Constraints in Cad Models for Reverse Engineering Using Photogrammetry

The paper discusses the use of geometric constraints in a CAD model-based photogrammetric system. The CAD models are represented by CSG (Constructive Solid Geometry) models. The geometric primitives that make up the CSG model are parameterized by their pose and shape parameters. Geometric relations, or constraints, between these primitives are expressed using these parameters. Constraints can be applied both between the geometric primitives in a CSG model and between CSG models. For example, continuity requirements between two connected piping elements constrain the relative position of the elements as well as their relative orientation. The constraints are formulated as weighted linearized observation equations and are included in a least-squares adjustment together with the photogrammetric measurements on the edges. The exterior orientation parameters of the images are also estimated in the adjustment. A pilot project was done on a gas-exploration site. Only the images and a couple of tape measurements were acquired on the site no markers were used. The model-oriented photogrammetric system, Piper, combines image observation and geometric constraints in a bundle adjustment to reconstruct an accurate CAD model of the installation.