Using high resolution DSM data to correct the terrain illumination effect in Landsat data

Transcription

1 19h Inernaional Congress on Modelling and Simulaion, Perh, Ausralia, December 2011 hp://mssanz.org.au/modsim2011 Using high resoluion DSM daa o correc he errain illuminaion effec in Landsa daa F. Li a, D. L. B. Jupp b, M. Thankappan a a Naional Earh Observaion Group, Geoscience Ausralia, GPO Box 378, ACT, 2601, Ausralia Fuqin.Li@ga.gov.au Medhavy.Thankappan@ga.gov.au b CSIRO, Marine and Amospheric Research, GPO Box 3023, ACT 2601, Ausralia David.Jupp@csiro.au Absrac: Terrain affecs opical saellie images hrough boh irradiance and bidirecional reflecance disribuion funcion (BRDF) effecs. Slopes facing he sun receive enhanced solar irradiance and appear brigher compared o hose facing away from he sun. For anisoropic surfaces, he radiance received a he saellie sensor from a sloping surface is also affeced by surface BRDF which varies wih combinaions of surface landcover ypes, sun, and saellie geomery (sun and sensor view, and heir relaive azimuh angle) as well as opographic geomery (primarily slope and aspec angles). Consequenly, o obain comparable surface reflecance from saellie images covering mounainous areas, i is necessary o process he images o reduce or remove he opographic effec so ha he images can be used for differen purposes on he same specral base. The mos common mehod of normalising for he opographic effec is by using a Digial Surface Model (DSM) and / or a Digial Elevaion Model (DEM). However, he accuracy of he correcion depends on he accuracy, scale and spaial resoluion of DSM daa as well as he co-regisraion beween he DSM and saellie images. A physics based BRDF and amospheric correcion model in conjuncion wih he 1-second SRTM (Shule Radar Topographic Mission) derived DSM produc released by Geoscience Ausralia in 2010 were used o conduc he analysis repored in his paper. The resuls show ha arefacs in he DSM daa can cause significan local errors in he correcion. For some areas, false shadow and over correced surface reflecance facors have been observed. In oher areas, he algorihm is unable o deec shadow or rerieve an accurae surface reflecance facor in he slopes away from he sun. The accuracy of co-regisraion beween saellie images and DSM daa is imporan for he opographic correcion. A mis-regisraion error of one or wo pixels can lead o large error of rerieved surface reflecance facors in he gully and ridge areas (rerieved reflecance facors can change from 0.3 o 0.5 or more). Therefore, accurae regisraions for boh saellie images and DSM daa are necessary o ensure he accuracy of he correcion. Using low resoluion DSM daa in conjuncion wih high resoluion saellie images can fail o correc some significan errain effecs. A DSM resoluion appropriae o he scale of he resoluion of saellie image is needed for he bes resuls. Key words: DSM, Saellie images, Topographic correcion. 2402

2 1. INTRODUCTION Topographic correcion of saellie images over mounainous or hilly areas is very imporan (Liang, 2002). Hill and mounain errain affecs opical saellie images hrough boh irradiance and BRDF effecs (Dymond e al., 2001). Slopes facing oward he sun receive more solar irradiance and appear brigher in saellie images han hose facing away from he sun (Shepherd and Dymond, 2003). For anisoropic surfaces, he radiance received a he saellie from sloping surfaces is also affeced by surface BRDF which resuls from surface landcover srucure ineracing wih he sun and saellie geomery (sun and view and is relaive azimuh angle) as well as opographic geomery (e.g. slope and aspec angles). All of hese affec he inversion of land surface parameers and applicaions ha aim o deec land surface change hrough ime series analysis. The availabiliy of good qualiy Digial Surface Models (DSMs) and Digial Elevaion Models (DEMs) o he public enables he rouine applicaion of opographic correcion. A variey of advanced opographic correcion algorihms based on DSM/DEM daa have been developed (Teille e al., 1982, Richer, 1997 and Shepherd and Dymond, 2003, Li e al., 2010a). However, for he DSM/DEM based algorihms, he accuracy of he correcion depends on he accuracy of he DSM daa, is spaial resoluion and scale, and he unavoidable levels of mis-regisraion beween he DSM and saellie daa. In his sudy, he impac of DSM on he opographic correcion of saellie daa was invesigaed. 