How Accurate Is 3D Scanning?

Are you asking yourself how accurate 3D scanning is? Have you found until now confusing numbers that you cannot understand and that vary from one source to another one? This is because there is not a unique answer and many factors come into play. But we are here to help!

For aerial uses such as terrain surveying, both LiDAR and photogrammetry can provide an accuracy of 1 to 3 cm (0.4 to 1.2 inches). When scanning at very short distances like 10 cm (4 in) away from the object, the accuracy of LiDAR can be as precise as 10 μm, less than the thickness of a hair.

To learn more, we invite you to dive deep into the details that will let you determine what accuracy can be achieved in 3D scanning depending on the use case, technology and other factors. Keep reading as we show you those details and how they can help you realize whether it is worth it to invest in one scanner option or the other.

How Is Accuracy Measured

By definition, accuracy involves comparing a measured value to what is considered the real value. The different ways how accuracy is measured are described below.

  • Sigma. The most common way to measure accuracy is by taking a series of measurements and using what in statistics is called normal distribution or Gaussian distribution. This method provides what is known as relative accuracy and is usually expressed as 1σ (to be read as “one sigma”) value. The 1σ is known as “standard deviation”, a statistical value meaning that 68% of the measurements will be inside a given range. For example, if 1σ = 1mm, that means that 68% of the measurements taken will be less than 1mm to the real value. There are also values for 2σ (95%) and 3σ (99.7%).
Sigma Percentage in range
  • Millimeters. Directly measuring the deviation of the real values is a way of measuring absolute accuracy. In this case, millimeters (mm) are the units of measurement typically used..
  • Parts in X. In addition, the absolute accuracy of photogrammetry will depend mainly on the scale used and how it is used, since the technology does not have a scale by default. So, when speaking about the accuracy of photogrammetry, it is common to express it in the form of relative accuracy. For example, the lowest accuracy is usually described as 1 part in 100 and a higher accuracy would be 1 part in 30,000.
  • Some manufacturers provide other accuracy measurements. For example, Nikon gives the accuracy as the Maximum Permissible Error (MPE), following more complex formulas.

Due to how differently accuracy can be measured, it might be difficult to compare equipment from different manufacturers. However, in the following section we have gathered some numbers so you can get a rough idea.

Accuracy According To Specifications

Manufacturers provide their own specifications in order to highlight what they consider the features that make their product stand out from the rest. This is also true for describing accuracy.

Here are some popular 3D scanning models and their specified accuracy to give you a general picture:

  • The scanners ModelMaker MMDX from Nikon have a relative accuracy 1σ from 7 μm for the model MMDx50 to 24 μm for the model MMDx160.
  • The Faro Freestyle 2 is a handheld scanner with an absolute accuracy of up to ± 0.5 mm (0.02 in).
  • The Faro Laser Scanner Focus X130 has an absolute accuracy of ± 2 mm (0.08 in).
  • The accuracy of the Nikon Laser Radar APDIS MV4x0 is expressed in MPE, as explained before. Its accuracy at 2 meters (6.6 feet) distance is 29 μm.
  • The Faro Focus 350 is a long range scanner, being able to detect points as far as 350 m (383 yards) with an absolute error of ± 1 mm (39 in).

The important thing to do with specifications like the ones above is to consider the case in hand and the aspects that may affect the accuracy described. Let us explain these aspects better in the next point.

Aspects Impacting Accuracy

There are several aspects that have a significant impact on accuracy. When thinking about how accurate 3D scanning is, you should also think about the following points.

How Far Away Will Be The Object Or Area To Be Scanned?

When the accuracy is given as an absolute value, for instance in mm, the distance of the scanner to the object is very important. This distance is usually related to the size of the object or area to be scanned. If a big ship is being scanned and has some of its points 100 meters (109 yards) away, a 3 cm or 1 inch deviation might not be a problem. However, if you are scanning an object that is in total 20 cm large, even 0.5 cm or 0.2 in will represent a big deviation from reality. Typically, there is a compromise between accuracy and distance reached. The further away it can scan, the less accuracy it will have. The closest it can scan, the more accurate it will be.

How Does The Technology Used Affect Accuracy?

As it happens in many other fields, even the best technology can provide bad results if it is not used for the application and for the conditions it was designed. For example, while standard tires might work perfectly on the streets of a city, they might not work as well on roads with ice or snow after a tough blizzard. And the same goes for 3D scanning.

