# Suitability of Aerial Photogrammetry for Dump Documentation and Volume Determination in Large Areas
## 4 Discussion
When comparing GNSS and AP from a height of 350 m AGL, the standard deviation was approximately 60 mm. The values correspond to our achieved results. When comparing TLS and AP, a standard deviation value of up to 76 mm was reached. The comparison of the specified volumes (Table 7) confirms the results of previous analyses. Volume difference values were specified for the comparison of TLS vs. AP as 0.09% at Heap No. 1 and 0.08% at Heap No. 2, and for the comparison of TLS vs. GNSS as \(-3.36\%\) for Heap No. 1 and \(-1.67\%\) for Heap No. 2. Regarding the relatively large size difference of both heaps, these results are at the level of expected values and correspond to small values of the mean error of residuals to the reference surface. The determination of volumes by contact methods such as the spatial polar method using total stations and the RTK method using GNSS was summarized by [PERSON] [3] as comparable. This is mainly due to approximately the same density of measured points. This has the most significant impact on the quality of the model and the derived results. By comparing volumes with a reference value, he determined an error in determining the volume with a value of 2.9% [3], which is in line with our results of the GNSS comparison. [PERSON] et al. [59] achieved a volume difference of 0.002% between TLS and AP. For AP data acquisition, a UAS was used at the height of 100 m AGL. The results show that the AP method for volume determination and documentation is in its precision comparable with the reference method TLS (the calculated volumes differ by less than 0.1%) and is significantly more accurate and reliable than the RTK GNSS method (volumes differ in ones of percentages). The advantage is the high density of surveyed points and thus almost true shape capture of the measured object. In areas with poorer texture, e.g., smooth surfaces and shadowy places, the error points occur, but these can be effectively removed using appropriate filtering tools. In the case of multistage measurements, it is appropriate to make a flight with the same parameters and external orientation using the same control points [60]. Although the TLS measurement method achieves excellent results, it has some limits: in particular, the need for field measurements and the incompleteness of the point cloud and 3D model. It is, therefore, suitable for smaller areas or separate objects [34]. Furthermore, the AP method with a correct measurement setup provides a higher work efficiency and model quality [5; 17]. [PERSON] [3] also compared the time required for field measurements by the different methods. If we do not consider the time required for flight and GCP preparation, conventional measurement is more time consuming than AP. In our case study, a large area was surveyed. Therefore, fieldwork is only a small part of the surveying process. The total time required for AP acquisition is, then, shorter than using terrestrial measurement [36]. However, with a large number of images, it is necessary to take into account the longer time of post-processing software processing [37]. A disadvantage of AP use is that it only allows for measuring objects visible from above. It depends on suitable weather, which allows the flight to be made and provides good lighting conditions. Measurements can be made only in daylight, ideally in the midday hours. Risk factors in terms of industrial plants also include the direction and strength of the wind, which could impair visibility by smoke from nearby chimneys or swirling dust from heaps. These risks should be considered during mission planning and scheduling. Nevertheless, the accuracy, efficiency, speed, and economic view of AP compared with other methods make it the first choice for solving problems associated with data collection for terrain modelling and volume calculation with larger scale territory and objects.