2. METHOD SELECTED FOR THE STUDY A physics based coupled BRDF and amospheric model for boh fla (horizonal) and rugged surfaces developed by Li e al. (2010a) is used in his sudy. For anisoropic surfaces over mounainous or hills area, he radiaive ransfer model from Li e al., (2010a) can be expressed as: L TOA 2 T V dir dif S( ρ) = L + E f ρ i e δϕ + f ρ i + E f ρ e + f ρ + E 0 [ V S (,, ) (1 V ) '( )] [ V ( ) (1 V ) ] (1) π 1 Sρ Where L TOA is sensor radiance, L 0 is pah radiance, T V is oal ransmiance in he view direcion, E, E dir and E dif are oal irradiance, direc componen of irradiance and diffuse componen of irradiance on a sloping surface, respecively, i and e are inciden and exiing zenih angles beween he sun and view direcions and surface normal, δφ is he relaive azimuh angle beween inciden and exiing direcions in he slope geomery and δφ = φ i φ v. φ i and φ v are he azimuh angle for inciden and exiing direcion in he slope geomery, ρ,ρ' and ρ are surface hemispherical-direcional, direcional-hemispherical and hemisphericalhemispherical reflecance (or bi-hemispherical) facors, respecively, and S is amospheric albedo which is he bi-hemispherical raio beween upwelling radiaion from he surface and he downwelling radiaion. f V and f S are defined as: f V = V /( V + d (θ V ))= V /T V and f S = S /( S + d (θ S ))= V /T S. Where S and V are direc ransmiance in he solar and view direcions. d (θ S ) and d (θ V ) are diffuse ransmiance in he solar and view direcions; T S is oal ransmiance in he sun direcion. 3. STUDY AREAS AND DATA USED The Ausralian Alps and Blue Mounains in New Souh Wales (NSW) were he wo areas seleced for his sudy as hese wo regions cover some of he highes errain in Ausralia. A clear Landsa 5 image (Pah 91 and Row 85) sensed on February 25, 2009 wih moderae solar zenih angle (~45 o ) was used o conduc he analysis. A small subse of he Landsa 5 TM image covering he Ausralian Alps locaed in he norheas of Vicoria was used. The cenral geographical locaion of he area is o S and o E. The Ausralian Alps have he mos significan opographic effecs in Ausralia, is elevaion is also he highes, reaching 2100 m above mean sea level. The Blue Mounains is locaed in NSW; a clear Landsa 5 image (Pah 90 and Row 84) sensed on Sepember 22, 2006 wih moderae solar zenih angle (~45 o ) was seleced for he sie. A subse of he Landsa 5 TM 2403

3 image covering he souh of he Blue Mounains was used. The cenre coordinaes for he area are o S and o E. The ancillary inpu daa needed o run he algorihm included amospheric waer vapour, aerosol opical deph, BRDF parameers as well as he DSM. Amospheric waer vapour was derived from radiosonde daa measured by he Ausralian Bureau of Meeorology. For he Ausralian Alps, waer vapour daa were obained from he Wagga Wagga meeorological saion ( o S and o E), locaed wihin he same Landsa scene. For he Blue Mounains area, no radiosonde daa was available for locaions wihin he same scene, herefore daa from he neares meeorological saion (Wagga Wagga) was used. The oal precipiable waer conen comparison beween he NOAA NCEP (Naional Ceners for Environmenal Predicion) reanalysis produc (Kalnay e al., 1996) and Wagga Wagga radiosonde daa shows ha hey are very close. Since he waer vapour has a relaively minor impac on he Landsa visible and shorwave reflecance rerievals, he Wagga Wagga radiosonde daa is sufficien o be used in he souh of Blue Mounains areas. The aerosol daa were obained from he local Aerone saion in Canberra (hp://aerone.gsfc.nasa.gov). The geographical locaion for his saion is o E and o S and was appropriae for use in analysis of boh images due o similariy of he amospheric sysems in he sudy sies. (a) (b) (c) (d) Figure 1. False colour composie images (bands 4, 3, 2, whie colour is deeced as shadow): (a) wih BRDF and amospheric correcion only for he Vicorian Alps image of Feb.25, 2009 (b) wih BRDF and amospheric correcion plus opographic correcion for he Vicorian Alps image of Feb.25, 2009 (c) wih BRDF and amospheric correcion only for he souh Blue Mounains image of Sep. 22, 2006 (d) wih BRDF and amospheric correcion plus opographic correcion for he souh Blue Mounains image of Sep. 22, BRDF parameers were obained from he MODIS BRDF model produc (Schaaf e al., 2002) which is he resul of fiing a model o amospherically correced MODIS daa over a 16-day composiing period. A 2404

