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Everything (or almost) you need to know about 3D scanning
Depending on the nature of your site or your installations, the type of survey to consider will not be the same.
What is the objective of the survey? How much precision do you need? What level of detail should the scan achieve? Should the campaign be georeferenced?
It's not easy to see clearly in this profession which seems so new and yet has existed since the 90s.
I'm giving you some keys to see things more clearly, because scanning is my battle!
1 - Your needs
1.1 - Industries: remodeling, revamping, extension, transfer
2 phases must be distinguished: the preliminary project and the execution studies.
In the preliminary design phase, when we are only looking to position blocks, mobile 3D scanning may prove sufficient. It will be precise enough to capture the general shape of the equipment. However, it will be insufficient on pipes of small to medium diameter tubes (15 to 60mm), connection flanges, and on metal profiles.
During the execution study phase, it is better to carry out a scan with a so-called “terrestrial” scanner (TLS), i.e. on a fixed tripod. This is the most precise scan which will allow you to go below 3mm per station. It is also the best way to capture the pipes to prepare a prefabrication (isometrics).
My personal opinion: for technical areas, favor terrestrial scanning from the preliminary project stage. Indeed, if you start with a mobile scan, you will have to carry on a second campaign with a TLS once the project is more advanced.
The good compromise: you can do the campaign in 2 steps. During the preliminary project, when there is no need to capture all the details, carry out a campaign using a terrestrial scanner, but with relatively spaced positions.
During the execution phase, do additional stations to target the details.
1.2 - Industrials: scan entire site for up-to-date documentation
In this case, you need a campaign that is a mix between exterior areas, other interior areas with low technical density (corridors, storage), and other more technical areas (production, technical premises).
In this case, we can combine mobile and terrestrial scanning technologies. Mobile scanning will be reserved for exterior and low-technical areas. For roofs, it can even be useful to use a drone survey. Be careful, however: if very technical elements are on the roof, this may require precise scans to be carried out.
As the site can be large, it is recommended to add control points with a total station to avoid any deviation, and also to record GPS points for georeferencing.
It is not necessary to scan everything in detail, including technical areas. This global scanning is more of a framework*. I recommend subsequently carrying out small targeted scanning campaigns according to the needs of the work, or to update areas.
*my vision: in my opinion, in the future, the scan will be done in 2 steps. The first will be a global campaign done by a professional. This survey will form a backbone. Then, detailed scans and updates will be carried out by the manufacturer or its stakeholders. Indeed, scanners are becoming more popular and prices are falling. More and more engineering companies and contractors are equipping themselves. They will be able to add scan complements to the core campaign.
1.3 - Geometric controls
Check the flatness of a floor, the parallelism of overhead crane rails, the deformation of a tank, etc…
All these operations require an accurate scan, therefore carried out with a high-performance terrestrial scanner.
For high-precision operations (example: positioning a turbine axis), requiring precision to the millimeter or less, terrestrial scanners are insufficient, and it will be necessary to use a laser tracker and precision prisms.
For the inspection of small parts requiring very high precision (0.1mm or less), we enter another range of scanners, not suitable for scanning large surfaces.
1.4 - Heritage buildings
These buildings are often quite imposing and have many ornaments.
The winning combo here is terrestrial scanning and photogrammetry. Terrestrial scanning will provide accuracy, and photogrammetry will provide completeness and a very qualitative photographic texture, useful for carrying out diagnostics (presence of moss, mold, small cracks). As this type of campaign is often georeferenced, GPS points and total station work are required.
Depending on the need, we will keep the point cloud or convert it into a mesh so as to have textured closed surfaces.
1.5 - 3D point clouds and modeling
When people first discover 3D scanning, they think it's such an innovative tool that they'll automatically obtain a CAD or BIM model. However, for a whole range of operations, it is necessary to transform 3D surveys into a model. Today, this process is still largely manual, although some AIs are starting to produce results.
The more detail you require in the model, the greater the modeling effort. For a 3D survey that takes a day, modeling can take 2 days or 2 weeks. Indeed, if you need a digital model with the minimum (civil engineering, main structure, general equipment shapes), it will take much less time than if you need to model all the small networks and cable trays.
So make sure you define the scope of your CAD / BIM digital mock-up needs, as this is the element that is likely to cost you the most.
And before anything else: ask yourself if the operation really requires modeling, and if you can't achieve the objective with the right point cloud and the right tool. Sometimes, modeling is "overkill" for a need that is really limited to taking measurements or pre-positioning elements in the preliminary design phase.
