# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 3 Results
### Concordance among PlanetScope, Near-Surface and Visual Phenology

Table 3: The accuracy statistics of comparisons between the PlanetScope NDVI-based transition date estimates and corresponding visual interpretations using all the PlanetScope site-year observations. Figure 8: The comparisons between the PlanetScope NDVI-based transition date estimates of the greenness rising period and the visually observed emergence for **(a)** both corn and soybean, **(b)** corn, and **(c)** soybean, using the four phenophase analysis methods. To further assess the degree to which the PhenoCCam-derived metrics aligned with the PlanetScope-based ones, we retrieved the CUR-based Greenup and DER-based EOS measures from both PhenoCam and PlanetScope using all the shared site-year observations (Figure 10). For the CUR-based Greenup, most of the PhenoCam-PlanetScope pairs were distributed along the one-to-one line with the corn in red color and the soybean in green color in Figure 10. About 78.5% of the variance in the PlanetScope-derived Greenup measures was explained by the PhenoCam-based ones. The average measuring difference was 7.137 days using RMSE and 4.63 days using bias. With regard to the DER-based EOS, the PhenoCam-PlanetScope pairs were also aligned along the one-to-one line, with the corresponding R square being 0.657, bias being 6.56 days, and RMSE being 8.395 days. For both measures, the high degree of concordance between PlanetScope and PhenoCam demonstrated the linkage between near-surface and high-resolution satellite-based phenology and indicated the coherence of those fine-scale sensors in retrieving crop phenological characteristics. The RMSEs of about one week between PhenoCam- and PlanetScope-based measures were possibly attributed to the difference in the sensor field of view, viewing angles, and sensor configurations, as well as the difference in crop seasonal dynamics captured by the different vegetation indices. Figure 9: The comparisons between the PlanetScope NDVI-based transition date estimates of the greenness falling period and the visually observed maturity for (**a**) both corn and soybean, (**b**) corn, and (**c**) soybean, using the four phenophase analysis methods. Finally, we evaluated the concordance among PlanetScope, PhenoCam, and visual phenology with all the shared site-year observations using the CUR-based Greenup and DER-based EOS (Figure 11 and Table 4). Both PhenoCam- and PlanetScope-derived metrics showed a good relationship with crop visual phenological observations. By comparison with visual crop emergence, PhenoCam-derived CUR Greenup estimates were within one week difference (RMSE = 6.43 days), almost no bias, and attained the R square of 0.73. The PlanetScope-based Greenup estimates showed a slightly positive bias (5.28 days), partly due to the use of PhenoCam imagery for visual phenological assessments. Similarly strong correspondence with visual crop maturity was found for both PhenoCam- and PlanetScope-based DER EOS (RMSE = 5.31 days and R square = 0.70 for PhenoCam; RMSE = 9.92 days and R square = 0.58 for PlanetScope), with PlanetScope estimates slightly positive biased (bias = 8.16 days). The congruence among PhenoCam, PlanetScope, and visual crop phenology demonstrated the potential of employing the PhenoCam GCC time series from the RGB imagery to identify critical crop phenological stages, as well as leveraging high-resolution satellite time series for crop phenological detection. The direct comparisons between PlanetScope and visual phenology, along with those between PhenoCam and visual phenology, indicated the linkage of these fine-scale sensors in terms of scale and representation for the coupled ground and sensor-based crop phenological characterizations.