A Marking Residual is a residual error relating to an object marked, by the user, on a photograph. It is the difference between where the user marked it and where the program expects it to be.
The most common marking residual is the Marked Point Residual. When a point has a 3D position, PhotoModeler can project it back onto the photograph (along its light ray) to determine where, in the ideal case, that point should be on the photograph. Due to various errors and averages etc. the x, y point that the user marked and this projected x,y position will not be the same. The difference between these two is called the Marking Residual for the point. You can see the projected location by turning on Projections. You can also see the residual in its vector form by using the Marking Residual Display setting (see Visibility on Photos). The residuals can be seen in numeric form and summarized on the Point Table.
Marking residuals are one of the best indicators of project quality and should always be checked after processing. All projects should have the largest point marking residual of under 10 pixels. For projects with known and calibrated cameras the largest residual should be less than 3 pixels and for projects done with all sub-pixel targets, the largest residual should be less than 1 pixel and smaller if possible.
A large marking residual means that the 3D location of the point and the mark position of the point do not “agree”. The lack of agreement points to problems in the project that should not be ignored. If there are only a few points with high residuals then they are probably not marked well (or misreferenced). If there are many points with high residuals then perhaps there is a camera calibration (or parameterization problem) or one or more of the camera stations are not oriented correctly.