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The quality and accuracy of Unmanned Aerial System (UAS) products greatly depend on the methods used to define image orientations before they are used to create 3D point clouds. While most studies were conducted in non‐ or partially‐forested areas, a limited number of studies have evaluated the spatial accuracy of UAS products derived by using different image block orientation methods in forested areas. In this study, three image orientation methods were used and compared: (a) the Indirect Sensor Orientation (InSO) method with five irregularly distributed Ground Control Points (GCPs); (b) the Global Navigation Satellite System supported Sensor Orientation (GNSS‐SO) method  using  non‐Post‐Processed  Kinematic  (PPK)  single‐frequency  carrier‐phase  GNSS  data (GNSS‐SO1); and (c) using PPK dual‐frequency carrier‐phase GNSS data (GNSS‐SO2). The effect of the three methods on the accuracy of plot‐level estimates of Lorey’s mean height (HL) was tested over the mixed, even‐aged pedunculate oak forests of Pokupsko basin located in Central Croatia, and validated using field validation across independent sample plots (HV), and leave‐one‐out cross‐ validation  (LOOCV). The GNSS‐SO2 method produced  the HL estimates of  the highest accuracy (RMSE%: HV = 5.18%, LOOCV = 4.06%), followed by the GNSS‐SO1 method (RMSE%: HV = 5.34%, LOOCV = 4.37%), while  the  lowest accuracy was achieved by  the  InSO method  (RMSE%: HV = 5.55%, LOOCV = 4.84%). The negligible differences in the performances of the regression models suggested that the selected image orientation methods had no considerable effect on the estimation of HL. The GCPs, as well as the high image overlaps, contributed considerably to the block stability and  accuracy  of  image  orientation  in  the  InSO method. Additional  slight  improvements were achieved  by  replacing  single‐frequency  GNSS  measurements  with  dual‐frequency  GNSS measurements and by incorporating PPK into the GNSS‐SO2 method.

Impact of UAS Image Orientation on Accuracy of Forest Inventory Attributes