How do I review the quality, or troubleshoot and correct problems in my project?
The following is a basic guide to troubleshooting a points-based project, especially one that is marked using natural feature marking (i.e. no targets) and manual referencing.
- First, check that your best camera file has been used. Preferably a calibrated camera and that the calibration is of good quality. A camera calibrated in PhotoModeler 6 shows some quality indicators (on the Cameras dialog). The basic general indicators of a well calibrated camera are a low “Overall Residual RMS” (say 0.1 or less), a low maximum point mark residual (say 1.0 or less), and good point mark coverage (say 90% or more).
- Open a 3D Viewer to see how the model solved in general. Does the model look warped or out of proportion? When camera stations are displayed, do they look like they’re positioned reasonably and pointing in the right direction? If the model doesn’t look right, it could be because of referencing and/or marking errors (assuming the calibration is good).
- Open a Point Table (View menu, then click “Point table – Quality”), sort by “Largest Residual”. Generally a project done without sub-pixel target marking should have residuals less than 10.0, ideally less than 5.0. Compare this to projects done entirely with high contrast sub-pixel-marked circular targets, which should have residuals less 1.0. A project using ‘natural’ features (not circular targets) may have relatively high residuals because the the points may not be easily marked in the exact location on each photo. Residuals well above 10.0 indicates referencing or other problems.
- Select the point in the table with the highest residual. Examine the point on each photo where it was marked and check its marked position. Is it referenced correctly? You can right-click on the point in the table to bring up the menu and select “Open Photos Showing Selected”. This opens all photos on which the point was marked. Referencing and/or marking errors and inaccuracies may became clear when you see the marked position on each photo. Misreferences, especially subtle ones that are misreferenced on points that are close to each other, will start to throw the solution off as PhotoModeler tries to reconcile the conflicting marked positions.
- Unreference the misreferenced points then re-reference them correctly. Also zoom in on your photos and make any necessary mark point position adjustments so that points are marked at as close to the exact location on each photo as possible. Misreferences are quite common at early stages of a project, especially on symmetrical objects, as it can sometimes be tricky to orient visually in the photos and mark the right points. The troubleshooting process should help you find and fix these early. It is much easier to find and fix problems early in a project’s life, so it is important to monitor quality indicators and catch the problems early.
- Reprocess and repeat steps 3-5, looking for marking and referencing problems. Note that you can also click the “Max residual…” section on the status bar to open and select the point on the photo with the largest residual, which is often the quickest way to pinpoint problem points.
- After making changes reprocess and review residuals until they drop below an acceptable level for your project and accuracy needs. You may wish to go over your point marks in each photo and fine tune their location when zoomed in and/or remove some points that are marked approximately, but it depends how much time and patience you have and how important accuracy is in the project.
- Go back to the 3D viewer to see how things look. Check marked lines to see if they look right, as they may have been drawn at some point between misreferenced points.
- Next check for doubled-up points. These may be points that are referenced in more than one group. For example, say a point is marked on Photos 1,2,3,4. If it is referenced on photos 1&2, and photos 3&4, but no references between the two groups. This results in two 3d points very close to each other. If this is the case, create a reference between the two groups (e.g. reference the point on photo 2 with that on photo 3). Note you could also use the ‘Weld Unreferenced’ tool on the Referencing menu to have PhotoModeler search for these kinds of points within a defined tolerance.
- At this point you should have a project of reasonable quality with low residuals and an accurate shape.
You’ll want to get used to troubleshooting a project and navigating around the model using the point/surface/curve tables, the 3D Viewer, the status bar, and various right click and other menus to view your features in photos. You’ll have better and more accurate models if you monitor as you go so that you catch problems early and problems don’t get compounded.