# Comparison of SNAP-Derived Sentinel-2A L2A Product to ESA Product over Europe
## 2 Study Sites and Materials  
Three study sites, corresponding to existing ESA land products validation sites and available ESA-L2A products, were selected to span a range of terrain and land cover conditions (Figures 1 and 2). Barrax is a sparsely vegetated site located in Spain between 38.76\({}^{\circ}\)N and 39.75\({}^{\circ}\)N and 1.73\({}^{\circ}\)W and 3.00\({}^{\circ}\)W. It is predominantly flat with a mean elevation of 850 m above sea level (a.s.l.), although there is some rugged terrain in the northeast and southeast reaching 1185 m a.s.l. Harth is a forested site located in continental France between latitudes 46.83\({}^{\circ}\)N and 47.85\({}^{\circ}\)N and longitudes 6.32\({}^{\circ}\)E and 7.82\({}^{\circ}\)E. It has mountainous terrain, with elevations ranging from 245 m and 1670 m and a relatively uniform distribution of aspects. La Crau is a pasture and savannah site located in maritime France between latitudes 43.19\({}^{\circ}\)N and 44.22\({}^{\circ}\)N and longitudes 5.46\({}^{\circ}\)E and 6.87\({}^{\circ}\)E. It has mountainous terrain, with elevations ranging between 43 m and 2749 m, but aspects are predominantly oriented north and south facing.  
For each site, five clear sky Sentinel-2A L1C products and the corresponding ESA-L2A products (Processing Baseline 02.05) were selected at different periods between April 2017 and October 2017 (Table 1).  
Figure 1: Nominal L1C-TOA granules corresponding to study sites (boxes) and nearest AERONET station (pyramid). Solid lines indicate intercomparison granules (and also validation for La Crau). The dashed lines indicate validation granules.  
Figure 2: (**a**) Sample L1C-TOA products for intercomparison granules containing Barrax, La Crau and Harth shown as a linearly stretched false colour composite; and (**b**) the Digital Elevation Model for each tile.  
AERONET measurements were used to validate AOT and WVP. For La Crau, the Observatoire de Haute-Provence (OHP) station (5.71 degE, 43.93 degN, 680 m a.s.l.) was used. For Barrax and Harth, AERONET sites with clear sky Sentinel-2 data were only available in adjacent ESA-L2A product grid tiles (Aras de Los Almos at 1.10 degW, 39.94 degN, 1280 m a.s.l. for Barrax and Laegeren at 8.36 degE, 47.48 degN, 735 m a.s.l. for Harth) as indicated in Figure 1. For Barrax and Harth, AOT and WVP measurements were extracted from ESA-L2A or S2C-L2A products processed over adjacent reference grid tiles. For all AERONET sites, Level 1.5 cloud cleared spectral AOT and WVP from sun-photometer measurements were used as in situ reference values. Validation experiments indicate a total uncertainty (bias) for clear sky (air mass of one) AERONET measurements of \(+/-0.015\) (\(+/-0.01\)) for AOT at 550 nm [32] and \(+/-15\)% (6%) for WVP [33]. To math in situ measurements to satellite products, linear temporal interpolation was performed using AERONET measurements within a \(+/-15\) min interval of the nominal overpass time. Following Doxani [20], AOT at 550 nm was spectrally interpolated using the Angstrom exponent, and AERONET measurements were compared to a 9 km \(\times\) 9 km average of satellite product estimates centred on the AERONET site.