There are different technologies for 3D scanning. As we mentioned at the beginning of this article, in certain cases the technologies can offer similar accuracies. However, there are also some differences…

LiDAR (Light Detection And Ranging) systems, sometimes also called laser scanners, have many different uses and ranges. In addition to the 1 to 3 cm accuracy of the best scans for aerial uses, other studies show 15 cm (6 in) accuracy for terrain elevation data in open areas. However, this accuracy might be affected when using the technology airborne over dense forests. When it comes to scanning at very short distances like 10 cm (4 in) away from the object itself, as we explained at the beginning of the article, the accuracy of LiDAR equipment can be as precise as 10 to 100 μm (which is 0.01 to 0.1 mm; or 0.0004 to 0.004 in). That is even less than the thickness of a human hair!
Photogrammetry has the advantage that it is able to capture both shapes and textures, since it uses photographs. Its accuracy depends not only on hardware, where very high-quality cameras and great computing power to process the photographs properly are needed, but also on having the right software, all things that may not be always accessible.
Contact Scanning
Contact scanning is a technology that may sound like the go-to option when it comes to accuracy. What could possibly be more accurate than directly obtaining the profile of the object by touching it? The truth is that, while it can actually be very accurate, some accuracy problems can arise while using this technology. Achieving the highest quality possible with contact scanning depends on having a trained operator to control the probe and give it the right motion. Also, the speed of the movement is important, and manually moving the probe at a constant speed is not always easy. Of course, there are solutions like using a robotic arm, but this can be quite expensive.

How Important Are Supports?

Although technical specifications are the first thing anyone would verify, this is a factor which has a high impact on the accuracy of 3D scanning. Consider the following questions:

  • Will the scanner be handheld or placed on a tripod?
  • Will it be controlled by a person or a robotic arm?

Similar to what happens with standard photography, handheld 3D scanners require a higher level of skills and experience to achieve the best possible result. On the other hand, when using special supports such as tripods or articulated arms, it is easier to keep consistent accuracy.

Nevertheless, even when using articulated or robotic arms, there are small deviations from the specified maximum accuracy that must be considered and added to the one of the scanner itself.

For example, Nikon provides some examples in their specifications: the laser scanner model MMDx50 working with their robot arm model MCAx20 gives a 1 sigma deviation of 50 μm.

While it may look as a very small value, this kind of deviations can be significant in high precision applications.

Is Lighting Relevant?

When it comes to assessing the environment around the object being scanned, lighting is an important factor to consider before choosing one technology over another.

For example, photogrammetry depends on having the right lighting set up to achieve the highest accuracy possible. When taking pictures in a very bright environment, it will be more difficult for the software to match and overlap different pictures, and therefore the accuracy is reduced. A similar thing happens when the light is too low or there are shadows.

In addition, incorrect lighting also affects the accuracy of texture representations in photogrammetry.

In the case of laser scans, it is not only the light in the environment that has an impact on the accuracy of the scan, but also the one emitted by the scanner. Light is a mixture of different wavelengths and some of these have more problems dealing with interference.

It has been proven that infrared light, which has a longer wavelength ranging from 700 nm to 1 mm, is a better option to obtain high accuracy as it is more resistant to interference.

Do I Need To Calibrate My 3d Scanner For Optimal Accuracy?

The answer is yes. Although 3D scanners come factory calibrated for the highest possible accuracy, it is important to verify their calibration periodically and perform any correction required.

To better illustrate what this involves, consider the case of photogrammetry. Calibration in photogrammetry means:

  • Determining the focal length
  • Setting the format size
  • Setting the main point
  • Configuring the distortion of the lens

Keep in mind that some cameras with very wide angles or fish-eye lenses will not calibrate as expected, which results in a reduced accuracy.

What Is Photo Redundancy And How Does It Affect Accuracy?

This is a special aspect when using photogrammetry. Redundancy makes reference to the number of images where the same points of data appears. In simple words, the higher the number of images with the same data points, the easier it is to process such data and the more accurate the final result will be. For example, if points are only visible in 2 images, the resulting scan is usually in the lowest accuracy range (1 part in 100), while including the same points in 8 or more photos will increase the accuracy to the highest ranges, somewhere around 1 part in 30000 or even better. Of course, this comes in expense of time, so you might want to find the best balance between accuracy and time spent to obtain your 3D model.

Accuracy Or Resolution, Which Is More Important?

It is important to clarify that accuracy and resolution are two completely different things. Although many people believe these terms can be used indifferently, accuracy represents how close to the real object is the resulting model after the 3D scanning process. On the other hand, resolution represents the number of points considered to create the model and how distant are those points to each other. It is also important to highlight that a higher resolution does not necessarily mean a higher accuracy. There are scanning technologies that only require capturing a small number of points from the object being scanned (low resolution) to generate an almost exact 3D model of the object (high accuracy). However, it has been found that there is an improvement in accuracy when the resolution of the camera is higher when it comes to photogrammetry. Moving from an 8 MP – 3264×2468 camera to a 20MP – 5400×3600 camera may result in an increase of relative accuracy from 1 part in 5000 to 1 part in 30000. These numbers are approximated, but they definitely give a good idea on how big the accuracy improvement can be. Therefore, both accuracy and resolution are important, but you may want to consider what your requirements are. Perhaps you may do well with a low resolution scanner if you only require to quickly scan small and simple objects.

Final Words

And with this, we have arrived at the end of the article.

Do you have any remaining questions about the accuracy of 3D scanning? Did we miss any points that you think are very important and have not been added to the post? Do you need consulting advice?

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