The classification accuracy was assessed by comparing the results with those obtained from ground data acquired as described in the previous section. The validation of the prescription maps was performed by using the data from all the 60 rectangular plots from the four fields as ground truth, calculating a confusion matrix (Congalton 1991). The latter, also known as error matrix, is obtained by placing in the rows the number of pixels of each class from ground truth data and in the columns the corresponding results from the UAV image classification. It allows to calculate the overall accuracy of the classification (% of the pixels correctly classified) as well as producer and user accuracy. The producer accuracy indicates the probability that the classifier has labeled an image pixel into the weed class given that the ground truth is actually weeds. Conversely, the user accuracy indicates the probability that a pixel is actually weeds given that the classifier has labeled the pixel as weeds. A low producer accuracy would imply a high omission error, i.e. a high proportion of true weedy pixels not identified in the detection method and thus contributes to underestimation of the weed patches. A low user accuracy corresponds to a high commission error, a high proportion of pixels misclassified as weedy, i.e. the classification overestimates the weed patches providing a conservative estimation. As remarked by Lo ´pez-Granados (2011), from the farmers’ perspective it is desirable have lower omission errors (high producer accuracy) and higher commission errors (low user accuracy), so that the weed patches are less likely to be missed. All the validation data were bulked to have a more representative sample.
The classification accuracy was assessed by comparing the results with those obtained from ground data acquired as described in the previous section. The validation of the prescription maps was performed by using the data from all the 60 rectangular plots from the four fields as ground truth, calculating a confusion matrix (Congalton 1991). The latter, also known as error matrix, is obtained by placing in the rows the number of pixels of each class from ground truth data and in the columns the corresponding results from the UAV image classification. It allows to calculate the overall accuracy of the classification (% of the pixels correctly classified) as well as producer and user accuracy. The producer accuracy indicates the probability that the classifier has labeled an image pixel into the weed class given that the ground truth is actually weeds. Conversely, the user accuracy indicates the probability that a pixel is actually weeds given that the classifier has labeled the pixel as weeds. A low producer accuracy would imply a high omission error, i.e. a high proportion of true weedy pixels not identified in the detection method and thus contributes to underestimation of the weed patches. A low user accuracy corresponds to a high commission error, a high proportion of pixels misclassified as weedy, i.e. the classification overestimates the weed patches providing a conservative estimation. As remarked by Lo ´pez-Granados (2011), from the farmers’ perspective it is desirable have lower omission errors (high producer accuracy) and higher commission errors (low user accuracy), so that the weed patches are less likely to be missed. All the validation data were bulked to have a more representative sample.
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