ASSESSMENT OF DEM ACCURACY GENERATED FROM ALOS PRISM HIGH RESOLUTION STEREO-OPTICAL IMAGERY USING LPS

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1 ASSESSMENT OF DEM ACCURACY GENERATED FROM ALOS PRISM HIGH RESOLUTION STEREO-OPTICAL IMAGERY USING LPS S.P. Chamida 1, Xiaoyog Che 2, Seishiro Kibe 3, Lal Samarakoo 4 1 Mister, Departmet of Earth Resources Egieerig, Uiversity of Moratuwa, Moratuw, Sri Laka: ; Fax: chamidaspc@yahoo.com 2 Doctor, 3 Professor, 4 Doctor, Remote Sesig ad GIS divisio, School of Egieerig ad Techology, Asia Istitute of Techology, Pathumthai, Thailad; Tel: ; Fax: , 2 xyche@ait.ac.th, 3 kibe.seishiro@jaxa.jp, 4 lal@ait.ac.th KEY WORDS: DEMs, ALOS PRISM, accuracy assessmet, LPS, error map. ABSTRACT: Low resolutio ad high icosistecy i digital topographic data are a crucial problem of their practical usage i various egieerig ad scietific applicatios. A ew Japaese Pachromatic Remote sesig Istrumet for Stereo Mappig (PRISM) which is a sesor that is carried out o Advaced Lad Observig Satellite (ALOS) could be cosidered as oe of the solutios to overcome the topographical data quality problem. This research was aimed to assess the accuracy of ALOS geerated DEM data ad to evaluate the practical usage of the Leica Photogrammetric Suit (LPS) software package as a tool for geeratig ALOS based DEM. The Study was carried out i a selected test site i Sri Laka. Geerated DEMs were compared for accuracy usig differet Check Poits (CPs) were observed i the field i three distributio patters; liear, curved ad arbitrary ad they were further categorized accordig to terrai distributio of the study area. Error maps were geerated to preset a spatial correlatio of the error distributio over the area. The lowest RMSE ad U.S. Natioal Imagery ad Mappig Agecy (NIMA) Absolute Liear Error (LE)90 of the height for the whole study area was 2-3 pixels depedig primarily o surface roughess, vegetatio, image texture ad image quality. The RMSE ad NIMA Absolute LE90 values of height for the specific areas raged from 2 pixels (lowlads) to 4 pixels i a tree-covered moutaious area. This study reveals that the accuracy of the geerated DEMs varies sigificatly with a? image pair( or pairs) used i DEM geeratio, terrai type, umber of Groud Cotrol Poits(GCPs) ad CPs distributio patter. INTRODUCTION Digital elevatio models (DEMs), which are digital represetatios of the earth's relief are oe of the most sigificat data structures used for geoscietific ad geomatic aalysis. It is ecessary to icorporate GIS data base ito elevatio data i the forms of DEM for the productio of geocoded, ortho-rectified raster images (Hohle,1996). However, DEMs with usable details are still ot available for much of the earth, ad whe available, they do ot always have sufficiet accuracy. To fill this gap, the Pachromatic Remote sesig Istrumet for Stereo Mappig (PRISM) sesor that was carried o the Advaced Lad Observig Satellite (ALOS) icludes creatio of digital elevatio maps as oe of its purposes. PRISM was lauched i Jauary 24, 2006 ad it s belogs to the class of push broom sesor which acquire data by a liear CCDs

