Abstract
The accurate assessment of tree crowns is important for agriculture, for example, to adjust spraying rates, to adjust irrigation rates or even to estimate biomass. Among the available methodologies, there are the traditional methods that estimate with a three-dimensional approximation figure, the HDS (High Definition Survey), or TLS (Terrestrial Laser Scanning) based on LiDAR technology, the aerial photogrammetry that has re-emerged with unmanned aerial vehicles (UAVs), as they are considered low cost. There are situations where either the cost or location does not allow for modern methods and prices such as HDS or the use of UAVs. This study proposes, as an alternative methodology, the evaluation of images extracted from Google Maps (GM) for the calculation of tree crown volume. For this purpose, measurements were taken on orange trees in the south of Spain using the four methods mentioned above to evaluate the suitability, accuracy, and limitations of GM. Using the HDS method as a reference, the photogrammetric method with UAV images has shown an average error of 10%, GM has obtained approximately 50%, while the traditional methods, in our case considering ellipsoids, have obtained 100% error. Therefore, the results with GM are encouraging and open new perspectives for the estimation of tree crown volumes at low cost compared to HDS, and without geographical flight restrictions like those of UAVs.
Highlights
Citrus cultivation is worldwide widespread [1]
The results with Google Maps (GM) are encouraging and open new perspectives for the estimation of tree crown volumes at low cost compared to High Definition Survey (HDS), and without geographical flight restrictions like those of unmanned aerial vehicles (UAVs)
The traditional method is based on assuming that the tree crown is a regular geometric figure like a spheroid [58], in our case we will adopt an ellipsoid that has been used in orange trees [59]
Summary
Citrus cultivation is worldwide widespread [1]. Among citrus fruits, the cultivation of the orange tree (Citrus sinensis (L.) Osbeck) is considered the most important from a commercial point of view and the Mediterranean climate is especially suitable for its cultivation [2]. All crops are challenged by multiple biotic and abiotic stresses [3], and citrus is no exception [4]. Whitney [5] reported that for orange production, soluble solids and net yields were associated with tree canopy volume. Among the factors used in adjusting or modelling ET in crops are the leaf area index or tree volumes [9]. When there are different types of crops or sizes of trees, optimization techniques can be used to make more efficient use of water [10]. As an example, accounting for the variability of tree size within a row in irrigation allows the adjustment of discharge rates to individual tree size, and this could result in significant water savings [11]. Reducing irrigation reduces groundwater contamination by e.g., nitrate leaching [12]
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