Translated with DeepL.com (free version)
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2 - How to choose a service provider
Many actors in AEC and industry have difficulty approaching the consultation of scanning companies, because there are no real specifications.
I sometimes see specifications drawn up by external consultants who proclaim themselves scanning specialists, but we often start with technical unpacking aimed mainly at justifying fees, with unrealistic requirements (very high resolution on each station, precision of 2mm on the entire site...). Below, I give you the keys to properly carrying out a consultation for industrial installation scanning. I will not venture into heritage buildings or topography, areas in which I am less relevant.
2.1 - Scope of the project
Make an estimate of the surface area to be covered. If there are platforms, add the areas.
If you need to scan several separate areas (interior - exterior - attic - different production areas) you must ensure that they can be linked to each other. Indeed, imagine needing to scan a process area and the attic located above, but to reach the attic, you have to go out, walk 100m, climb a hoop ladder, then reach the attic. This creates real complexity for the scanning operator, with many junction stations, or even the use of a total station. Accessibility between areas is therefore an important point.
Very useful for consultation: if possible, send photos and any existing drawings (even if they are obsolete). This allows you to better estimate the work to be done.
Don't be too restrictive in thinking about saving money. Experience has often shown me that unplanned areas ultimately need to be captured. Out of habit, I always scan a little more than planned, just in case... But it's better to anticipate.
2.2 - Technical density
It is necessary to specify the technical density (or the congestion) of the area. The fuller the volume, the more scanning stations will be required.
A completely empty 5000m² warehouse can be scanned in less than 2 hours. If there are racks everywhere, this can easily triple the campaign.
An indicator: if I consider the scans that I have carried out on several tens of thousands of sqm of industrial installations during my career, I get an average of 1 scanning station for 10 to 15 m² of surface (knowing which I use terrestrial scanner). It is an average between very technical zones and sparsely dense zones. In production rooms (energy, process) it is not uncommon to have to do 2 stations 1m apart.
For a refrigeration compressor room, I am regularly at one station for 8m².
2.3 - Security, coactivity, severe conditions
It must be clarified whether areas are subject to special security precautions, and whether staff are present during the campaign. This can significantly slow down a campaign. Sometimes it is better to scan at night or on weekends, when equipment is down and staff are away.
In the food industry, areas are regularly washed with plenty of water. It would be a shame if a €45k scanner was sprayed.
Significant vibrations from the ground can also degraded the capture quality (the scanner head will move).
The scanners also have a range of operating temperatures. If you have to scan a cold room at -20°C, most scanners will give up the ghost unless you keep them at the right temperature.
An atmosphere full of dust or vapor will also be a problem, leading to missing or distorted data. Dust (cement works, silos) can damage the scanner's ventilation, settle on the optics, or become embedded inside.
2.4 - Items to be scanned
An area can be very dense, but you may only need the civil engineering, and not the detail of each piece of equipment.
This is especially true when an area needs to be completely emptied.
You must therefore specify what you need (civil engineering, structures, equipment, tubes with minimum diameter to capture), and the desired accuracy.
Also try to anticipate other possible needs, because as long as the operator is on site, it often does not represent much work beyond capturing a few dozen additional m².
If you need a 3D model (CAD / BIM) it is imperative to have a good capture density to achieve the required level of detail (LOD). Modelers have the worst difficulty modeling elements captured with too few points or too partial. It is therefore better to make too many stations than too few.
If the campaign is in a non-georeferenced local coordinate system, specify the origin and orientation. For example, define a corner of the building as 0,0, and define a wall as X or Y orientation. This will enable you to work with correctly oriented elements on screen.
2.5 - Color or intensity scanning
Color scanning, especially in HDR mode, is currently clearly the preference of many customers. However, this leads to longer campaigns, heavier data, and there may be photo failures (a person passes in front of the scanner when it takes photos). This requires spending more time per station, and sometimes redoing stations or photos. But scanners are improving and color scanning is getting faster and faster.
Intensity scanning makes elements visible even in the dark (the laser works in complete darkness). It can also reveal elements invisible to human eye (like paint touch-ups, patching).
In very dark areas, such as technical attics, if you want color scanning, you need good lighting. Otherwise, the scanner will take pictures in dim light.
Some scanners also include thermal imaging (option from Z+F). If you have this need, contact me, I will direct you to a specialist.
2.6 - 3D Modeling
The result of 3D scanning is a point cloud that can contain several billion points. When you want to integrate new equipment or piping, you need to be able to design the components using CAD / BIM tools. In this case, you need to transform the point clouds into a 3D model, or at least partially.
