# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 3 Results
### Concordance among PlanetScope, Near-Surface and Visual Phenology  
For the PhenoCam sites that had PlanetScope observations, we further evaluated the degree to which the transition date metrics from the PlanetScope NDVI time series aligned with the visually assessed phenological events from corresponding PhenoCam photos. With the 31 PlanetScope site-year observations, comparable characteristic patterns as those in PhenoCam were found among the transition date metrics, though the PlanetScope-based measures tended to be positively biased with delayed estimations of visually observed phenological events. As for crop emergence, the CUR-based Greenup and Gu-based UD estimates showed better alignment with visual assessments compared to TRS- and DER-based SOS measures (Figure 8). The RMSEs and biases ranged from 1-2 weeks (for Greenup and UD metrics) to 3-4 weeks (for SOS metrics). In particular, the CUR-based Greenup achieved the lowest RMSE (8.348 days) with most of the estimates falling within the one-week interval of emergence observations (bias = 5.276 days) and explained about 65% of the variation in the observed ones. We further conducted the analysis for the corn and soybean sites, respectively. For both species, the CUR-based Greenup estimates attained better agreements with the visual ones (RMSE = 8.624 days and bias = 5.105 days for corn; RMSE = 7.797 days and bias = 5.6 days for soybean) among all the derived transition date metrics. With regard to crop maturity, the TRS- and DER-based EOS estimates from the PlanetScope time series had about one week difference upon comparison to visual ones in terms of both RMSE and bias (Figure 9). The CUR-based Dormancy and Gu-based RD measures, on the other hand, were positively biased with more than four weeks delayed (bias = 34.208 days for Dormancy and RMSE = 29.701 days for RD). The EOS estimates, with an R square of about 0.58, explained more variance in the maturity observations compared to Dormancy and RD ones (R square of about 0.25). Similar metric patterns were also observed in the separate analysis of the corn and soybean sites, with the crop maturity more approximated by the EOS measures. For corn, the DER-based EOS measure achieved the lowest RMSE (8.327 days) and bias (7.6 days) and explained about 73% of the variance in the corn maturity observations across sites and years. As regards soybean, the RMSE and bias of the DER-based EOS were also the lowest (RMSE = 11.908 days and bias = 9 days) and its R square value was 0.465. The comparisons conducted for PlanetScope, PhenoCam, and visual phenology indicated the divergent correspondence between transition date metrics and visually observed phenological events. For both PhenoCam and PlanetScope, the CUR-based Greenup exhibited a strong relationship with the photo-based crop emergence observations (Tables 2 and 3). The DER-based EOS reconciled well with the crop maturity observations. The comparable correspondence patterns between PhenoCam and PlanetScope suggested the potential of connecting fine-scale sensor-derived phenological measures with crop phenological events of biological implications. The CUR-based Greenup and DER-based EOS measures, with their respective good performance, were utilized for the subsequent analysis of the coupled ground and sensor-based crop phenological characterizations. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{3}{c}{**Crop Emergence**} & \multicolumn{5}{c}{**Crop Maturity**} \\ \hline
**Phenological** & **RMSE (Days)** & **Bias (Days)** & **R2** & **Phenological** & **RMSE (Days)** & **Bias (Days)** & **R2** \\
**Metric** & & & & & & \\ \hline TRS-SOS & 26.70 & 25.93 & 0.72 & TRS-EOS & 10.56 & 8.96 & 0.58 \\ DER-SOS & 27.96 & 27.20 & 0.74 & DER-EOS & 9.92 & 8.16 & 0.58 \\ CUR-Greenup & 8.35 & 5.28 & 0.64 & CUR-Dormancy & 36.04 & 34.21 & 0.26 \\ Gu-UD & 11.79 & 9.76 & 0.62 & Gu-RD & 31.53 & 29.70 & 0.22 \\ \hline \hline \end{tabular}
\end{table}