (equation 4). ' 0 0 0 0 0 (, θ, θ, h, ) = ( + δ, θ + δθ, θ + δθ, h + δh, δ) = + S V S S V V = 0 + ( δ ) + ( δθ ) + ( δθ ) + ( δ ) + ( δ) S V (4) he partial difference (x), where x tand for the deiation around each reference alue, are computed by linear interpolation uing the correponding one-dimenional LU. he computational implementation of thi approach i much eaier than uing multidimenional LU, a the total number of cenario to imulate i only a few hundred. 3.2 Performance ealuation - implified method. 6S RC he method wa preiouly teted on AVHRR iible and near-infrared channel data (Nune et al.). he pectral location of the SPO/HRVIR and ASER VNIR band 2 and 3 i imilar to AVHRR band 1 and 2, and a comparable reult of the ealuation would mot likely be obtained for the enor. For thi performed tet, the atmopheric and aerool model were et to mid-latitude ummer and continental, repectiely (Vermote et al. 1997b). he iewing and illumination condition were choen appropriately for the AVHRR enor and the tet area the continental area of Portugal (latitude 37 N to 42 N; longitude 7 W to 9 30 W). According to thee condition and baed on a et of AVHRR image, one thouand cenario were generated at random. For that et of cenario, the OA reflectance alue etimated by the ued method (black croe) are plotted againt the 6S direct computation reult for AVHRR channel 1 (figure 1). he input OA reflectance alue were alo plotted for reference (grey circle). It i clear from thi figure that een a implified correction i much better than uing the uncorrected data. Mot of the imulated cenario fall cloe to the identity line (black) except for the ery high and ery low reflectance alue, far from reflectance reference alue, 0 = 0.15. It alo reeal ome difficultie in dealing with low ground horizontal iibility alue, caued by the dual behaiour of the etimated OA reflectance on uch circumtance. he oerall accuracy in comparion to the 6S RC wa ery encouraging with root mean quare error around 1% on both channel and for the whole et of cenario (Nune et al. 2004). A greater accuracy i expected for real ituation, a occurrence of extreme alue for input ariable i not a frequent a in the completely random et of cenario.
0.5 implified method etimation OA input alue 0.4 reflectance 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 6 OA reflectance etimation Figure 1. Ealuation of the accuracy of the implified atmopheric correction method: comparion with 6S RC direct etimation. 4 Reult and dicuion 4.1 Application to coatal zone monitoring he atmopheric correction method decribed in the preiou ection wa at firt deigned to be applied mainly oer land, where ground horizontal iibility meaurement are obtained with reaonable patial and temporal coerage from Meteorological Office, airport and aerodrome. For the coatal zone project COSA (COatal zone monitoring uing remote ening SAellite data), the purpoe i to etimate the OA reflectance oer water around the ea breaking zone. he dependence of the water reflectance with the amount of upended matter i a well-known fact in remote ening (e.g. Doraxan et al. 2002). In order to hae a relationhip between SM and OA water reflectance, accurate calibration and atmopheric correction of the image ha to be made. he latter i with no doubt the mot challenging tak, particularly due to the ery low alue of water reflectance. wo ASER image (8 and 24Oct.2001) and one SPO/HRVIR (14Oct.1998) image of the tudy area were ued within thi application. he firt three band of each image, in the iible and near-infrared pectral region, hae been calibrated to OA reflectance alue and atmopherically corrected uing the implified method. he reference cenario were etablihed for each image, with the input ariable et to the mot likely alue (able 1). he atmopheric and aerool model were et to mid-latitude ummer and 90% maritime and 10% continental, repectiely.
ASER I (08 Oct 01) ASER II (24 Oct 01) HRVIR (14 Oct 98) θ ( ) 47.7 53.5 49.9 φ ( ) 167.0 169.0 165.0 (km) 15 [5;100] 12 [5;100] 10 [5;100] h (m) 10 10 10 1 OA 0.07 [0;0.50] 0.07 [0;0.50] 0.08 [0;0.40] 2 OA 0.05 [0;0.60] 0.05 [0;0.60] 0.045 [0;0.40] 3 OA 0.03 [0;0.90] 0.03 [0;0.90] 0.03 [0;0.40] able 1. ypical cenario ued for generation of LU (typical [ minimum; maximum]) he iibility i a parameter which i prone to wide ariation in both pace and time, een for relatiely mall area. It i known (e.g. Zielińki and Zielińki, 2002) that aerool compoition and loading i ery ariable oer coatal water, trongly depending on wind behaiour characteritic (direction, elocity and duration). Right aboe the breaking zone there i alo an extra input of aerool in the atmophere due to the amount of water releaed by the breaking wae. Unle field data i aailable for the particular location and time deired, uch a detailed characteriation i ery difficult to obtain. In the preent application, the ground horizontal iibility alue were taken from meteorological data from the national network of meteorological tation (Intituto de Meteorologia), in which meaurement are made eery 3 hour. here are other poible ource for the horizontal iibility data, uch a the on-line he Weather Underground, Inc (www.wunderground.com) which gather data not only from meteorological office but alo from airport, aerodrome, and other location. A iibility map for the whole area i generated for the time of image acquiition by temporally and patially interpolating the aailable iibility meaurement. 4.2 Atmopherically corrected data 4.2.1 he corrected atellite image For the atmopheric correction of the three atellite image decribed in ection 4.1, the implified method wa applied according to the parameter range hown in table 1. Special attention wa gien to the ea water area, for which the typical alue had been choen. he method perform an atmopheric correction on a pixel-by-pixel bai, and each ariable i prone to take different alue at different pixel. Due to the mall dimenion of the target area, ome of the parameter (iewing and illumination geometry) preent negligible ariation within the whole interet zone, and were therefore et to their typical alue throughout. he effect of ground height ha to be conidered in atmopheric correction oer land a it can widely ary een within a mall cene. For thi application oer ea water, the target height wa taken a contant and et to the typical alue of 10m. he ground horizontal iibility, the mot critical parameter and the one more prone to ary on a pixel cale. Unfortunately, for thi pecific tudy area, only one tation wa proiding iibility meaurement for all of the image date, and thu the iibility had to be taken a homogeneou in the whole area, although taking different alue on different date (ee able 1). Under thee circumtance, the only input ariable actually corrected at the pixel cale i the meaured OA reflectance. Figure 2 how the ASER I channel 1 image before (left) and after (right) the atmopheric correction. A peudo colour table wa generated with the help of PCI Geomatic oftware (PCI Geomatic 2001) for an enhanced iual interpretation.
