*2.5. Statistical Analysis*

The Orfeo Toolbox (OTB Development Team, 2018) [29] was used for the calculation of vegetation indices for pixels located nearest to the soil and plant sampling points in 2017 and 2018. The indices was subjected to statistical analysis and the mean, standard deviation, coefficient of variation (CV), minimum, maximum were determined (Tables 3 and 4). Relationships between VI values and grain yield as well as the number of spikes per square meter were evaluated using Pearson's correlation and a linear regression analysis. These analyses were conducted for the dates from mid-March to mid-July (Table 2). The choice of dates remained strictly dependent on the availability of cloudless Sentinel-2 satellite images. Depending on location, there were 4 to 6 dates (obtained, on average, every 3–4 weeks) in which the images were available without clouds.

**Table 3.** Grain yield (t ha−1) and number of spikes per m<sup>2</sup> m in sampling points at harvest in Bro ˙zówka (A), Zdziechów (B), and Kryłów (C) in 2017 and 2018.



**Table 4.** Mean values and ±SD of vegetation indices for the dates when the highest NDVI values were observed.

The correlation coe fficient values for each date are presented over time in a graphical form for better visualization of the relationship change between NDVI and grain yield (Tables 5 and 6). For periods when maximal values of the correlation coe fficients were observed, the diagrams of linear regression are presented (Figures 5 and 6), together with regression equations and coe fficients of determination (R2).

Spatial analyses were conducted, and maps were prepared using QGIS 2.18 software (QGIS Development Team, Gossau, Switzerland) [30], while the statistical analyses were performed using GNU R and TIBCO Statistica software.

**Table 5.** Correlation coe fficients (r) between NDVI values and grain yield (t·ha−1) for three locations in 2017 and 2018. The background color of the cells with the correlation coe fficient values indicates the strength of the relationship (significant correlations at α = 0.05 are given in bold).


\*—approximated growth stages of the whole field crop.


**Table 6.** Correlation coefficients (r) between NDVI values and number of spikes per square meter for three locations in 2017 and 2018. The background color of the cells with the correlation coefficient values indicates the strength of the relationship (significant correlations at α = 0.05 are given in bold).

> \*—approximated growth stages of the whole field crop.
