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.
What relative and absolute accuracies are?
The accuracy of photogrammetric outputs can be distinguished between absolute and relative; both of them are important to assess the quality of the results.
Relative accuracy tells you how accurate measurements are in relation to each other. It is the difference between a feature reconstructed in a 3D model— also known as the “digital twin”—and the feature in reality.
In this case, you have a house with 6x5x3m dimensions, and your goal is to capture and reconstruct the dimensions as close as possible to the original, to scale but not necessarily related to their exact position on Earth.
For a correct reconstruction and to achieve the best possible relative accuracy, the photogrammetry software needs:
- Images with excellent ground sampling distance (GSD)
- Images captured with high overlaps
- Images featuring a consistently high overall quality
To be able to do a comparison between the location data collected and the actual location of any captured feature on Earth, checkpoints are required. For more information on how and when to use them, and how to distinguish them from GCPs, read this helpful article.
What you need to know is that absolute accuracy is correlated with:
- The accuracy of geotags
- Ground control points (GCPs) if you use them
- GSD
As a rule of thumb, final absolute accuracy is generally 1-2X the GSD horizontally and 2-3x the GSD vertically. So always check what the required accuracy of your project is in order to define GSD wisely. You can only measure and be as accurate as what you captured in the images!
In the paragraphs below, please check the parameters upon which the absolute and relative accuracy of a model depend, how to improve them and how to assess the results.
Geolocation accuracy
Without PPK, the geotagging accuracy of WingtraOne aerial images is in the range of a few meters. When using WingtraOne PPK, the accuracy of the image geotags depends a lot on how accurately the base station is logging the correction data. This is why we mention in all literature to take special care with base station setup.
In the optimal case, when flying with PPK, an absolute geotagged horizontal and vertical accuracy in the range of a few centimeters can be achieved. In the case of both PPK and non-PPK data capture, the resulting products have a high relative accuracy, which allows you to measure lengths or volumes accurately, due to the good image quality.
Spatial resolution
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 the actual ground captured in one pixel and is an important factor that affects the quality of an image. It is expressed as ground sample distance (GSD). The better the resolution of the images, the better the chance of achieving high accuracy because items can be more precisely located. The bigger the GSD—i.e., the more actual space captured in one pixel— 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.
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 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 of 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 (quality) of the camera model and on the calibration procedure.
The camera models in WingtraOne payloads are well-defined in Pix4Dmapper and other photogrammetry software through an optimization process using good datasets. In other software, a calibration process should be performed after creating a project.
How to assess the results?
Once you are done with processing in photogrammetry software, check the report that the software automatically generates.
In case you used Pix4D, please refer to this article to check how to determine whether the outputs are accurate or not, and what to do if you face low-accuracy results.