A Tie point, such as a GCP or MTP, is an input which the software explicitly trusts when making calculations. When a Tie point does not fit within the calculated position it is then considered an outlier. In PIX4Dmatic, this is indicated by a red circle in the Tie points table found in the Status center. In general, this red indicator should not be ignored and should be removed. The steps to remove an outlier are described in this article.
An example of an outlier in the Status center.
How to remove an outlier
If an outlier is present after running the calibration process, either the calibration was faulty, or there is still a mark that is not correctly located in the images. In this case, it is imperative to first check the position and reprojection errors in the Tie points table in the Status center for the different GCPs in order to isolate problematic marks. The Tie points table displays which GCPs have an unusually high error.
After determining which GCPs have a high error, it is recommended to check the marks again and remove or improve the ones that are not sufficiently accurate. Removing or improving the marks should improve the reprojection errors that are shown provided the project is initially calibrated well. Once satisfied, it will be necessary to reoptimize cameras under the Calibrate step.