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Photogrammetry is the science of making measurements from photographs. The output of photogrammetry can be in the form of points, maps, drawings or 3D models. Photo means 'light', gram means 'drawing' and “-metry” means 'measurement'.
The history of photogrammetry is almost as old as the invention of the camera itself when early images were used for surveying. As a simple example, the distance between two points that lie on a plane parallel to the photographic image plane can be calculated by measuring their distance on the image if the scale (s) of the image is known. This is achieved by multiplying the measured distance by 1/s.
The invention and development of digital cameras have increased the number of applications of this technology and the potential accuracy that can be achieved.
The GOM TRITOP system used by highly-trained and experienced engineers is a photogrammetry measurement process which captures reference points placed on or around the object. This system can be used as a standalone measurement system or to capture a reference point cloud which can be used by an optical scanner.
For all applications, the GOM TRITOP system requires a DSLR camera with a fixed focus and flash, reference points, coded markers and two or more scale bars. Many different camera models can be used but each must use a fixed focus and flash. This is so that each image will have the same exposure and pixel definition which is essential to maintain consistency throughout the photogrammetry session. Due to the minimal amount of equipment required, this measurement system is easily portable.
The photogrammetry system is light and portable
The TRITOP software is developed to identify the coded markers and associate the optical barcode with a unique number. Each image can be orientated within the project as long as five or more of these coded markers are visible. Once all the images are orientated together the software will identify the un-coded markers by identifying the change in contrast from the black to white areas. The scale bars are identified using specific coded markers and are inspected against a nominal value. Using two or more bars will give a deviation of the measurement series.
Calibration crosses and scale bars ensure the accuracy of the TRITOP photogrammetry system
There are two main applications of the GOM TRITOP photogrammetry system. The first application is using the system as a standalone measurement solution and the second is capturing a reference point cloud to reduce the possible inaccuracies of an optical 3D scanning system.
1. Photogrammetry as a standalone measurement system
As an independent system, photogrammetry can be used to complete many different types of projects where full surface data is not required, specific geometry is being inspected or where parts are being moved during the measurement process. One example where used photogrammetry as an independent measurement solution is a torsion test recently completed on a Formula Student race car chassis.
Photogrammetry was used to test the displacement of the chassis under stress
Reference points were placed on the chassis and a photogrammetry session was completed to calculate the 3D coordinate of each point. Stress was then applied to the chassis and measurements were taken at set intervals. Each measurement was inspected against the baseline chassis calculation to generate an inspection of the strength of the chassis.
2. Photogrammetry as a basis for optical scanning
Using the captured reference points as a base for optical 3D scanning is another application for the technology. The GOM ATOS optical 3D scanning systems use reference points to locate the position of the sensor in relation to the part.
Photogrammetry enabled the measurement of a large satellite
By using a second measurement system, the potential scanning area can be vastly increased whilst not incurring large deviation errors. This method is used to scan full size cars, boats, planes and other large objects.
A photogrammetry adaptor is a specific object designed for repeat inspection of a geometry which can be captured during the TRITOP process. The adaptor uses a unique pattern of un-coded markers which is recognized by the software and associated with the CAD geometry for quick and easy inspection. The most common adaptors are used to inspect holes, planes, cylinders and other uniform geometry but an adaptor can be designed to measure almost any feature.
Engineers used photogrammetry to capture measurements of the last remaining battle of Britain airworthy MkIIa Spitfire