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Digital Videogrammetry is the video equivalent of photogrammetry. A video camera takes 1500 'pictures' per minute - each of which can be independently viewed, printed and measured as an individual 'photograph'.

Precise digital videogrammetry is a combination of precision instruments with one or more video cameras and a laptop computer. The pictures taken are recorded as digital images on the laptop, which can display combinations of the images on screen in real time. In this way the view through one or more theodolites, together with the theodolite readings and other information, can all be reviewed and recorded simultaneously at relatively high speed and then examined in detail at leisure.

The video data capture system allows:-

  • Accurate recording of whole structures on a 'brick by brick' basis
  • Comparison of videos from different surveys, such that movements can be detected wherever they occur rather than at discrete survey points as would be the case with a conventional survey using targets or telltales
  • The survey to be made with only line of sight access to the structure being surveyed

Benefits

Precise digital videogrammetry combines the high information content of photogrammetry, the speed of electronic measurement, the precision of the best optics, an indisputable record of what was measured and the ability to compare and communicate results digitally.
It is relatively cheap, flexible and highly efficient.

Equipment linked to laptop on-site for instant monitoring

Total Station Videogrammetry

Using a total station with video cameras, the position of a reflective target can be recorded automatically along with the view. This is particularly efficient when tracking as it is conventionally necessary to identify each string of points by a coded label. The unnamed points can now be plotted and then identified from the video files with the added check on accuracy. Using a reflectorless EDM the instrument is simply pointed at any chosen target and the distance and bearing are recorded directly onto the video picture of the chosen target.

Video-Intersection

The precise positions of remote points can be obtained using two theodolites and comparing and matching or overlaying the images simultaneously. By using the view from one theodolite to track a second one, a large number of intersections can be obtained very efficiently without the need to identify specific targets.

Lenses mounted through and above the lens for comparative measurements

General Video Levelling

One camera looks through the telescope of the levelling instrument recording the cross-hairs and staff reading. A second camera records a general view to show where the staff is and what it is sitting on and the third camera records the bearing of the telescope. Distances are subsequently calculated by tacheometry giving all the necessary information for a general level survey.

Continuous readings can be taken by tracking the staff - for paved areas, the staff can be mounted on a trolley and moved at speed. Since the cross hairs and view are permanently recorded at 1500 readings per minute, it is possible to go back and extract information to almost any desired accuracy of level or interval. If precise positioning is required a theodolite is used with an EDM reading off reflective tape on the staff, with all the results combined on the same file set.

Monitoring gradual movement in structures

Level

The level telescope is panned across the structure so that the cross-hairs superimpose an 'invisible' horizontal line, usually placed on a brick course joint. Any subsequent movement can be detected by comparison of recordings, brick by brick. The image files also provide records of crack patterns and crack widths. The comparison is done by comparing images from recordings at different times. An accuracy of 1mm is easily achieved in level monitoring.

Verticality

The same procedure as for level monitoring is used with a theodolite to record a series of verticals imposed on the structure. Any subsequent changes are easily detected. Provided the theodolite can be relocated changes in length or height of structure can also be determined. The accuracy depends simply on the instruments used.

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Digital VideoGrammetry : building detail

Image taken 3rd March 2005

Digital VideoGrammetry : building detail

Image taken 20th May 2005

Transient deflections can readily be measured to an accuracy of about 1mm at intervals of 0.04 seconds. Improved accuracy can be obtained using travelling microscopes. A example of this is the measurement of railway bridge deflection under train loading.

Digital VideoGrammetry : the equipment Digital VideoGrammetry : iris Digital VideoGrammetry : iris