The photogrammetric markers
The photo crack meter is one of the three main components of the complex. It performs the following main functions:
- fixing the monitoring location;
- setting the scale and coordinate system for monitoring;
- contains the source data for photogrammetric calibration of images.
Currently, three types of markers are produced, designed for different indicators of measurements accuracy.
The value of the root-mean-square error in determining the mutual position of the centers of two markers was taken as a measure of accuracy, obtained from processing a pair of images taken at a distance of 2 meters and with an intersection angle of 90 °.
Markers of different types look the same. The difference is in the accuracy and, accordingly, the complexity of the program for their calibration, the results of which are recorded in the calibration file and supplied with the markers.
Currently, we have mastered the production of markers from the following materials:
• PVC plastic (letter P)
• aluminum (letter A)
• fiberglass (letter T)
Information about the material of production is indicated in the type of beacon to the right of the accuracy indicator, for example, M-0.1 P, M-0.02 A or M-0.005 T – the first is made of PVC, the second is made of aluminum and the third is made of fiberglass.
All these materials have sufficient mechanical strength and stability, but have a different coefficient of thermal expansion (in aluminum, it is about 3 times higher than fiberglass, and in PVC-three times higher than aluminum). And, although, the software takes into account the corrections for the temperature – it cannot always be known with sufficient accuracy. Therefore, when performing high-precision observations in conditions of a strong temperature difference, it is desirable to use markers made of a more stable material in this respect.
Markers are attached on both sides of the crack, either with glue or with screws.
In the first case, any glue can be used that will provide a reliable fixation during the entire monitoring time: epoxy, "liquid welding", "two-component glue-plasticine", etc. The use of mounting tape can be quite reliable and convenient in some cases.
If the attachment with glue, for some reason, does not suit, - markers can be fixed on the screws, using a special type of washer, which we have specially developed for attaching photo markers.
Also some dealers offer their own mounting options based on their experience in monitoring.