I am happy to announce that the full text of Making People Believe is now online (without images due to restrictive copyright agreements).

Feel free to discuss the article in a comment below.

The use of terrestrial 3D laser scanning devices is increasing in all surveying areas including topographic surveys and building recording. However, a dichotomy exists in that the deliverables produced by such surface-rich acquisition devices generally tend to be surface-less clouds of points.
This article discusses the use of a Callidus 3D laser scanner on a multi-phase site at Knowlton, Dorset. This site originally featured a Neolithic henge earthwork (circular bank and ditch) with two causeways crossing the ditch. Later, in mediaeval times, a church was built in the centre of the henge to effectively Christianise the pagan monument.

The purpose of the exercise was to acquire not only a complete dataset of the earthwork for topographical analysis, but also a high-resolution scan of the church fabric itself.

Figure 1: Knowlton Church and Henge. The Callidus 3D laser scanner can be seen to the left edge of the photograph

Historical and Archaeological Background

Knowlton Henge as we know it today is in fact part of a larger group of henges, which date from the Late Neolithic (approx. 3000-2400BCE). The Church Henge, the subject of this study, is the best preserved, with substantial earthworks surviving to this day. It is an oval enclosure, roughly 107m by 98m with a ditch surrounded by a bank. The bank is up to 1.8m high, and the ditch up to 11m wide and up to 1.2m deep in places. The remaining henge monuments have been ploughed out or are no longer directly visible, although their locations are known through aerial photography and excavation.

The Church Henge is particularly interesting, since the remains of the medieval church stand in the middle. The chancel and nave were constructed in the 12th century, with the subsequent addition of the north chapel and tower in the 15th century. It is constructed mainly of flint, with ashlar dressings of Greensand and Heathstone. In the 18th century the roof collapsed, and the church fell into disuse, leaving us the ruined structure we have today.

Creating an accurate record is essential to enable archaeologists to effectively manage the preservation of such a monument. Surveying the henge monument by laser scanner gives us a great advantage. Even with DGPS (differential global positioning system), the highest practicable resolution obtainable is 0.5m. Whilst this method is useful, it is possible for more ephemeral details to be entirely missed. As shown with Knowlton, laser scanning allows a much greater amount of information to be collected. This may be meshed and used for analyses such as cross sections, contour plots and lighting analyses by archaeologists to gain a greater understanding of the form and function of the monument or object in question.
The structure of Knowlton Church is badly eroded in places, and emergency conservation works have been undertaken from time to time. It is important before any work begins to a historic building such as this, that a survey takes place as a record to allow future researchers to understand what has happened to the building, its state before and after restoration/repair, and to allow a better understanding of the structure.
Over the centuries the lime mortar has eroded, and flints have fallen away, leaving few straight edges for the surveyor. This presents a particular theoretical problem – the surveyor must make a decision as to what to record as an edge on the plan. Where there is no definite edge, an interpretive process begins; an interpretation is often recorded where time on-site is limited. This again is where the laser scanner can begin to become essential to archaeologists in the future, enabling an objective record (governed by the accuracy of the equipment) to be captured rapidly in the field.

The henge complex has been the subject of a series of ongoing investigations by Bournemouth University. The most recent survey undertaken being a GPS survey in 1995. This appears to have a resolution of around 1 metre with an unknown vertical accuracy. Additionally, due to the constraints of GPS technology, the church itself was not included in this survey.

Scanning Methodology

The methodology typically used for terrestrial 3D scanning revolves around the time-honoured use of prisms or targets. These are typically surveyed using conventional means, such as a TotalStation, and then scanned with the laser scanner.

This methodology enables the processer of the data to quickly register the separate scans to an existing control network using the known position of the targets within the control and the scanned position of the targets. A transformation to move scans from their local coordinate system to that of the control network is then trivial to generate.

The downside to this approach is that the actual scan data itself is generally ignored in favour of targets. As such, the registration between scans is really only as accurate as the surveying of the targets. Additionally, it requires the positioning of targets around a structure of site that might not be suitable for the use of targets. A scheduled monument, such as Knowlton, is such a case.

As such, we decided on a target-free acquisition of data for the following reasons:

1. The Callidus scanner has a high resolution (or sampling of data) and a good accuracy (+/-8mm), although we did not think the accuracy good enough for high-quality sphere-fitting.
2. The scanner can acquire around 1000 points/second. This would enable us to take multiple 360 degree scans of the church and henge with enough overlap to use an alternative data-driven registration technique.
3. The church and henge are both scheduled monuments. The use of targets when scanning the church may have required targets being applied to the masonry. This may be completely unacceptable for a scheduled monument.

Therefore, our scanning strategy was defined such that we would:

1. Acquire three 360 degree scans from inside the church itself, one per �compartment� of the church
2. Acquire multiple 360 degree scans around the church between the church and earthworks
3. Acquire multiple 360 degree scans from the bottom of the henge ditch
4. Acquire multiple 360 degree scans from the top of the outer henge bank

This strategy would enable us to ensure that we had considerable overlap between scans and acquire data for the entire church and earthwork from multiple viewpoints.