4 combinaion of he Ross Thick volume kernel and he Li-sparse Reciprocal (Schaaf e al., 2002) geomeric kernel models was used o approximae hese daa. Following Li e al. (2010b), he BRDF shape is defined for large regions (regional scale); for his sudy, he average value over he Landsa scene exen was used. High qualiy DSM daa are very imporan for physically based errain illuminaion correcion. Geoscience Ausralia and CSIRO have released he 1-second Shule Radar Topographic Mission (SRTM) derived DSM and DEM of Ausralia in February, 2010 (Geoscience Ausralia and CSIRO Land & Waer, 2010). The DSM daa are designed o serve differen purposes and one of hese is where radiaion is obsruced by he roughness surface raher han he soil surface. However, we found ha DSM daa needed o be pre-processed before being applied in errain correcion. Various arefacs in he DSM daa, which can have a significan effec on slope and aspec, need be removed or reduced. In his sudy, he DSM daa were firs median filered o remove ouliers ha seem o occur in SRTM daa and have a serious effec on calculaions of slope and aspec. I was hen smoohed o a minimal level using a 3 by 3 Gaussian filer o remove digiizaion effecs. The digiisaion effecs are due o SRTM daa originally having 1 mere precision. I was hen resampled o Landsa resoluion using he bilinear mehod and finally Gaussian smoohed wih widh of 5 pixels o bring he scale o he level (50 meres correlaion lengh) judged bes in his case for he correcion. The appropriae scale for he correcion is hillslope scale raher han a he micro-relief scale and his was aken ino accoun. DSM daa were also resampled o mach he Landsa resoluion and coordinaes. To analyse he impac of DSM spaial resoluion, he 3-second global SRTM daa se (derived from aggregaion of he original 1-second SRTM daa) was also used. The pre-processing mehod for he 3 second daa was he same as for he 1-second daa excep for he final re-scaling by he Gaussian filer as he daa are already a a much coarser scale han he 1-second daa. Landsa 5 images were provided by he Naional Earh Observaion Group, Geoscience Ausralia. The Landsa images were orhocorreced wih a laiude and longiude (geographic) projecion as in Li e al. (2010b) where he BRDF and amospheric correcion algorihm is described. 4. RESULTS AND DISCUSSION Figure 1 shows he errain correced resuls for he wo sudy sies using he 1-second DSM daa. The images were processed using wo differen algorihms: (1) wih BRDF and amospheric correcion only and (2) wih BRDF and amospheric correcion plus opographic correcion. Comparing Figures 1a, b, c, and d, we can clearly see he errain effec a hillslope scale has been significanly reduced in Figures 1b and 1d when compared wih BRDF and amospheric correcion only (Figures 1a and 1c). The resoluion and qualiy of DSM daa is very imporan o ensure he accuracy of curren errain correcion. The opographic correcion error from DSM daa included hree differen ypes, i.e., error from he DSM arefacs, error from mismached pixel coordinaes beween DSM and Landsa images and error from wihin pixel DSM variaion. (a) (b) (c) Figure 2. The impac of DSM daa on he accuracy of errain correcion for he souh Blue Mounains image of Sep. 22, 2006 (a) False colour composie images (bands 4, 3, 2) wih BRDF and amospheric correcion only, (b) False colour composie images (bands 4, 3, 2, yellow colour is deeced as shadows) wih BRDF and amospheric correcion plus errain illuminaion correcion, and (c) DSM daa of he same area. In he processing, he DSM daa were smoohed o filer ou some arefacs (see Secion 3). However, his echnique could no remove all arefacs. A ypical example is in he Blue Mounains area where he SRTM 2405

