An Investigation into the photogrammetric process Christopher F. Ryan, Falmouth Marine School, UK
Abstract An investigation into the photogrammetric process was carried out, accuracy and ease of use were commented on. A model of a boat, CNC milled to 0.02mm accuracy was measured using Rhinophoto 3.0. The pointcloud created by the software was compared to the original model sent to the CNC milling machine, the standard deviation (0.34284 mm); and mean (0.33378 mm) and median (0.28196 mm) distances of the points from the original model were measured. The obtained pointcloud was found to be of moderate to good accuracy, and moderate ease of use. A number of potential improvements and recommendations were made.
Introduction Reverse engineering designs from existing vessels has been a common practice for many years, in the past it was essential to be able to produce a lines plan from an existing vessel as lines were often lofted directly from scale models in the workshop. In more recent times with the widespread use of CAD (computer aided design) software a need for easier, faster and more accurate methods has arisen. The scope of this article is to investigate the photogrammetric process and comment on accuracy and ease of use. There were two main methods used to take lines from a boat hull before photogrammetry and 3d scanners existed. These were the string triangulation method (this was referred to as the triangulation method by Gardener [1] but it will be referred to as the string triangulation method to avoid confusion later on); and what will be referred to as the grid method. The methods are as follows: String triangulation method: The boat is divided into ten stations and these are marked on the hull with tape. Points to be measured at each station are marked on the tape. A line parallel to the vessel is marked on the floor a known distance from the longitudinal centreline of the boat and the stations are marked on this line (each of these points will be referred to as point B for their corresponding stations). A point is marked on a stick (point A). The stick with a point marked on it (point A) is leant against the vessel at each station, a piece of string is stretched between point A and a point on the hull, the string is then measured and the measurement recorded. This is repeated for each point on the hull surface. The distances from point B to each of the points marked on the hull are measured in the same way. Other measurements taken from each station are: shear height, height of keel from workshop floor, rabbet height, half beam, horizontal distance of point A from the longitudinal centreline of the boat and vertical height of point A from the workshop floor. (Fig 1)