Photoship: measuring a ship hull by photogrammetry

This module is designed to reconstruct existing hull shapes by calculating the 3D coordinates of measuring points by means of photography and reference points. Photoship finds itself somewhat on the edge of the natural PIAS functionality because it seldom occurs that a vessel exists, while drawing or other data are lacking. Furthermore, the photogrammetric process requires some insight and experience, so that it only will pay off when applied on a regular basis. In order to provide a notion of the operation a short introduction is presented here. A much more detailed separate manual is available on request.

The role of Photoship in the reverse engineering process

The goal of reverse engineering is to obtain an unambiguous 3D model of the hull shape. The role Photoship plays in this process consists of the construction of a wireframe model, which consists of measured points and user-defined line segments between those points. A solid model based upon such a wireframe can be generated with Fairway (see Convert Wireframe to Solid). Also all other Fairway facilities can be applied on a photogrammetrically measured hull shape, notably post-processing of the hull lines ( Wireframes) and export of the 3D hull model.

The basics of photogrammetric measurement

The principle of photogrammetry is based on stereovision. The 3D coordinates of measuring points are calculated on basis of the depictions of these measuring points on multiple photos. A comparison with human vision can be made here. In order to see depth, two eyes are necessary. The principle of stereovision is illustrated in the picture below:

Lines of sight intersect in space

What we see here are the depictions a1 and a2 of point A on two different photos. From a1 and a2 two lines of sight are drawn through the foci O1 and O2. The point of intersection of these two lines of sight defines the 3D coordinates of A. This principle is named `intersection'. For this principle to work, the orientation of each photo has to be known. These are calculated with the aid of reference points. Reference points are measuring points with known coordinates. They are also being used to give the model the right scale and orientation. Because we are working with measured values, the lines of sight will not intersect exactly. This is why it is impossible to find an exact solution. For this reason, the whole system of measuring points and external orientations is optimized numerically (by iteration) by means of the method of least squares. To be able to do this, initial values for the measuring points need to be known. The result is the most accurate set of values for the 3D coordinates of the measuring points. This way the hull shape is mapped completely by use of multiple photos and measuring points.

Measuring a ship hull by Photoship

1. The following steps are to be taken:
2. Place the landmarks on the hull.
3. Measure the coordinates of the reference points by conventional methods.
4. Make photos, while making sure that every landmark appears on minimal three, but preferably more shots.
5. Open the pictures in PhotoShip, give the coordinates of the reference points and de camera parameters.
6. Point out and connect the photo points.
7. Give the relations between the measuring points by defining line segments.
8. Calculate the external orientation of each photo.
9. Calculate the 3D coordinates of the measuring points
10. Save the 3D coordinates in an .SXF file, so it can further be processed with Fairway. Now the file is ready to be applied in PIAS, or e.g. to be exported to a general-purpose CAD system.

This process is elaborated step by step in www.sarc.nl/images/stories/photoship/article_photoship_en.pdf

Photo's, points and connections in the GUI of Photoship

The resulting 3D model in Fairway