Digital terrain model (DTM) generation is essential to recreating terrain morphology once the external elements are removed. Traditional survey methods are still used to collect accurate geographic data on the land surface. Given the emergence of unmanned aerial vehicles (UAVs) equipped with low-cost digital cameras and better photogrammetric methods for digital mapping, efficient approaches are necessary to allow rapid land surveys with high accuracy. This paper provides a review, complemented with the authors’ experience, regarding the UAV photogrammetric process and field survey parameters for DTM generation using popular commercial photogrammetric software to process images obtained with fixed-wing or multicopter UAVs. We analyzed the quality and accuracy of the DTMs based on four categories: (i) the UAV system (UAV platforms and camera); (ii) flight planning and image acquisition (flight altitude, image overlap, UAV speed, orientation of the flight line, camera configuration, and georeferencing); (iii) photogrammetric DTM generation (software, image alignment, dense point cloud generation, and ground filtering); (iv) geomorphology and land use/cover. For flat terrain, UAV photogrammetry provided a horizontal root mean square error (RMSE) between 1 to 3 × the ground sample distance (GSD) and a vertical RMSE between 1 to 4.5 × GSD, and, for complex topography, a horizontal RMSE between 1 to 7 × GSD and a vertical RMSE between 1.5 to 5 × GSD. Finally, we stress that UAV photogrammetry can provide DTMs with high accuracy when the photogrammetric process variables are optimized.

Water deficit is one of the major factors limiting the production of sugarcane (Saccharum officinarum L.). A study of the effects of limited water condition on yield components and their relationship with productivity can aid breeding programs in selecting for high yielding genotypes under this condition. The objective of this study was to investigate the relationships among the parameters stalk number, stalk height, stalk diameter, and stalk weight with cane yield in sugarcane growing in a field under moderate water stress during its grand growth period, in order to provide information to help breeders in adopting traits for selecting drought tolerant varieties. Seventy-eight genotypes plus two controls, one drought-tolerant and one drought-susceptible, were grown under a moderate water deficit condition in the field in 2005/2006 at Weslaco, TX. Productivity and yield components were measured. Under stress, the tolerant control (TCP93-4245) showed higher productivity, stalk number, stalk height and stalk weight than the susceptible one (TCP87-3388). However, the susceptible control showed higher stalk diameter. Linear association was found between productivity and its yield components, but stalk diameter showed to be fairly unstable among genotypes. Stalk height showed significant correlation with stalk number, stalk diameter and stalk weight. Stalk diameter also showed positive correlation with stalk weight. Therefore, during the selection procedure, when one of these traits is enhanced by drought tolerance, the correlated trait should also increase, making it feasible to select genotypes with high productivity, stalk number, stalk height, and stalk weight under water deficit.