The more detail and accuracy you ask for, the more expensive and time-consuming it will be. I've known of projects that required several months of modeling, while the on-site survey only took a week.
The choice of model type (CAD, BIM + LOD) depends on your habits and internal policy for managing your models. BIM is gaining ground in industry, but traditional CAD still has quite a following.
Personally, I encourage customers to proceed in 2 steps, even if this leads to 2 quotations. The first step is scanning. Then, after examining the scans using a visualization tool, the customer points out the elements to be modeled and transmits his targeted modeling requirements to the service provider. This allows me to provide a clear framework for the service and avoid over-modeling.
Artificial Intelligence: companies are working on AIs capable of segmenting and classifying point clouds. On fairly repetitive elements and very clean point clouds, they are starting to achieve respectable results. Most of the AIs known to the general public work on 2D data such as photos or videos. The task is much more complex when it comes to 3D point clouds with occlusions and artifacts.
At the time of writing, there is still no magic AI that automatically transforms a point cloud into a CAD / BIM model.
2.6 - What to ask in the service provider's quote?
If you want to put scanning providers (including me) in competition, be sure to ask for the following details, otherwise the offers will not be comparable:
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the equipment used,
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the approximate number of stations planned and the resolutions (the most usual is 6mm at 10m from the scanner, or around 40 million points per station),
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color mode or only intensity,
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always request the point clouds, one file per station, no decimation, e57 format (the most neutral format, accepted by all scanning softwares). Ask that the scans be cleaned well. This point is important, otherwise the service provider will keep the scans only for its modeling needs, deliver you a model, and you will lose all the non-model details contained in the scans,
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the price for modeling and what the model contains,
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deadline for the campaign, and the delivery of data.
3 - The equipment
Above, I discussed the needs you may have. Here I give you some information on the equipment, with its advantages and disadvantages.
3.1 - Terrestrial laser scanners
In my opinion, this remains the king tool for industrial scanning. Accurate, whether in measuring or leveling, and delivering dense data suitable for examining details and modeling, it remains the tool of choice.
Principle:
This type of equipment includes a laser emission cell, a rotating mirror, inclined at 45°, and a head also rotating, on the other axis. As the device rotates, the beam hits the mirror and is sent in all directions, reflecting off surfaces. The principle is that of the laser range finder, but there, several hundred thousand points (or even more than a million) are captured per second. Each point has xyz coordinates. At the end of a station, several tens of millions of points are captured: this is what we call a 3D point cloud. Each station captures what it has in its field, up to the maximum range of the scanner (from a few dozen to several hundred meters). You must then move the scanner to another location to cover areas not covered by previous stations.
The stations must then be registered with each other. The latest scanners allow on-site pre-registration, but you have to refine it once you return to the office. The scanners can also take photos, leading to a photorealistic point cloud. An explanatory video (credits: Leica) is given at the end of the chapter.
There are different competing brands delivering quality data: Faro (S and Premium series), Leica RTC 360, Leica P series, Zoller & Fröhlich (5016, 5010), Trimble TX7-8-9-FX, Riegl VZ-400i, Surphaser.
The accuracy is often less than 3mm and the leveling is very precise, useful for obtaining a leveled campaign and for controlling horizontality.
The Leica BLK 360 is more compact, lighter, delivers honorable data, but still falls behind the scanners above.
There is also one thing to take into account: digital noise. High noise will give a grainy appearance to a smooth surface. On small pipes, this can give a "hedgehog" and lead to diameter detection errors.
Most scanners have various additional sensors (altimeter, compass, GPS) but they are not very precise. The scanner's internal GPS does not allow georeferencing (at least, not at the time of writing).
3.2 - Mobile scanners
For several years now, it has been possible to scan while moving. But we easily understand that the accuracy will deteriorate, because the data must be positioned in real time during walking. The point clouds are also less dense.
However, the material has progressed well. For classic buildings with few details and non-technical areas, this type of scanner can take the advantage of the terrestrial scanner due to its ease of use.
Brands: Navvis VLX, Leica Pegasus. These devices are worn like a harness. There is also the Leica BLK 2GO and the Geoslam Zeb Revo, which you hold in your hand.
3.3 - Hybrid scanners and smartphones
Real estate agencies frequently use a tripod-mounted device that may look like a terrestrial scanner, but it is more of a 360 camera with a basic measurement function: the Matterport.