2 array. It cotais 6 CCDs for viewig at the Nadir, ad 4 CCDs for viewig i the forward ad backward directios. A image file is provided for each CCD whe dealig with 1A ad 1B1 level imagery (ucorrected imagery) Presetly, algorithms ad software for creatig a DEM product from the stadard product of PRISM have bee developed. Oe group has developed the software package SAT-PP (Satellite Image Precisio Processig) for the precisio processig of high-resolutio satellite image data (Grue et al., 2005). The mai module of the package is the automated image matcher for the geeratio of DSMs (Zhag, 2005). They used SAT-PP software package to geerate DEMs form ALOS PRISM stereo images for the four mai test areas ad assessed their accuracy. The RMS error of the elevatio was betwee 5.5 m to 6.6 m, which meas better tha three pixels. Further, GEOIMAGE recetly coducted a test DEM geeratio from a PRISM triplet over Sydey to assess the accuracy of the DEM ad showed a mea differece of 3.7m ad a RMS error of 5m. This research was aimed to assess the accuracy of ALOS geerated DEM data usig differet Check Poits (CPs) observed i the field i three distributio patters; liear, curved ad arbitrary ad to evaluate the practical usage of Leica Photogrammetric Suit (LPS) software package as a tool for geeratig ALOS based DEM. METHODOLOGY Data used ALOS PRISM Backward, Nadir, ad Forward stereo images (Triplet - Level 1B1), these are the image data that performed radiometric correctio to Level 1A data ad added the absolute calibratio coefficiet. Acillary iformatio such as radiometric iformatio, observatio modes ad scee sizes ad ceters required for processig superior to Level 1B2 is added. A stadard scee i triplet mode sizes 35 km width for a pixel groud resolutio is aroud 2.5 m. Five meter cotours ad sub-meter level accuracy mass poit data which was extracted from photogrammetric images (1:10,000) of the study area were obtaied from the Survey Departmet of Sri Laka ad were used to geerate referece DEM. GPS settigs ad surveyig High accurate Novatel GPS receiver ad Magella explorist 600 GPS uits were used to collect GCPs ad CPs. Novatel readigs were post-processed by Geodetic Survey Divisio, Caada Cetre for Remote Sesig, Natural Resources Caada. Magella explorist 600 GPS readig were post-processed with Differetial Global Positio System (DGPS) techique. Positio data were collected at referece ad rover positios at the same time ad referece poit error was applied to rover readigs for differetial correctios. All GPS readigs were filtered with the referece DEM ad aomalous readigs were removed. The CPs were collected i lowlad ad uplad at arbitrary poits, alog lies (roads ad profiles i moutais) ad i areas (alog a curved lies) to compare the accuracy of the DEM related to various CPs distributio patters.

3 Dem geeratio ALOS PRISM Level 1BI stereo - optical image frames, preprocessed with the radiometric coefficiets used to mosaic the scee area ad subset the study area from each image (Backward, Nadir ad Forward). The absolute DEMs were geerated from stereo image pairs (Backward- Nadir, Nadir-Forward ad Forward-Backward) with differet umber of Groud Cotrol Poits (GCPs) usig the Leica Photogrammetric Suit (LPS), which is developed by Leica Geosystems (Figure 1). The Automatic Terrai Extractio (ATE) i LPS facilitated fast DEMs geeratio for all terrai areas. Accuracy assessmet process Figure 1: Schematic flow chart of the Research Methodology. The study area was subdivided ito low-lad : up-lad- based o homogeeous topography ad lad use characteristics. Whe, DEMs were extracted with differet umber of GCPs arragemet ad image pairs, its accuracy was also computed by ATE with referece to each CPs distributios patter. The accuracy of the DEMs was also computed for GCPs arragemets uder oe CPs distributio for specific terrai with oe image pair. The elevatio error was determied by calculatios of the vertical differeces betwee geerated DEMs ad CPs. Max error, Mi error, Mea error, Absolute Mea error, Root Mea Square

4 Error (RMSE), Liear Error (LE) 90 ad NIMA Liear Error (LE) 90 were comprised i the error report which made by ATE with referece to give CPs for each extracted DEM. Max error represeted positive maximum elevatio differeces betwee extracted DEM ad CPs, while mi error represeted maximum egative elevatio differeces. As a quality degree RMSE of height was determied for all poits (CPs) by usig Equatio 1. 2 ( Z GEN, i Z REF, i ) Where, i= 1 RMSEZ = Z GEN,i = elevatio of poit i i the geerated DEM Z REF,i = referece elevatio of poit i (Check Poit s elevatio) = total umber of 3D referece poits used Equatio 1 I additio, the NIMA LE90 statistics was computed ad it was based o the assumptio that a ormal distributio exists with the set of observatios. I this case, the set of observatios is the DEM errors computed usig CPs. The Equatio 2 was used to calculate NIMA LE 90 ad the value of represets a 90 percet cofidece level derived from statistical tables. Stadard deviatio of the error was calculated by usig Equatio 3. NIMA LE90 = ± σ Equatio 2 σ = i= 1 ( e i e ) Where, σ = stadard deviatio of the error e i = absolute error of referece poit i e = mea absolute error for the etire set of referece poits = total umber of 3D referece poits used 2 Equatio 3 Error maps were geerated by computig elevatio differeces betwee extracted DEM ad referece DEM. These error maps were used to calculate percetage of the error statistics (Figure 4). RESULTS AND DISCUSSIONS Elevatio, slope ad vegetatio cover of a terrai have a differet degree of complexity. Objects o the terrai ad the terrai itself i urba or suburba areas have a quite differet appearace from those i forest ad ope rural areas. The Error statistics, The RMSE ad NIMA Absolute LE90 values vary alog with the umber of GCPs uder differet CPs distributio patters for lowlad ad moutai areas (Figures 2&3). The moutai areas give more errors as terrai complexity ad thick forest. Accuracy of the DEMs uder Liear CPs distributio i lowlad areas give higher deviatios with the umber of GCPs but i a moutaious area gave lower deviatio. Hece it reveals that accuracy assessmet results of the DEMs maily deped o the CPs distributio ad terrai type.

