Assessing the accuracy of your surveying results is very important for determining the applications that they can be used for. This article presents factors that influence the accuracy of the results.
Without PPK, the geotagging accuracy of WingtraOne aerial images is in the range of a few meters. In the case of a WingtraOne PPK system, the accuracy of the image geotags depends a lot on how accurately the base location is logged and the correction data it provides. In the optimal case, when flying with PPK, an absolute geotagging accuracy down to 1 cm can be achieved. With both options PPK and non-PPK, the resulting products have a high relative accuracy, which allows you to measure lengths or volumes accurately, due to the good image quality.
Accuracy depends on the quality of the images, which depends on the camera as well as the flight height and environmental conditions. Spatial resolution refers to the size of a pixel on the ground and is an important factor that affects the quality of an image. It is expressed as ground sample distance (GSD). The higher the resolution of the images, the better the chance of achieving high accuracy because items can be more precisely located. The bigger the number of the GSD, the lower the spatial resolution of the image and the less visible the details.
In photogrammetry, the image resolution defines the highest accuracy that can be achieved with data. It is impossible to get an absolute or relative accuracy better than the image resolution. If for example, the GSD of the project is 1 cm/pixel, the final accuracy cannot be better than 1 cm.
Camera calibration accuracy
Calibration is the process of determining the camera’s focal length, principal point, and lens distortions, as well as the position and orientation of the camera at the time of the image acquisition. The initial parameters are given by the camera manufacturer and with the geotags. Approximate values can be good enough, but in case high accuracy is required, these values should be accurately defined. Optimizing these parameters is the first step during photogrammetric processing in, for example, Pix4D or Agisoft. A high-quality lens and stable camera will produce better results since the images are less distorted. Small consumer-grade cameras are more sensitive to vibration and temperature. The quality of the reconstruction relies on the accuracy of the camera model and on the calibration procedure.
The camera models of WingtraOne payloads are well-defined in Pix4Dmapper through an optimization process using good datasets. In other software, a calibration process should be performed after creating a project.
The accuracy of photogrammetric output can be distinguished between absolute and relative. Absolute accuracy refers to the accuracy of the model or map with respect to the Earth, while relative accuracy refers to how well located any given features are relative to other features.
Relative accuracy refers to the quality of the reconstruction and depends on the type of camera used for image acquisition, the overlap between the images, and the visual content of the images. The more matching points between the overlapping images, the better the connection between images and the relative accuracy.
It is generally admitted that relative accuracy is expected to be 1-3 times the GSD. The relative accuracy can be assessed by checking the reprojection error of the 3D points on the images.
The absolute accuracy of the WingtraOne survey depends on the PPK data or ground control points (GCPs) measured in the area. The accuracy of the photogrammetric project can never be better than GCP accuracy. It can be assessed using the GCPs as control points or comparing the resulting output against other data that has been verified.
High absolute accuracy presupposes high relative accuracy of the model, as this defines the overall quality of the stitching.