Structural Surveying

in Laser

Introduction
The idea of surveying buildings and other manmade structures is nearly as old as the pyramids. Combining the elements of measurement and observation, structural surveying is important for everything from initial construction to historical preservation. Methods have changed over the years as technology has emerged to provide faster, easier and more accurate results.

Methodology
Drawings of 2D and 3D building façades, plus three-dimensional representations of all sorts of other structures, allow engineers to create, repair or restore them while keeping track of progress in real time. The nature of surveying is to create lines of sight that reproduce an image of the object in question. For new construction, this means the proper alignment of footings on the ground that matches the blueprints. In a restoration project, this means re-creating the original design in three dimensions and then using those drawings to exactly repair or replace structural elements. The accuracy one can achieve by using surveying methods and equipment ensures that everything is in its proper location. This is not only important from an aesthetic point of view; in some cases, the structural integrity of the building may suffer if repaired elements are not in exact alignment.

Traditional Benchmarking
In building restoration, surveyors use a total station-the ultra-modern, electronic version of the transit that measures angles and distances with incredible accuracy-to locate the centers of the target object. From these center points, additional points are measured that represent edges, corners, line end-points, and other remarkable features. For an average building, it is not unusual to see a surveyor collect hundreds of points to achieve an appropriate level of accuracy. The purpose of benchmarking is to locate particular surface features and insert them into some sort of map or drawing. With the advent of CAD (computer-aided design), it is possible to download data collected by the total station directly into a drawing program. With a level of accuracy that ranges from between 2mm and 5mm, traditional methods are generally sufficient for most projects.

Laser Scanning and Benchmarking
The introduction of the laser scanner as a tool for structural surveying has resulted in three direct benefits. First, the time a surveyor spends onsite is greatly reduced. A surveyor who takes two or three days to collect data with a total station can easily see his or her work hours cut in half-or more-with a ground-based laser scanner mounted on a tripod. Second, the number of points a surveyor can collect with a laser scanner will jump from the hundreds into the thousands. The more points that are collected, the better the final picture appears in a CAD environment. Third, an accuracy level in the neighborhood of 1mm to 2mm is clearly achievable with scanners, especially given the point density that is a result of laser technology.

Borescopes and Endoscopes
Another tool available to the structural surveyor is the borescope. Also called an endoscope-a term borrowed from medicine-this device is used for observing and measuring the insides of structures that are not readily accessible, all without negatively affecting structural integrity. In this process, a borehole is drilled into the outer wall of the structure-generally 12-13mm in diameter-through which the scope is inserted. All sorts of structures can be observed this way, including ductwork, voids beneath and between floors, and spaces in ceiling and roof areas. Most borescopes come equipped with fiber optics for illumination purposes and are oftentimes connected directly to a digital camera to preserve the observed images.

Author Box
Peter Brittain has 1 articles online

Land Surveys Pty Ltd are an engineering surveyor based in Perth & Karratha, Western Australia who can provide you with a geological survey.

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Structural Surveying

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This article was published on 2010/03/31