Skip to 0 minutes and 6 seconds GREGORY DUNN: Hello. I’m here with James, and we’re going to talk about laser scanning. James, what is laser scanning?
Skip to 0 minutes and 12 seconds JAMES MILES: It’s a method to record objects in a 3D environment.
Skip to 0 minutes and 15 seconds GREGORY DUNN: Interesting. What kind of laser scanning do you do here at Portus?
Skip to 0 minutes and 19 seconds JAMES MILES: Well, we’ve done several different types of laser scanning. The one here is called phase scanning, but we’ve also used triangulation, and time-of-flight, and LiDAR. Phase scanning is used here simply because it allows for quick recording method here, from which we can then produce quite accurate models very quickly.
Skip to 0 minutes and 39 seconds GREGORY DUNN: So it’s an accurate method, then, to record.
Skip to 0 minutes and 41 seconds JAMES MILES: Oh, it’s very accurate. The settings we’re using here, this has a resolution of 10 metres to record an object every 1.5 millimetres.
Skip to 0 minutes and 48 seconds GREGORY DUNN: So, what kind of information does it actually record, then?
Skip to 0 minutes and 52 seconds JAMES MILES: It records everything it can see. So, basically, this goes round 360 degrees, and has a field of view from minus 60 to 90. It goes around in a circle, and it records everything it can see within it. So, we have to take a series of different scans along different parts to make sure we capture everything.
Skip to 1 minute and 10 seconds GREGORY DUNN: And are there any downsides to laser scanning?
Skip to 1 minute and 12 seconds JAMES MILES: There’s a few downsides. Depending on the scan that you’re using, it takes a very long time to capture the data, and it also takes a very long time to process the data as you’ll see in a bit.
Skip to 1 minute and 22 seconds GREGORY DUNN: Interesting. And I notice that there’s a, like a little orb back there. What does that do in relationship to the laser scanner?
Skip to 1 minute and 28 seconds JAMES MILES: Each scan is unique, and they need a combination of different targets. So these spheres are targets, and then from this we can then stitch the multiple scanned data together.
Skip to 1 minute and 39 seconds GREGORY DUNN: Interesting. Thank you. What can we do with the data?
Skip to 1 minute and 42 seconds JAMES MILES: Well, because it gives us a very accurate representation, we can do very precise measurements. We can create animations of the data coming through, we can manipulate the viewpoint from different angles. And then we can use the data we have here for the beginning of the virtual reconstruction process for the building.
Skip to 1 minute and 58 seconds GREGORY DUNN: Now I’m back inside with James, and we’re going to look at the data that he has downloaded. So, now that you’ve downloaded it, what do you with the data?
Skip to 2 minutes and 6 seconds JAMES MILES: Well, we’ve downloaded the data. And what we’re doing now is, we process the data here. But there are several stages to process the scan data. First is, that you have to pre-process every single scan. What this does, is it finds the targets within the scan data. And it’s also able to find lots of corners and straight edges. So, these features here, are overlapping features in the scan data, which the software then uses to process. After pre-processing has done, we then register all the scan data together. And then we’re left with an overview of the scans.
Skip to 2 minutes and 40 seconds GREGORY DUNN: So you can stitch together different, like when you do a laser scan of one area and then you go down and do a laser scan of another, you can stitch them together in here, so they’re seamless.
Skip to 2 minutes and 50 seconds JAMES MILES: Yeah. So, these targets, which we used here, so these little green blobs that we have here, those are the white targets that we used earlier on. Now, what the software does, as you can see from here, it has an initial scan data from one side. Because it knows the diameter of the sphere, it can then create a circular object for us. And through a series of these different targets going along, we can put multiple scans together.
Laser scanning buildings
In the previous steps you saw how much information is held in the fabric and form of our Roman buildings. Laser scanning allows us to record a dense cloud of three-dimensional points and colour values to capture some of this information, for buildings, excavated features, and objects.
The laser scanning at Portus has over the years developed from small object scanning to large architectural scans. The scanners employed at Portus have been a Vivid 910 triangulation scanner (small object and exterior tests), a Leica Scanstation 2 and a Leica Scanstation C10 both of which are time of flight scanners.
Testing a Minolta scanner at dusk on site, early in the Portus Project - Hembo Pagi © University of Southampton
We have even tried very experimental techniques such as using a Microsoft Kinect (normally used for gaming) to scan areas inacccessible to the team. In one experiment we mounted the Kinect on the end of a five metre pole that Peter used to scan a room deep underground in the Imperial Palace.
Scan data showing the inside of a deeply buried room, created using a Microsoft Kinect and KinectFusion as part of the PATINA Project - Peter Wheeler © University of Southampton
Each type of scanner offers a different resolution of data recorded. In 2013 we began to use a Faro Focus 3D which is a phase scanner. In 2014 we are using an updated version of this system providing longer range scanning, better colour calibration and integrated global positioning.
View of the basement level of the Imperial Palace as if the first storey and vegetation had been removed - James Miles © University of Southampton
The technology works by sending a laser beam out towards an object. Time of flight calculates the time taken for the laser beam to rebound back to the scanner and converts this to distance via our knowledge of the speed of light. Phase scanners compare outgoing and returning signals and provide better accuracy.
Faro Focus scanner at Portus, provided by Opti-cal Survey - Hembo Pagi © University of Southampton
Using a laser scanner allows for an accurate three-dimensional representation of the object scanned. This can then be used as a tool for investigation: line data can be extracted, volumes calculated, animations (such as those at the bottom of this page) can be created, and changes over time can be observed. Laser scanning, like LIDAR, relies on line of sight but by setting up the scanner in multiple positions we are able to create a complete model. We can then place ourselves virtually within this in any position and look in any direction, including through surfaces.
The Faro Focus 3d that was used on site in 2014 has an accuracy of ±2mm, a range of 0.6m to 120m and the measurement rate of 976,000 points per second. It has an integrated camera to capture RGB values per point, which allows colour to be added to the model and can be used in both light and dark conditions. The laser scan models already completed have over a billion points and are linked together with the Total Station data and the photogrammetry data to create a detailed model of the buildings and excavated areas.
The laser scan I produced of the basement level allows us to record details when we need them, from the comfort of the lab. At 45 seconds in you can see a photo of same view of the basement in the sixth image of this slideshow from the BBC News website (2009).
Laser scan of a room in the Imperial Palace - we can revisit these data as needed to measure and draw specific components - James Miles © University of Southampton
© University of Southampton