Figure 2. ASER channel 1 image (08 October 2001) of the tudy area: OA (left) and OA (right) reflectance A we can ee in figure 2, the difference in the SM pattern between OA and etimated OA reflectance image are not ery noticeable. here i a difference in the range of alue, but thi doe not affect the ditribution pattern. he limited aailability of information about the ground horizontal iibility alue, which forced thi ariable to be taken a homogeneou, might be the main caue of thi reult. Conidering that the OA reflectance wa the only ariable actually arying within each image, the atmopheric correction method reult in a imple linear tranformation function between input (OA) and output (OA) reflectance alue. hi would not be the cae if the iibility wa allowed to ary patially, a it i expected to happen. 4.2.2 Performance ealuation with ground data Another ealuation of the atmopheric correction performance wa carried out on thee image. Figure 2 how a plot where the range of reflectance alue for a number of and area identified in the image i repreented with atmopheric correction (olid line ellipe) and without atmopheric correction (dahed line ellipe), for both ASER and HRVIR ued band. Alo diplayed on the graph i a plot of the aerage reflectance pectrum of and, obtained from field urey at eeral beache in the tudy area. Under thee condition, the method howed to perform reaonably well (epecially in the near-infrared region) for medium to high reflectance alue. Although thi implified method had hown a good accuracy when compared to the 6S RC, it performance oer water i rather poor, when compared to ground data. he water reflectance i ery low, both in the iible and near-infrared band. For at-enor reflectance alue in the range 0-6%, mot of the recorded ignal i due to the effect of the atmophere. Under low iibility condition, the 6S RC eem to oercorrect thi effect and a a conequence, the etimated OA reflectance ometime take null or een negatie alue for coatal water. If thi i found to be a drawback of the 6S RC alone, maybe the ue of another RC would be enough to oercome thi limitation. Neerthele, another improement of the atmopheric correction method i planned to be carried out in a near future, combining a dark target approach (uing deep ea water reflectance) and other table reflectance target. A new et of field meaurement i cheduled for the ummer/autumn 2004, which hould allow for a better ealuation of the error aociated with the ue of and area a table reflectance target.
Figure 3. Atmopheric correction ealuation for SPO (blue) and ASER VNIR (red) band. 5 Concluion Atmopheric correction of Earth obering atellite image are not often ued due to the high computational effort and the lack of auxiliary data. Some modern high patial reolution enor (e.g. ASER) already proide atmopherically corrected data. Other enor like Landat/M and SPO/HRV(IR) are a aluable ource of preent data and, in many cae, of the only hitoric record. hee dataet need to be corrected for mot application. A implified pixel-by-pixel atmopheric correction method baed on the 6S RC (Radiatie ranfer Code) wa applied to SPO/HRVIR and ASER atellite image oer ea coatal water. hee atmopherically corrected reflectance image are ued to etimate the amount of SM (otal Supended Matter). he ery low reflectance alue of the ea water in the iible and near infrared pectral zone make atmopheric correction a challenging tak. he method performed well when compared to the 6S RC (rme around 1%) but not o well againt ground data. he ery low reflectance till poe ome problem a they are ometime corrected to negatie alue. When comparing the atmopherically corrected and non-corrected reflectance image, a difference in the range of reflectance alue i clear. Noticeable difference in the SM pattern ditribution are not detected, but that might be caued by the ue of a ingle ground horizontal iibility alue for the whole image. Further improement of the implified method are currently being deeloped. he ue of ground reflectance data of known table target (e.g. deep ea water, ea and area, etc) i to be ued together with thi approach. he ue of ASER urface reflectance data for comparion with urface reflectance etimated by the implified method might allow for it further calibration and more effectie performance on image from other atellite enor.
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