5 radar ofen could no reach ino gorges due o he depression angle. This resuls in some areas being filled in by he radar so ha he valleys seem shallow and some areas no having validaed DSM daa. In such cases, oher sources (exising DSMs) were used o fill in he values as in he 1-second DSM produc. The consequence is ha some brigh areas in he original images were even brigher or masked ou as compued shadows. By conras, some dark areas were no masked as shadows or appeared even darker. Figure 2 shows an example of his. Some of he brigher and masked pixels (yellow colour) of Figure 2b are associaed wih arefacs in he DSM daa (Figure 2c). In his area, he DSM creaes arificial slopes and shadows while he real shadows are no masked. However, i is impossible o avoid his kind of error unil higher qualiy DSM daa ses become available in he fuure. Table 1. Comparison of errain correced surface reflecance beween accuraely co-regisered Landsa and DSM daa, and mis-regisered daa ses. Slope condiion Wih BRDF only Wih errain and accurae DSM Wih errain and shifed DSM Facing sun Away from sun Away from sun Facing sun Facing sun Mismach beween DSM and Landsa pixel coordinaes can cause significan error for he correcion, paricularly for pixels around he ridges and gully floors. If pixel coordinaes beween DSM and saellie images are no consisen hen someimes he DSM daa may show he pixel o be in he slope away from he sun, whereas, in Landsa, i may be locaed in a slope facing he sun wih relaively large a-sensor radiance. In his case, afer correcion, he correced reflecance will be very large. On he oher hand, if he DSM has he pixel in he slope facing he sun, bu in he Landsa, i is locaed in he slope away from he sun wih small a-sensor radiance, hen afer correcion, he correced reflecance will be smaller. To es his, we shifed wo DSM pixels in he wes direcion using he Sepember 22, 2006 image o see how he resuls changed. Table 1 shows some resuls from five seleced pixels which were boh facing and away from he sun. The resuls from Table 1 clearly show how pixel mismach affecs he accuracy of he produc. The reflecance facor can change from 0.30 in correcly mached DSM and Landsa daa o 0.50 in poorly mached coordinaes. In some areas, he difference will be larger han his. The resuls can also be seen clearly from he images (see Figure 3) where he well regisered image removed mos of he shadow, bu he poorly co-regisered image did no. (a) (b) Figure 3. The impac of co-regisraion beween DSM and Landsa images on he accuracy of errain correcion for he souh Blue Mounains sie (a) accurae co-regisered false colour composie images (bands 4, 3, 2, whie colour is deeced as shadow) (a) mis-regisered false colour composie images (bands 4, 3, 2, whie colour is deeced as shadow). 2406

6 For some pixels, alhough DSM coordinaes mach perfecly wih Landsa coordinaes, due o he wihin pixel DSM variaion, in some areas, eg. gully and ridge areas, he reflecance facors can no be fully correced. As a resul, shadows canno be removed compleely in some areas and surface reflecance facors are overcorreced in oher areas. In analysing his, we applied he opographic correcion algorihm using wo differen DSM daa ses based on he 1-second SRTM daa and he 3-second global SRTM daa. The same sub area was exraced from he Landsa images. Figure 4 shows he comparison. The figure clearly shows ha he 1-second DSM daa can remove more shadows (Figure 4a), whereas 3-second DSM removed fewer shadows (Figure 4b). I can be seen from Figure 4 ha he 3-second DSM daa leaves a residual exure of ligh and shade ha is a he hillslope scale. Therefore, he 3-second DSM is oo coarse o achieve he accuracy of he correcion han he 1-second DSM daa. However, more work is needed o es wheher he residual shadows from correcion using he 3-second DSM affecs land cover mapping and oher applicaions. A presen, he bes SRTM DSM daa (1-second) are around 30 m resoluion and Landsa daa are also a 30 m resoluion. The 1-second DSM daa are sufficien o correc Landsa daa which have similar spaial resoluion wih DSM for mos areas. However, for some areas where errain variaions are a a finer spaial resoluion, i is difficul o remove shadows using he 1-second DSM daa and Landsa daa. In some areas, he coarse resoluion of boh DSM and Landsa daa resul in a larger value for he calculaed slope angle, his could imply low or no radiaion from he sun. However, here is sill a large amoun solar radiaion inciden wihin he exen of he pixel on accoun of finer errain variaions wihin he slope. Therefore, overcorrecion can no be avoided in his siuaion. Wheher beer han 1-second DSM daa, or a modified algorihm, can improve he accuracy of correcion for he errain variaions a a finer resoluion and high resoluion saellie images needs furher invesigaion. (a) (b) Figure 4. The impac of DSM spaial resoluion on he accuracy of errain correcion for he souh Blue Mounains sie (a) using 1-second DSM daa (a) using 3-second DSM. 5. CONCLUSIONS A physics based