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Online waveform processing in Terrestrial Laser Scanning- what exactly is it?

By 15/01/2016
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Echo digitization scanners are not new to the market, introduced as state of the art technology first in 2004 for Airborne Laser Scanning (ALS) then implemented into Terrestrial Laser Scanning (TLS) in 2008. In recent years RIEGL has introduced many models of these new generation terrestrial laser scanners in its V-line, which make use of echo digitization and online waveform analysis, and are some of the only terrestrial laser scanners to do so.

Whilst you might have come across echo digitization and online waveform analysis before, do you know exactly what it means, or what those expressions stand for? Let me explain...

Echo digitization is the process that happens before target detection can take place so also before any online waveform analysis. An analogue return from a single emitted laser pulse (or echo signal) is being sampled and converted into digital form; this process is performed by analogue-to-digital converters (ADC). Echo digitization allows us to then proceed with more advanced signal processing, referred to here as online waveform processing, which is used to derive multiple pieces of information from the data.

Still a bit confusing? Let me go to the online waveform processing in more detail.

Online waveform processing is a digital signal processing specific for laser scanning. What is so special about it? All the necessary information about our targets can now be extracted, thanks to this process. It delivers target range, amplitude, reflectance and pulse shape deviation. All this information is acquired for each of the detected echoes. They can be used further in many different applications and functions. As the name ‘online’ indicates, the whole process is done in a real time within the scanner hardware.

Now, let’s get back to RIEGL and their amazing V-Line scanners, capable of all the processing mentioned above and much more. By uniquely implementing the online waveform processing, RIEGL hardware is capable of determining the range of all targets that the single laser pulse meets on its way. It is possible to discriminate them as long as the echo signal amplitude is above the detection threshold within the instrument. This feature is called Multi-Target Capability and it brings terrestrial laser scanning to a completely different level. It enables us to acquire 3D data that is much more detailed, reliable and robust. This was not previously available for laser scanners working with analogue signal detection.

With the capability of performing 42000 measurements per second in a long range mode (100kHz at 2000m range) and 396000 measurements per second in super high speed mode (950kHz) the new VZ-2000 from RIEGL (launched just in 2014!) is able to capture a practically unlimited number of targets per pulse. That is pretty impressive!

The pictures below will help to understand the processes described better. They are taken from the RIEGL VZ-2000 manual. (RIEGL, 2014).

Figure on the left shows echo signals of different types of target. Graphs represent data acquisition in more detail. We can see digitization process and target extraction. (RIEGL VZ-2000 manual - General description and Data Interfaces 2014)

Figure on the left shows echo signals of different types of target. Graphs represent data acquisition in more detail. We can see digitization process and target extraction. (RIEGL VZ-2000 manual - General description and Data Interfaces 2014)

Maybe now we can try to understand the information that can be derived from the RIEGL V-Line scanners. I mentioned the target range previously, which can allow for Multi-Target Capability. There is also Amplitude and Reflectance which have a lot of very useful applications, and Pulse Shape Deviation - highly important. I would like to show you examples and short explanations for each of the mentioned features. It is amazing what we can do with all of them.

Let start with Multi-Target Capability. As I described before, V-Line scanners are able to capture multiple targets that intersect the laser beam. This is useful for a number of different reasons; scanning and capturing useful data from highly vegetated areas, scanning cultural heritage sites with multiple obstructions, or for better surface investigation when used for monitoring purposes. There are many more but for now let’s focus on the example of the Multi-Target Capability on a cliff face. The data shown was captured by 3D Laser Mapping.

Multitargetcapability

Above you can see a scan of the cliff face coloured by echo (or target). For measurements where only one target has been detected, the points are coloured green. For measurements with multiple targets, the first target is yellow; the last target is blue; any intermediate targets are light blue.

Clearly all the vegetated area is picked up and can be deleted from further operation. For instance we can use it while creating surface out from the point cloud.

Oblique view of the edges area. Traditionally, laser scanners will record one point midway between the first and last echoes recorded by the VZ-400, Image on the right shows the same area, coloured from photos for reference.

Oblique view of the edges area. Traditionally, laser scanners will record one point midway between the first and last echoes recorded by the VZ-400, Image on the right shows the same area, coloured from photos for reference.

The correct detection of the two following echoes depends on the sampling rate, and the time taken for the receiver to “reset” after each echo is received. Within RIEGL V-Line laser scanners, the multi-target resolution is equal to around 0.8 m. Targets situated closer together than this value might result in 'false points' in between.