5 (a) Lowlad (b) Moutais Figure 2: Variatio of RMSE of elevatios with umber of Groud Cotrol Poits (GCPs) for extracted DEMs from Backward Nadir image pairs with lie, curved ad arbitrary Check Poits (CPs) distributio. (a) Lowlads. (b) Moutai (a) Lowlad (b) Moutais Figure 3: Variatio of NIMA Absolute LE90 of elevatios with umber of GCPs for extracted DEMs from Backward Nadir image pairs uder lie, curved ad arbitrary Check Poits (CPs) distributio for differet terrai. (a) Lowlad. (b) Moutai Computed elevatio maps for best PRISM DEMs geerated with differet image pairs show spatial distributio of the errors i terrais (Figure 4). Leged Error map of Backward - Nadir images pair Elevatios (m) Error map of Forward - Nadir images pair Leged 4 Elevatio errors(m) Kilometers Kilometers Elevatio error 5m> 5m 10m 10m< No of pixel Percetage of error 78.28% 15.07% 6.65% (a) Lowlad Elevatio error 5m> 5m 10m 10m< No of pixel Percetage of error 72.83% 25.27% 1.9% (b) Moutais Figure 4: Error maps of highest accurate DEMs based o liear CPs accuracy assessmet (a) Lowlad (b) Moutaious

6 Table 1: Summary of the lowest error statistics of DEMs uder differet terrai types ad CPs distributio patter Image pairs Forward - Backward Backward - Nadir Nadir - Forward Check poits selectio RMSE NIMA 5m> 5m - 10m< (m) LE90(m) (%) 10m (%) (%) lowlad - lie moutaious - lie Whole area-arbitrary lowlad - lie moutaious - lie whole area -arbitrary lowlad - lie moutaious - lie Whole area-arbitrary Accordig to Wolff (2007) the height RMSE values computed for DEMs, which were extracted usig SAT-PP software from ALOS PRISM images, raged from 4.7 m (ope areas) to 12.8 m (trees) without ay post-processig for bluder removal. I this study it raged from 2.70m (lowlad) to 7.97m (moutai with tree covers) with best DEMs extracted by LPS uder differet GCPs setup. Average values for the full sub-areas spaed from 5.5 to 6.6 m (Wolff, 2007) where as this study gave 6.77m for DEMs geerated from backward-adir image pair, CONCLUSIONS I this study, DEMs were geerated from ALOS PRISM stereo images for a selected study area by LPS software usig differet umber of GCPs (8 to 35). The error statistics (RMSE ad NIMA Absolute LE90) of height o geerated DEMs were computed based o differet parameters icludig Check Poits (CPs) distributio patter, terrai type ad image pair used i DEM geeratio. The lowest RMSE ad NIMA Absolute LE90 values of height i lowlad area raged from 2.7m to 5.9m ad from 2.5m to 7.2m respectively ad i moutaious area that raged from 7.97m to 11.7m ad from 8.6m to 14.8m respectively with all image pairs uder every CPs distributio patters; liear, curved ad arbitrary were cosidered i this study. The lowest RMSE ad NIMA Absolute LE90 values for the whole study area are 6.8m ad 8.9m respectively. Therefore, it ca be geeralized that ALOS data ca be used to create a DEM with a accuracy of 2 to 4 pixel. I additio, this study reveals that the backward-adir image combiatio i the DEM geeratio offers the highest accuracy tha the other combiatios for the study area. REFERENCES Grue A., Wolff K. (2007), DSM Geeratio with ALOS/PRISM Data Usig SAT-PP. IEEE Iteratioal Geosciece ad Remote Sesig Symposium (IGARSS) 2007, July HOHLE J., 1996, Experiece with the productio of digital orthophotos. Photogrammetric Egieerig ad Remote Sesig, 62, pp Haover Workshop 2007, "HighResolutio Earth Imagig for Geospatial Iformatio", Haover, Germay, May 29- Jue 1, Zhag L., Automatic Digital Surface Model (DSM) Geeratio from Liear Array Images. PhD Dissertatio, Mitteiluge Nr 88, Istitute of Geodesy ad Photogrammetry, ETH Zurich.