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A layperson’s guide to 3D laser scanning

Home » A layperson’s guide to 3D laser scanning Published: 15th December 2016 This Article was Written by: Richard Furlong - City Surveys Group

Laser scanning is a high-precision method of producing a virtual three-dimensional image of a surface and of items on that surface.

A kind of very detailed dot-to-dot puzzle that maps out the contours of the objects within the laser’s field of view.

The technique used is LiDAR, or Light Detection and Ranging, which essentially involves aiming a laser at the surface, measuring the light reflected back from it, and calculating the position of the target object or surface from this with a high degree of accuracy.

In some instances, the instrumentation is also equipped with a digital camera to take corresponding photographs of the target. When combined this allows for the virtual contour mesh or ‘point cloud’ to be aligned with a true visual image for ease of interpretation and use.

Colour data from these photographic images can be imported into the digital software to apply a virtual texture to the contour mesh – combining the real-world visual information into the virtualisation.

Laser scanning is commonly known by several names:

• 3D laser scanning
• Three-dimensional laser scanning (3DLS)
• Terrestrial laser scanning (TLS)
• High-density survey (HDS)
• High-definition survey (HDS)

While the last two share the same acronym, they are essentially the same process – so the terms can be used interchangeably.

The silver lining

Laser scanning data is obtained as a ‘point cloud’ which, as mentioned above, is like a highly detailed, three-dimensional dot-to-dot puzzle.

From this, it is possible to retrace lines, objects and surfaces in the form of a contour mesh, turning these individual specks back into usable 3D items.

With the option to add colour data from photographs, it is possible to reproduce a very highly detailed image of an object remotely.

This ability means laser scanning is not just useful for high-precision measurements but also for those where the object itself is in an inconvenient position to measure manually.

Laser scanning is fast and reliable and the point cloud that is produced offers a way to interact virtually with an environment that might be too hazardous for extended contact in real life.

The results are used for all manner of applications including (but not limited to)

• Producing three-dimensional models of structures for design, engineering or feasibility purposes
• Producing 3D walk-through models and visual impact assessments
• Generating virtual reality models of structures or areas of land
• The production of virtual, on-screen objects or the rendering of a virtual, three-dimensional object
• Recording historical structures or objects for heritage or archaeological recording purposes

A point on precision

Whilst each individual point in the cloud is highly precise, and can include a measurement of the RGB (Red, Green, Blue) colour value of the target at that position too, the overall precision of the mesh as a whole depends on the number of points of measurement.

This is expressed as the ‘point density’ – the average proximity of the coordinates to one another. By ensuring measurements are made at least this close together the necessary level of detail can be produced.

During measurement there are certain surface characteristics that might negatively affect the level of precision obtained, although many of these can be accounted for in the results.

Wet surfaces are generally less suitable for laser scanning, due to the different way they reflect the light, and certain high-gloss or metallic surface finishes may introduce a similar uncertainty.

Measurements should be taken close to the perpendicular where possible as the greater the angle of incidence, the more noise there is likely to be in the reflection.

Distance is also a factor and, whilst laser scanning is beneficial because it allows for remote measurement of physically inaccessible objects, these should still be carried out as close to the target as can reasonably be achieved.

A related concern is the presence of atmospheric contaminants and precipitation as airborne particles like dust (as well as raindrops, fog and snowflakes) can produce a false positive by reflecting the laser light back to the detector before it has reached the target object.

Extreme temperatures may affect the equipment used. As a result many laser scanners come with a ‘safe’ operating temperature range to ensure they function as intended.

Stable ground is essential as the precision of the scan is high enough to be affected by relatively small vibrations – particularly where a high level of accuracy is essential to the project.

And finally, because the laser needs to reach the target, be reflected back and be detected back at the scanner, there must be a clear line of sight between the two; for obvious reasons, this means detection is impossible if buildings or other structures are in the way.

Large-scale laser scanning

On very large scales, a panorama of laser scanning point clouds may be produced – just as with visual photographs – by overlapping common features and ‘stitching’ the separate scans together.

With the help of computer software, multiple point clouds can be automatically joined, based on intelligent detection of common surfaces and features in neighbouring scans.

This process is called ‘cloud to cloud registration’ and can also be carried out manually, although the software approach is likely to be much faster, more accurate and less demanding.

Scanning over large areas has enabled laser scanning to be used on infrastructure projects from roads to pipelines, ambitious archaeological and architectural works, and on high-precision surfaces like airport runways.

When to use 3d laser scanning

Whilst not an exhaustive list and focusing primarily upon construction applications, using 3D laser scanning makes complete sense if your access to a site is limited by:

• Time (e.g. if only limited downtime is acceptable)
• Location (from remote targets to confined spaces)
• Safety (e.g. hazardous or high-altitude positions)
• Longevity (if you need to preserve a snapshot of a site before, during or after work)

However, it is a good general-purpose surveying tool as, once you have your point cloud, you can extract the data you need as you go along, for example, when circumstances may have made it impossible to take new physical measurements.

This approach means that, rather than developing a complete, detailed virtual model – or a physical model, for that matter – of the site, you can store the point cloud data and then generate smaller models of specific targets as they are needed.

As a result of this, your models can be easier to work with, specific to the task at hand, and more cost-efficient, while laser scanning can work alongside conventional surveying techniques where necessary for an even more complete picture.

Want more info?

If you’d like to learn more about 3D laser scanning and the value it can add to your project, why not contact City Surveys? As one of the UK’s leading 3D laser scanning survey firms, we’re ideally placed to advise you on how best to survey your site.