Tip 67: Two ways to use external XYZ data in PhotoModeler
External 3D XYZ point data can be imported into PhotoModeler and used in a few different ways. This tip video describes the two most common uses: multi-point transforms, and control points.
External XYZ data can be supplied by other measurement devices, such as survey total stations, GIS, laser scanners, or other photogrammetry projects. This data, when imported into PhotoModeler, can be used to define the coordinate system (to ensure the origin, scale and rotation of the 3D data in PhotoModeler matches that of an external system).
The first way to use external 3D point data is in a multi-point transform. The transform computed moves a PhotoModeler project into a coordinate system via a Helmert Transform. A key aspect of this transform is that it does not change the shape of the 3d model. Three or more external 3d points are assigned by the user to three or more matching points in the existing PhotoModeler 3D model. The model data is not affected and the transform can be turned off and on easily.
The second way to use external 3D point data is via Control Points. Control Points also put the project into an externally defined coordinate system but their main purpose is to overcome a weakness in the project. For example, control points help single photograph projects solve, the Inverse Camera process can recover unknown camera parameters for historical and archive photos using control, and control points can ‘straighten out’ a project when the photo overlap is weak (such as a long string of photos). Control points are included in the processing/adjustment algorithm, and can change the shape of the 3D model.
When do you use one method over the other? If your goal is to place a PhotoModeler project into a coordinate system matching some external data, use the Multi-point Transform method. If you need to strengthen a project or overcome a weakness, use the Control Point method.
Lastly, the tip video describes accuracy assessment of the coordinate system fit, and a function for converting an existing import between the two types.
Watch the Youtube video for an overview: