# Suitability of Aerial Photogrammetry for Dump Documentation and Volume Determination in Large Areas
## 2 Materials and Methods
### Data Acquisition and Processing
#### 2.3.3 Aerial Photogrammetry  
A priori analysis was performed to determine the expected accuracy of the determined volume. It depends mainly on the area of the surface and the accuracy of the height coordinates of the detailed points. When estimating the accuracy, a heap height of 10 m and the horizontal heap dimensions 100 m \(\times\) 100 m were assumed. Photogrammetric software producer declares the standard height error of DSM points as 1.5 times the size of the pixels on the ground. If the maximum error does not exceed twice the mean error, and one pixel has a size of 5 cm, then the maximum difference in heap volume determining has a maximum value of 1.5%. Therefore, this accuracy is fully acceptable.  
At the stage of flight plan creation for data collection, images with a resolution of 5 cm per pixel were proposed, which, regarding the parameters of the camera, corresponds to the flight altitude of 580 m above the ground. Four flight axes were realized (Figure 6). Mutual transverse overlap was 75%. Longitudinal overlap of images in the flight direction was 65%. Redundant number of frame pairs increases the accuracy of the resulting DSM and alternatively allows some images to be omitted: for example, blurred images due to turbulence during the flight.  
Figure 4: Surveying network (3D view on GCPs for 3D scanning—purple, scanner stations—green and blue; selected surveyed heaps are highlighted as red areas).  
Figure 5: TLS data: (a) raw TLS point cloud, (b) classified TLS point cloud.  
Ten ground control points were stabilised for absolute image orientation in the coordinate system (Figure 6). GCPs were signalized by square black and white plastic signs with a size of approximately 30 cm\(\times\) 30 cm. GCPs were determined by GNSS fast static method with post-processing and 20 min observation on each point. In addition, virtual reference station generated in the middle of the surveyed location (using SKPOS reference network data) was used.  
Overall, 56 images were made in the case study area. Flight time between first and last image was approximately 15 min. The images were radiometrically corrected and calibrated. Data recorded by onboard GNSS receiver and IMU devices were post-processed and attached to the coordinate system ETRS89 and, together with the coordinates of control points and camera calibration parameters (internal orientation), provided input data for the block alignment of images and calculation of external orientation elements of the analytical aerotriangulation using photogrammetric software. The resulting coordinates were transformed into the coordinate system S-JTSK. Residual deviations of the ground control point coordinates after the analytical aerotriangulation solution reached values smaller than 20 mm for positional coordinates and 30 mm for the height coordinate.  
From these modified aerial images, DSM was automatically generated in the form of a point cloud. Points were generated for every second pixel of the aerial image. About 1.7 million points (raw data) were processed. Unnecessary and erroneously generated points were filtered and removed similarly as described for TLS processing. Such modified point clouds contained about 168,000 points for Heap 1 and 445,000 points for Heap 2.