Staircases by Campbell James W. P. Tutton Michael

Author:Campbell, James W. P., Tutton, Michael
Language: eng
Format: epub
Publisher: Taylor and Francis

More complex instrument-based techniques

Where more detailed survey is required, normally because invasive works are proposed to the staircase, there are three main survey types which can be used: reflectorless TST, photogrammetry and rectified photography/orthophotography. Increasingly, laser scanning is also being used as it offers a relatively quick recording process with equipment which is now sufficiently mobile to be used in confined spaces. As with all instrument-based techniques, it is important that the person commissioning the work has the necessary means to view the results. If the output is simply a paper-based report, that will not be a problem, but with an output which is 3D, appropriate hardware and software will be required to view and manipulate the survey data. All techniques are described in detail in Measured and Drawn18 and are merely summarised here.

A TST is a computerised theodolite that uses a laser beam fired at a portable prismatic mirror to measure or calculate distances and can store measured co-ordinate data for transfer to graphics software. A ‘reflectorless’ TST dispenses with the prismatic mirror, relying instead on reflection of the laser by the object surface. Either is particularly useful for measurement of remote objects and 3D spaces, especially when combined with software such as TheoLt™, which that provides a ‘real-time’ interface with graphics software such as AutoCAD™. When used in this manner, the TST becomes the mouse/cursor and the drawing is generated on-site with the object in front of the surveyor and, as such, is a form of ‘direct plotting’ that allows errors and assumptions to be checked as they are made. This form of digital recording requires a reasonable level of specialist knowledge to operate and the costs are higher than more basic techniques. It is particularly useful for establishing 3D control frameworks around or within complex 3D structures. It would normally be used to create 3D models, from which 2D drawings could be extracted, if necessary, but it is not suitable for the measurement of intricate decorative detail.

Photogrammetry and rectified photography or orthophotography, on the other hand, are suitable for the measurement and recording of intricate decorative detail. Both can be done using ordinary hand-held cameras, but professional reliability requires they be done by specialist contractors using specialist cameras designed to eliminate distortion at the edges of the image. The former involves the tracing of detail from a scaled photograph to create a line drawing; the latter is simply the creation of a dimensionally accurate scaled photograph. Both can be imported into drawings, especially if generated in CAD, as a quick way of representing fine detail, such as baluster profiles, in 2D.

Laser scanning is the fastest way of collecting large amounts of very detailed 3D data which can then be used to display the results in a 3D format. There are many types of terrestrial laser scanners and each type records hundreds of thousands of 3D points from a known position of orientation, with the result being a data set known as a ‘point cloud’. These data sets

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