A residual in photogrammetry is short for “residual error” – that is, the error left over in some value after processing has completed. A “marking residual” is the difference between where PhotoModeler thinks a point should be on the photograph and where you have marked it. This is calculated by projecting the 3D position of the point onto the photograph and comparing it with the marked position. The Marking Residual Display on photographs and the Residuals lists in the Point Table can assist you with your project in a number of ways. The Marking Residual Display helps you study all the marked point residuals on a photograph at once. In a good project, residuals should not be visible at the default exaggeration factor. If you can see large residuals, this indicates serious problems with those points – they are likely misreferenced or mismarked. If you cannot see the residual vectors then increase the exaggeration factor until they are large. If you see a pattern in the residuals (such as all vectors pointing to the right), this may indicate poor camera parameters or another orientation problem. Some very high accuracy projects, done all with sub-pixel marks and a well calibrated camera, show short residual vectors at even very large exaggeration factors (900 times, for example!). You can contact firstname.lastname@example.org if you need further guidance.
To see your Marking Residuals in the Point Table, choose the configuration “All” or “Quality-slow”. You can see two columns, Largest Residual and RMS Residual. You can sort the table by largest residual and in a good project, you should see very few points with residuals over 3 pixels. If you are doing a high accuracy project, you will want most points to be under 1.0. For high residual points, you can select them in the table, right click, and choose “Open Photos showing selected”. This will open the photos containing the selected point, and the point will appear highlighted in red. You can then do various functions such as adjusting the point’s position, unreferencing it, deleting it, etc. Marking Residuals are a very important measure of your PhotoModeler project’s health. Study them after every processing and rely on them!