Again, the online waveform analysis enables us to reduce the issue by introducing information about 'pulse shape figure'- Pulse Shape Deviation, another example of the information that can be extracted from the online waveform processing. Each echo signal comes back with its pulse shape, this pulse shape is compared to the expected and undistorted pulse shape (system response pulse shape). This function forms additional attributes to each point within the point cloud. If there are low values it indicates that the returning pulse shape does not differ significantly from the expected one. If however there are high values of the deviation between the pulse shape of the returning echo and the system response then it means that it may be caused by merging echo signals from several targets that were close to each other. The system in that case can notice that the waveform between those points is slightly stretched and therefore the deviation values are higher. The values are stored and ready to use while filtering. This value of deviation caused by false points can be even 10 times higher than the regular value. This is all possible again only because of online waveform analysis.

Please take a look at this example of the staircase. The upper part is clearly distorted. The laser beam and the surface create a very sharp, small angle. It ends up with false points in between. You can easily notice that all the false points are picked up and filtered out within one simple tool.

'Conventional' laser scanners would not be able to detect the edge and the wall separately, but with the RIEGL V-Line, edge detection is possible.

'Conventional' laser scanners would not be able to detect the edge and the wall separately, but with the RIEGL V-Line, edge detection is possible.

Another very important feature existing within echo-digitizing scanners is the availability of Calibrated Amplitude and Reflectance for each target. The amplitude is defined as a ratio of the actual detected amplitude of the echo signal versus detection threshold of the instrument. Reflectance is equal to the ratio of the actual amplitude versus the amplitude of a white, diffuse target. Each point within the point cloud is assigned with the brightness value respectively to its reflectivity. Objects that have the same reflectivity will have the same brightness independently of the scanner distance - so it means that our Reflectance becomes distance independent. Wow!

This would be very useful to improve the object classification. Trunks of different species of trees can be recognized, lithological sequences in the cliff faces can be distinguished, and classification of vegetation and solid structures is possible. Let’s have a look on those images of cliff face coloured accordingly to reflectance.

The image above, shows a cliff face coloured by Reflectance. The Reflectance is distance independent and can be used for further processing.

The image above, shows a cliff face coloured by Reflectance. The Reflectance is distance independent and can be used for further processing.

Enlargement of an area at the right hand side, showing the even “reflectance” across the cliff face and harbor wall.

Enlargement of an area at the right hand side, showing the even “reflectance” across the cliff face and harbor wall.

Because it is independent of range, the V-Line Calibrated Reflectivity may be used to help identify non-continuous structural patterns and lithological sequences in separate areas of the scan.

Calibrated Amplitude will also allow us to perform measurements in many different weather conditions. Below there is an example of scan data influenced by strong rain. All points that represent rain drops are easily filtered and deleted thanks to online waveform processing. We can use either the Deviation filter (mentioned previously) or Reflectance filter. This makes it a fast and easy to use system.

Image source: Ananda Fowler and Tan Nguyen, RIEGL USA: ‘Laser scanning and recent 3D technology, online waveforms and attributes.’

Image source: Ananda Fowler and Tan Nguyen, RIEGL USA: ‘Laser scanning and recent 3D technology, online waveforms and attributes.’

It has been proven that outputs from ‘conventional’ scanners and those with online waveform processing differ between each other. The latter ones contain many more points which leads to better quality of the output. Other additional information captured during the scanning like Calibrated Amplitude or ‘Pulse Shape Figure’ play an important role as well.

This system saves time and therefore, money. It gives reliable and accurate data that can be used in many effective filtering operations. We are not limited by any vegetation condition nor a poor weather forecast.

I’m looking forward to seeing further developments of RIEGL. They have amazing ideas! - Joanna

 

3D Laser Mapping are proud to be premier distributors of RIEGL scanners, since 1999.

We will be hosting an event in November, that gives you the chance to learn more about 3D technology, including the RIEGL V-Line scanners. For more information, click here.

 

If you want to find out more drop us a message. You can also learn more by reading following papers. They helped me with better understanding this whole feature:

- M. Doneus, M. Pfennigbauer, N. Studnicka, A. Ullrich, “Terrestrial Waveform Laser Scanning for Documentation of Cultural Heritage”, CIPA 2009

- A. Guarnieri, F.Pirotti, A.Vettore, “Comparison of discrete return and waveform terrestrial laser scanning for dense vegetation filtering”, ISPRS 2012

- Ananda Fowler and Tan Nguyen, RIEGL USA: ‘Laser scanning and recent 3D technology, online waveforms and attributes’.

 

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