# A New Methodology for Estimating Surface Albedo in Heterogeneous Areas from Satellite Imagery
## 6 Conclusions  
The need to measure surface albedo at a global scale in different land cover types, with an improved spatial resolution regarding current existing satellite surface albedo products, is the main motivation for the methodology developed in this work. The combined methodologies using Landsat 8 or Landsat 9 imagery (resolution 30 m) were designed after the analysis and comparison of four existing methodologies regarding ABOVE surface albedo imagery (resolution 500 m) and punctual surface albedo values measured in SURFRAD stations. The analysis based on ABOVE imagery shows that the surface albedo obtained with methodology V01 in pixels with intermediate NDVI values between 0.20 and 0.50 (low vegetation) are the best values, although the V02 methodology generates close results. Methodology V03 generates the best results of surface albedo for pixels with NDVI values below 0.20 (urban areas). The three methodologies had a good performance in surface albedo computation for pixels with NDVI values over 0.5 (high vegetation), although methodology V03 performs slightly better. The evaluation considering SURFRAD punctual values shows that the most accurate methodology is V02, regardless of the NDVI value of the pixel. Provided that all SURFRAD stations are located in areas with low vegetation, these results are coherent with the results obtained after the analysis with ABOVE imagery. However, this result reinforces the application of the V02 methodology in pixels with intermediate NDVI values between 0.20 and 0.50. Figure 5: Visual evaluation of the surface albedo difference of the city of Champaign on 12 September 2016, between ABOVE and the different methodologies: (**a**) V02, (**b**) V03, and (**c**) Comb3. Graphical scale and colour palette corresponding to the difference between surface albedo ABOVE from Figure 4. North, coordinates, and scale are shown in the images from OpenStreetMap at the (**left**). In this way, three different combined methodologies were developed, consisting of different combinations of the existing methodologies V01, V02, and V03 applied to different pixel NDVI values. A final comparison with ABOVE images shows that the combined methodology 3 (Comb3) is the optimal combined methodology. Combined methodology 3 applies the V02 methodology to pixels with NDVI values over 0.20 and the V03 methodology for the other, and results in an RMSE constant for most types of terrestrial coverage, which is improved regarding the individual methodologies: combined methodology 3 is 85% better than methodology V02 and 52% better than the V03 methodology, when applied to heterogeneous areas (mixing urban and vegetation land uses) and considering the similarity to the ABOVE product as an improvement. A visual evaluation also shows the best results for combined methodology 3. Future research lines will deal with the combination of surface albedo and Land Surface Temperature satellite data towards the determination of the reflected and direct components of the incoming solar radiation, with application to UHI and solar energy quantification [9; 64]. To improve these future lines, it is worth highlighting the spatial-temporal resolution limitations of the proposed methodology, which could be solved and complemented with additional data provided with drones if the study required it. Furthermore, to address the spatial resolution constraints, exploration into alternative satellite missions becomes imperative. Integration of datasets from high-performance satellite platforms like Sentinel-2, boasting a resolution of 10 m for NDVI, or even Very High Resolution (VHR) platforms such as Planet with an approximate 3-m GSD for NDVI, could significantly enhance the BRDF estimation, subsequently refining the accuracy of surface albedo determination. In addition, the evaluation of the methodology and its refinement could benefit from the determination of RMSE for areas with very low NDVI values (below 0.2). This action will imply the establishment of pyranometers in complementary locations to the official networks.