coupled BRDF and amospheric correcion model can be applied o boh fla and inclined surfaces and has been shown o reduce mos of he BRDF, amospheric and opographic effecs a he scale of Landsa daa. However, he accuracy of he DSM based opographic correcion mehod depends on he qualiy of DSM daa, is co-regisraion wih Landsa daa as well is spaial resoluion. The presence of arefacs in DSM daa can cause significan error in he errain correcion process. On one hand, i resuls in false shadows and over correced surface reflecance facors. On he oher, i is unable o deec he shadow and rerieve reflecance in he slopes away from he sun. Forunaely, hese errors end o be localised and easy o idenify. The accuracy of co-regisraion beween saellie and DSM daa is imporan for he opographic correcion of saellie images. Mis-regisraion by one or wo pixels can lead o large errors in he gully and ridge areas. Therefore, accurae regisraion of saellie images and DSM daa are necessary o ensure he accuracy of he correcion. As SRTM regisraion may be near a half pixel compared wih GPS, his will probably require ha errain-correced producs be based on precision processed, orho-correced images raher han curren sandard precision images. 2407

7 The use of low resoluion DSM daa o correc high resoluion saellie images does no provide as high qualiy resuls as using he high resoluion DSM. However, furher invesigaion is needed o esablish how he residual shadows from using a lower resoluion DSM will affec landcover mapping or oher ime series applicaions of surface reflecance daa. Bu i is clear ha DSM daa a a scale appropriae for he saellie daa and he scale of hillslopes modifying he irradiance are necessary o correc high resoluion saellie images. ACKNOWLEDGEMENT The auhors acknowledge Geoscience Ausralia for he provision of Landsa images and DSM daa. MODIS BRDF model parameers were downloaded from he NASA MODIS websie. Sun phoomeer daa were from he NASA AERONET sie and radiosonde daa and precipiable waer were obained from NOAA. Advice provided by Dr Leo Lymburner and Mr Norman Mueller regarding he qualiy DSM daa and opographic correcion resuls is graefully acknowledged. Auhors would also like o hank Mr Lan-wei Wang and Mr Peer Tan for reviewing he paper. This paper has been published wih he permission of he CEO, Geoscience Ausralia. REFERENCES Dymond, J.R., Shepherd, J.D. and Qi, J., 2001, A simple physical model of vegeaion reflecance for sandardising opical saellie imagery. Remoe Sensing of Environmen, 77: Geoscience Ausralia and CSIRO Land & Waer (2010) 1 Second SRTM Derived Digial Elevaion Models User Guide. Version 1.0. Geoscience Ausralia. Kalnay, E., Kanamisu, M., Kisler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., Whie, G, Woollen, J, Zhu, Y., Chelliah, M, Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Leemaa, A., Reynolds, R., Jenne, R. and Joseph, D., 1996, The NCEP/NCAR 40-Year Reanalysis Projec. Bullein of he American Meeorological Sociey, 77: Liang, S., Quaniaive Remoe Sensing of Land Surface. Wiley-Inerscience, 534pp. Li, F., Wang, L. and Jupp, D. L. B., 2010a. A processing sysem for generaing sandard nadir BRDF adjused surface reflecance (NBAR) producs for Landsa. The 15h Ausralasian Remoe Sensing and Phoogrammery Conference, Alice Spring, Li, F., Jupp, D. L. B., Reddy, S., Lymburner, L., Mueller, N.,Tan, P. and Islam, A., 2010b, An evaluaion of he use of amospheric and BRDF correcion o sandardize Landsa daa. The IEEE Journal of Seleced Topics in Applied Earh Observaions and Remoe Sensing, 3: Richer, R., Correcion of amospheric and opographic effecs for high spaial resoluion saellie imagery', Inernaional Journal of Remoe Sensing, 18: 5, Schaaf, C., Gao, F., Srahler, A.H., Luch, W., Li, X., Tsang, T., Srugnell, N.C., Zhang, X., Jin, Y., Muller, J. P, Lewis, P., Barnsley, M., Hobson, P., Disney, M., Robers, G., Dunderdale, M., Doll, C., d Enremon, R.P., Hu, B., Liang, S., Privee, J.L. and Roy, D., 2002, Firs operaional BRDF, albedo and nadir reflecance producs from MODIS. Remoe Sensing of Environmen, 83: Shepherd, J.D. and Dymond, J.R., Correcing saellie imagery for he variance of reflecance and illuminaion wih opography. Inernaional Journal of Remoe Sensing, 24: Teille, P.M, Guindon, B. and Goodenough, D.G., On he slope-aspec correcion of mulispecral scanner daa. Canadian Journal of Remoe Sensing, 8: