VALIDATION AND INTER-COMPARISON OF FOUR GLOBAL SURFACE ALBEDO PRODUCTS USING THE SALVAL TOOL. www.eolab.es Contact: Jorge Sánchez-Zapero. EOLAB CTO & Research Engineer (+34)963769448 [email protected] https://www.linkedin.com/in/jorgesanchezzapero/ EOLAB – Earth Observation Laboratory Parc Científic Universitat de València. C/ Catedrático Agustín Escardino, nº 9, 46980 Paterna, Valencia, Spain ABSTRACT METHODOLOGY J. Sánchez-Zapero (1) , F. Camacho (1) , L. de la Madrid (1) , Z. Wang (2) , M. O. Roman (3) (1) EOLAB, Valencia, Spain. ([email protected], [email protected], [email protected]) (2) University of Maryland College Park, College Park, MD, USA. ([email protected]) (3) NASA/GSFC, Greenbelt,MD, USA.([email protected] ) Product Plataform/ Sensor Period Spatial Resolution Temp. Frec. Comp. Period Projection Spectral albedos C_GLS V1 SPOT/VGT 1999-2014 1km 10 days 30 days Plate carrée ▪VIS [0.4-0.7μm] ▪NIR[0.7-4.0μm] ▪BB [0.3-4.0μm] PROBA/VGT 2014-Present MCD43A3 C6 TERRA+AQUA/ MODIS 2000-present 500m Daily 16 days Sinusoidal ▪VIS [0.3-0.7μm] ▪NIR[0.7-5.0μm] ▪BB [0.3-5.0μm] GLASS NOAA/AVHRR 1981-1999 ~5km 8 days 17 days Sinusoidal ▪BB [0.3-3.0μm] TERRA+AQUA/ MODIS 2000-2010 1km EBF DBF NLF Cro p Shru b Her b. BA LANDVAL 9 6 10 26 21 16 11 Global 7 8 16 17 24 10 14 0 5 10 15 20 25 30 Percentage NOA M SOA M EUR O AFRI ASIA OCE A LANDVAL 25 12 14 21 22 5 Global 19 11 9 24 31 6 0 5 10 15 20 25 30 35 Percentage Quality Criteria Validation Metrics Completeness Gap size distribution (annual maps, temporal variations per biome/continent). Length of gaps. Spatial Consistency Visual inspection global maps. Monthly maps and histograms of residuals. PDFs & histograms of residuals per biome and continent. Indicator of spatial homogeneity and correlation (CV, MoranIndex) Temporal Consistency Qualitative inspection of temporal variations Cross-correlation and auto-correlation. Intra-annual Precision Histograms of the smoothness Inter-annual Precision Box-plots per bin and median absolute anomaly of 95 th and 5 th percentiles for two consecutive years Stability Box-plots per bin and median absolute anomaly of 95 th and 5 th percentilescomparing each year to the long term average The slope of the evolution of inter-annual precision Systematic errors (Bias) Mean differences (bias) Major Axis Regression linear fit (slope, offset) Histogram of differences Box plots of differences per bin Total Error (Accuracy) Pearson’s coefficient (power of the linear fit) Root Mean Square Deviation (RMSD) Box-plots of absolute bias per bin Surface albedo is defined as the fraction of incident solar irradiance reflected by Earth's surface over the whole solar spectrum, and represents an Essential Climate Variable (ECV) as established by the Global Climate Observing System (GCOS). The uncertainty of satellite-based albedo products must be evaluated over global conditions with the aim to determinate the compliance of user requirements (GCOS) and the usability. Moreover, the availability of numerous products creates a need to understand the level of consistency between satellite products. This work presents the validation and inter-comparison of four global surface albedo products: SPOT/VGT V1 of the Copernicus Global Land Service, NASA MODIS MCD43 C6, Global LAnd Surface Satellites (GLASS), and PROBA-V V1 for the continuity of the Global Land Service. The results are generated using the Surface ALbedo VALidation (SALVAL) tool. SALVAL has two main objectives: (i) to provide transparency and traceability in the validation procedure, designed to be compliant with the CEOS-LPV (Comitee on Earth Observations Satellites - Land Product Validation) sub-group and QA4EO (Quality Assurance for Earth Observation) recommendations, and (ii) to provide a tool to benchmark new products or update product validation results as the time serie expands, reaching Validation Stage 4 in the CEOS LPV hierarchy. designed to represent globally the variability of land surface types