This problem is not directly related to DSM-creation as it can apply to any PhotoModeler project. The problem of processing a project with many low-angle photos applies to non-DSM projects as well but happens more often with DSM work.
In DSM projects you often have one or more pairs of photos that have low angles. These pairs are suited to the stereo-processing done by DSM but can cause trouble with camera station orientation when the project is first being set up.
There are a few ways of handling this. The first is to combine enough high-angle (25 degrees and larger separation) photos in the project so the low angle pairs can orient against them. You would not use the high-angle photos for DSM work but they do help with orientation of the project. Also you may wish to do a field calibration (to improve camera parameters) and the high-angle photos (along with rolled photos) help with that.
The second method involves changing some of PhotoModeler default DSM settings. You will decrease some of the angle thresholds to allow lower angle photography to be used. In Preferences (Options / Preferences… menu item) in the Audit branch change these two values to zero: ‘Angle maximum (deg.) - Bad’, and ‘Angle maximum (deg.) - Weak’. You may also wish to uncheck ‘Perform Angle Checks’ under the Preferences Orientation branch.
The third method is to use control points. You import control points from an external source (a survey or a high accuracy PhotoModeler project done with other photos), mark them on the low angle photo pairs and then process (see Working with Imports and Coordinate Systems). Note some of the Preferences setting changes for angle thresholds, as described in method 2, may still be required when using control points. The control points will give you stronger orientations and may improve project quality is some circumstances.