Abstract
Unmanned aerial vehicles (UAV) are increasingly used for topographic mapping. The camera calibration for UAV image blocks can be performed a priori or during the bundle block adjustment (self-calibration). For an area of interest with flat scene and corridor configuration, the focal length of camera is highly correlated with the height of the camera. Furthermore, systematic errors of camera calibration accumulate on the longer dimension and cause deformation. Therefore, special precautions must be taken when estimating camera calibration parameters. In order to better investigate the impact of camera calibration errors, a synthetic, error-free aerial image block is generated to simulate several issues of interest. Firstly, the erroneous focal length in the case of camera pre-calibration is studied. Nadir images are not able to prevent camera poses from drifting to compensate for the erroneous focal length, whereas the inclusion of oblique images brings significant improvement. Secondly, the case where the focal length varies gradually (e.g., when the camera subject to temperature changes) is investigated. The neglect of this phenomenon can substantially degrade the 3D measurement accuracy. Different flight configurations and flight orders are analyzed, the combination of oblique and nadir images shows better performance. At last, the rolling shutter effect is investigated. The influence of camera rotational motion on the final accuracy is negligible compared to that of the translational motion. The acquisition configurations investigated are not able to mitigate the degradation introduced by the rolling shutter effect. Other solutions such as correcting image measurements or including camera motion parameters in the bundle block adjustment should be exploited.
Highlights
The derivation of geospatial information from unmanned aerial vehicles (UAV) is becoming increasingly ubiquitous [1]
The procedure consists of identifying common feature points between overlapping images and recovering their poses at first in a relative coordinate system, followed by the georeferencing phase with the help of, for example, ground control points (GCP) or the camera positions measured with global navigation satellite system (GNSS) [2,3]
To investigate how camera poses and 3D accuracy are affected by the erroneous focal length, an error which varies from −50 pixels to +50 pixels with a step of 10 pixels is added to the original focal length
Summary
The derivation of geospatial information from unmanned aerial vehicles (UAV) is becoming increasingly ubiquitous [1]. The image pose can be derived from aerial images with high accuracy. The bundle block adjustment (BBA) is a basic tool for photogrammetric pose estimation. Camera calibration parameters can be considered pre-calibrated and constant, or their values can be re-estimated in the self-calibrating bundle block adjustment [4,5]. Continuously overlapping images are taken in series. Unlike block configuration, it constitutes only few parallel strips and cross strips are often absent. It constitutes only few parallel strips and cross strips are often absent This unfavourable configuration makes the camera self-calibration less accurate. The lack of difference in Remote Sens. 2020, 12, 22; doi:10.3390/rs12010022 www.mdpi.com/journal/remotesensing
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