over optimal conditions in terms of spatial homogeneity and topography Evaluated over 2001 -2004 period: SPOT/VGT V1, MODIS C6 & GLASS SPOT/VGT V1 vs PROBA-V V1 during the overlap period (Nov’13-May’14) for the continuity of the Global Land Service Product Completeness Conclussions Conclussions Evaluation of the sampling per biome type and continental region Parameter Thresho ld Purpose Distance to open water bodies [km] 5 Avoid open water bodies and their changing reflectance behavior with viewing geometry Minimum fraction of majority land cover type at 5 km distance 60% Avoid areas with heterogeneous land cover. Land Cover Majority at 5km Exclude 'Water bodies' and 'Urban areas' Vertical range [m] within a distance of 5km <300m Avoid areas with significant terrain variability close to a site. Location (Latitude) 60ºS to 80ºN Exclude sites over extreme latitudes, where Global Land products does not provide data Network # of sites SAVS 1.0 521 (including256 BELMANIP2.1) Calibration Sites 20 Additional sites (from existing networks) 26 Additional sites (Geo-Wiki) 158 Total 725 Criteria of selection QA Criteria Perfor mance Comments Product Completeness - Main limitations over Northern latitudes in wintertime and Equatorial areas. Similar results than SPOT/VGT (and MODIS) Spatial Consistency + Global distributions showed systematic positive bias (PROBA-V > SPOT/VGT) for NIR and BB, and bias 0 for VI. Global distributions of residuals showed ~36% of cases within the optimal level for VI, and 50% for NI and BB Temporal Consistency + Reliable temporal variations for most of the cases compared with satellite reference products and ground observations. Intra-Annual Precision + Similar smoothness than both references (SPOT/VGT and MODIS C5), showing slightly higher δ values in NI. Overall Spatio- Temporal Consistency ± PROBA-V vs SPOT/VGT shows high correlation (R>0.93) and low scattering, with almost no mean bias in VI and systematic positive mean bias of ~5% in NI and BB (except in snow). 39%, 43% and 42% (67%, 73%, and 75%) of pixels showed optimal (target level) for VI, NI and BB. Comparison of PROBA-V and SPOT/VGT per biome type showed low bias (<3%, random sign) for VI, and positive bias for NI and BB in all biome types. The exception was the snow class, with negative bias. Accuracy Assessment - PROBA-V:N=274; B=0.032 (22.1%); RMSD=0.042; Snow free conditions. 4% of pixels within GCOS. Improved results for MODIS C5 using the same sampling: B=0.006 (4.9%); RMSD= 0.029; 18.1% of pixels within GCOS. Product Completeness Spatial Consistency Temporal Consistency Temporal Realism Accuracy Assessment Intra-Annual Precision Spatio-Temporal Consistency Spatio-Temporal Consistency SPOT/VGT V1 vs MODIS C5 SPOT/VGT V1 vs GLASS GLASS vs MODIS C5 Spatial Consistency Intra-Annual Precision Inter-Annual Precision Temporal Consistency QA Criteria SPOT/ VGT MODIS C6 GLASS Comments Product Completeness - - + SPOT/VGT & MODIS showed main limitations over Northern latitudes in wintertime and Equatorial areas. GLASS is gap-filled product Temporal Consistency ± + + Good temporal agreement between the three satellite products. SPOT/VGT : Some limitation observed to detect spurious snowfall events, and slight seasonality over desertic calibration sites. Intra-Annual Precision + + + Similar smoothness all products Inter-Annual Precision ± + + 2004 vs 2003 SPOT/VGT : RMSD 2,6% over calib. Sites. Median abs. Anomaly: 3% MODIS C6: RMSD 2,6% over calib. Sites. Median abs. Anomaly: 2,3% GLASS: RMSD 2,6% over calib. Sites. Median abs. Anomaly: 2,2% QA Criteria SPOT/VGT V1 Vs MODIS C6 SPOT/VGT V1 Vs GLASS GLASS Vs MODIS C6 Comments Spatial Consistency + ± + Residuals: VGT vs MOD: 15% GCOS / 75% Target VGT vs GLASS: 14% GCOS / 53% Target GLASS vs MOD: 39% GCOS / 82% Target Overall Spatio- Temporal Consistency + + + VGT vs MOD VGT vs GLASS GLASS vs MOD R 0,95 0,94 0,97 Bias 1,9% -0,4% 1,7% RMSD 0,04 (19%) 0,05 (22%) 0,03 (16%) %GCOS/Target 40/69 42/71 54/77