Often, clients are amazed by the quality of the photos. The 3D delivered is often shown from afar, and this is where I invite you to remain vigilant: the 3D quality is insufficient, whatever the subsequent use. The measurement uncertainty is close to 2cm, and the point clouds are too sparse. We are more on a first approach, but for the project, it will be necessary to redo a real scanning campaign.
This device is, however, very suitable for virtual tours of real estate.
The latest smartphones also include a 3D capture mode, which will surely progress. For now, we can mainly use it in a personal way, to rearrange our interior furniture for example.
In the future, a good avenue for smartphones will be to combine the 3D scanning function with video capture, for augmented reality uses for maintenance or training operations.
3.4 - Drones and photogrammetry
The use of drones has literally taken a leap forward in recent years.
They are often used in conjunction with a camera and carry out campaigns of thousands of photos around buildings and also to capture terrain.
Indeed, by combining photos of objects taken from different angles, specialized software is able to reconstruct a 3D image. However, additional targets are required - captured with a total station or a pro GPS - in order to be able to deduce coordinates and distances (otherwise, the photo is dimensionless).
Photo drones are the tool of choice for exterior areas of heritage buildings, especially when combined with ground scanner stations. In industry, they are mainly used for inaccessible roofs.
Some powerful drones are also capable of carrying a Lidar to carry out aerial laser scanning.
3.5 - Additional surveying equipment
If there is a need for georeferencing, if there is no surveyor's nail on site and access to coordinates, GPS points must be added in addition to the 3D scanning campaign. A professional GPS is then used, and which allows centimetric accuracy.
It may also be necessary to use a total station in addition to 3D scanning. Indeed, in 3D scanning, errors can accumulate over long distances. If we have to scan a long road, there will be a deviation, the measurement uncertainties accumulating over the stations.
The total station, due to its high precision over long distances, makes it possible to put the scanning campaign back on track.
3.6 - The limits of 3D scanning
You can scan a lot of things with a laser scanner, but every device has its limitations.
What cannot be scanned:
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glass (except to make it opaque with a special spray like Aesub), and liquids. The laser beam passes through...
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items behind steam or dense dust: you cannot scan through it, the laser beam being largely absorbed.
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moving objects*: this leads to trails of point clouds that are unusable (but it can be artistically interesting!). This produces a bit of the same effect as car headlights in a night photo: the photo shows a light trail.
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the inside of material. This may seem obvious, but when we don't know, we think of medical scanners. Scanners used in construction capture the surface. You will not have the 3D of pipes buried or hidden by a wall.
*there is now 3D motion capture, but it is very specific. To do this, we use dozens of synchronized cameras placed in a sort of cage. We can then capture athletes performing movements in 3D.
Which is very difficult to scan:
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reflective surfaces such as stainless steel. If the surface has lived a little and is covered with a light veil, we can achieve this. But polished stainless steel is a real mirror, and the laser will actually scan the reflections and not the object. We end up with points lost in space, not very aesthetic.
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very fine elements (capillary tubes, cables). At best we obtain a string of points, but detection of the diameter is often impossible,
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Extremely dark surfaces also give poor quality results. Indeed, the scanner works by reflection of the laser beam, the latter is strongly absorbed by dark surfaces, and the return to the sensor is weak.
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Ice: the beam will penetrate the surface layer, and the return to the sensor will generally not be good. Intensity scanning also gives almost black point clouds on frosted surfaces that are white! If we scanned in color mode, this will hide the misery, but the measurement remains quite poor.
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scanning elements in the rain: the laser beam may not reach its destination because it is absorbed, giving a partial result. Soggy surfaces will not be scanned correctly - or at all.
Which gives unreliable results, or even distortions: scanning through glazing. Refraction deflects the beam on the way out and back.
Level of detail: a terrestrial scanner will be able to capture fairly fine elements such as angle irons, pipes of 15-20mm in diameter (the scanner must be quite close to the pipe). On the other hand, capturing the thickness of an angle iron is very difficult, because a 40mm wide angle iron can be 4/5/6mm thick in standards. Current terrestrial scanners will not be able to tell the difference. If you absolutely want to measure these thicknesses, you need a vernier caliper. The same goes for the bolts. Even if it appears in the scan, it will be difficult to distinguish a 16mm nut from an 18mm nut.
When it comes to photogrammetry, it is imperative to have a good geometry and texture variations, otherwise assembling photos to recreate 3D is impossible. Capturing highly reflective elements is also complicated, because reflections are taken in photos.
Photogrammetry can go into great detail, but it requires a very large quantity of photos. If there is not enough, the edges will appear "fat", as if they had melted.
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