Accuracy of Pix4D outputs

The accuracy of the outputs can be distinguished into:

  • Relative accuracy: It is the accuracy that is defined by comparing individual features on a map / reconstructed model / orthomosaic with other features on the same model. For example, two points of the model can be 2 meters away from their real position on the earth but if their relative accuracy is high, then the distance measured between these points will be very accurate as it is related with the relative position of the points.
  • Absolute accuracy: It is the accuracy that is defined by the difference between the location of features on a map / reconstructed model / orthomosaic and their true position on the Earth.

Relative accuracy

The relative accuracy will depend on the quality of the reconstruction of the project, which itself depends on the overlap between images, the visual content of the images, and many other parameters. 

Generally, one can expect an error of 1-3 times the Ground Sampling Distance (GSD) of the original images for the relative position of a point in a project that is correctly scaled and reconstructed.

 
Important: This error is not global. Some points will be more accurate than others based on the number of images on which they are visible (overlap), how close they are to a GCP, what is the GSD in this area.
 
Note: This error can be reached at any scale. However, for sub-centimeter GSD projects, it is harder to get the ideal conditions. Problems such as the perspective, vibrations of the camera, blur effect, depth of field, etc. are magnified. It is recommended to do very careful testing and planning for such projects, in order to get the expected accuracy. Rolling shutter cameras are not recommended for sub-centimeter accuracy results.

Using Ground Control Points (GCPs), the relative accuracy may improve (especially in areas with lower overlap or with difficult image content).

 
Information: 

Absolute accuracy

Absolute accuracy increases significantly when using RTK GPS for the image geolocation or when using GCPs. When using GCPs, the accuracy depends on the accuracy of the measured GCPs, their number and their distribution.

 
Important: The accuracy of the results cannot be higher than the GCPs' accuracy.

Generally, the expected accuracy of a correctly reconstructed project is:

  • 1-2 GSD horizontally (X,Y coordinates). 
  • 1-3 GSD vertically (Z coordinate).
 
Example: For a project with a GSD of 5 cm, the expected horizontal accuracy is 5-10 cm and the vertical accuracy is 5-15 cm.
For a project with a GSD of 10 cm, the expected horizontal accuracy is 10-20 cm and the vertical accuracy is 10-30 cm.
 
Note: This error can be reached at any scale. However, for sub-centimeter GSD projects, it is harder to get the ideal conditions. Problems such as the perspective, vibrations of the camera, blur effect, depth of field, etc. are magnified. It is recommended to do very careful testing and planning for such projects, in order to get the expected accuracy.

Point cloud, DSM, orthomosaic accuracy

The point cloud, DSM and, consequently, the orthomosaic accuracy are also affected by the quality of the initial images and their visual content. Sharp edges, trees, reflective surfaces, and certain type of roads and rooftops may be locally less accurate.

For more information about how to use the rayCloud to improve accuracy: 202559119.

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3 comments

  • Takeyuki Suzuki

    In my sub-centimeter GSD project, polyline's length doesn't show sub-centimeter value. How can I show it?

  • Ina (Pix4D)

    Hi,

    At the moment in the mapper, you can measure distances at the minimum of cm level.

    Having measurements under centimeters would require accuracy at that level as well. Having consistent subcentimeter accuracy it might be challenging. The accuracy of your outputs you should expect is 1-2 pixels (GSD) horizontally (X, Y coordinates) and 2-3 pixels (GSD) vertically (Z coordinate). The GSD of the data set depends on the focal length, the "flight" height, the sensor and the image height. So, if you want to acquire a sub-centimeter accuracy, then your GSD should be half that. So, you should design the "flight" plan in such a way to acquire the adequate data for obtaining such an accuracy.

    Thanks,

    Cristina

  • Takeyuki Suzuki

    OK. Thank you, Cristina.

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