Figure 2: Plan view of the registered point cloud data. The individual station locations can be clearly seen as blank circles

Acquisition

Once the scanning strategy was decided, the acquisition process is simply a case of moving the scanner to its new position and setting it running. We decided to use a 0.25 degree angular stepover for our scans. This enabled us to acquire a full 360 degree scan in around 15 minutes. We also acquired a few 360 degree scans with a higher horizontal resolution to compare the results.

To support our target-free alignment strategy, we ensured that our scans had around 10-15% overlap between neighbouring scans. This would enable us to use a data-driven registration based on matching points between overlapping scans. This did not considerably add to the acquisition time, in our opinion, and generated a far denser survey than ever before.

In total, we spent around 10 hours on-site, around 8.5 acquiring data and 1.5 for setup and teardown of the equipment and moving the scanner between scan positions. We acquired 32 scans (plus 3 test scans at a higher resolution) containing a total of 17,086,377 points.

Processing

Once the dataset was acquired, it was necessary that we process the raw Callidus data into a format more useful for metrology and visualisation purposes. Our ultimate goal was not to produce the almost obligatory point cloud from our dataset, but to generate a fully surfaced mesh. The benefits of this approach include:

1. The finished dataset is visually superior for interpretation and visualisation purposes
2. Metrologically, the topological information present within a meshed dataset is critical for making accurate measurements, especially sectional information
3. Registration inaccuracies are clearly visible as �cracks� or �breaks� in the mesh whereas registration error in point clouds can easily go unnoticed
4. The mesh can be built with only the �best� points acquired with the scanner, as opposed to all points, a good proportion of which will be of low quality

Our first stage was to produce accurate registrations between the scans. This was achieved by using a least-squares fit algorithm firstly on manually selected matching point pairs on overlapping scans and secondly by running an iterating algorithm which automatically converges the overlapping scans. This process, running under our own Demon3D software, took approximately 3 hours to complete and produced registrations of around 4mm RMS deviation.

Once our registrations were as good as we could make them, we meshed the dataset using Demon3D‘s high-performance 3D fusion routines. This process was done in a piecewise manner enabling us to discard points that we regarded as being of low confidence.

An Aside on Quality

It is assumed that all points measured by a laser scanner are empirically accurate. This is incorrect,especially in the case of time-of-flight terrestrial scanners. The metrics that affect the accuracy of points include the distance of the scanner from the object (for non-collimated beam scanners or scanners within autofocus capability); the angle at which the scanner points at the surface; the signal return intensity and the optimal distance at which the scanner works.

For example, a point of high confidence (i.e., one which we are fairly certain is accurate) can be classified as one which is around 10 metres from the scanner, is directly facing towards the scanner and is of a non-reflective substrate. This ensures the optimal signal return and minimises the possibility of a �bouncing pulse� in which incorrect range data is generated. A point of low confidence could be classified as being at the maximum or minimum range of the scanner and at a glancing angle. Sloping roofs when scanned from the ground are excellent examples of potentially low confidence points. Additional weighting of a point’s confidence can also be introduced by the light conditions at the time of the scan. A scan acquired during the day may be less accurate than a scan acquired during twilight or night time.

All these factors lead to a notional degree of confidence in the accuracy of any given point and this confidence weight should be used during both registration and merging of scans into a complete mesh.

Processing (contd..)

The final stage of processing was simply to clean the mesh by removing redundant triangles, triangles that intersected others, isolated vertices and other topological incongruities. We also filled any small holes present in the mesh. Once the main processing stage was completed, we decimated the dataset down from around 48 million triangles down to just under 4 million. This was done simply to enable the model to be deployed on standard PC hardware.

Final Thoughts

In summary, the Callidus scanner made it straight-forward to survey the church and henge to a very high resolution and accuracy.
In short, we have demonstrated that 3D laser scanning is a powerful and relatively inexpensive technique for accurately surveying archaeological topographic features, especially extremely subtle earthworks such as the churchyard boundary found at Knowlton Henge. Additionally, scanning provides an excellent snapshot of exactly the condition the site was in during the survey as evinced by the paths worn into places on the henge bank and the unmown grass in the ditches.

Figure 3: Plan view of the surfaced and rendered Knowlton Church and Henge. The internal earthworks associated with the church boundary wall are clearly visible as are pathways round the top of the bank and bottom of the ditch.

This density of data affords the archaeologist huge potential for monitoring deterioration in these fascinating and sometimes threatened landscapes.

Figure 4: Perspective rendering of the surfaced dataset showing the church fabric

Acknowledgments

The survey at Knowlton Henge would not have occurred without the assistance of David Hadden of Trimble for loaning Archaeoptics the Callidus scanner used in this survey and Duncan Coe of West Berkshire Council (originally the English Heritage Inspector for Ancient Monuments of Dorset) for granting us permission to undertake the survey.

Knowlton Henge earthworks and church are under the guardianship of English Heritage and are a scheduled ancient monument.

Bibliography

Reference: An Inventory of the Historical Monuments in the County of Dorset. RCHME, pp. 111-116. Volume 5, East Dorset. London: HMSO.

More to follow

Further renders and possibly an animation of the model.
Books on henge monuments at Amazon…

Technorati Tags: 3D, archaeology, article, church, earthworks, henge, history, laser scanning, lasers, surveying, topographic, topography

Static sites, stagnant sites

In the year 2000, when I begain my masters degree in archaeological computing (and coupled with a connection to SuperJaNET – the high speed academic network), I began to experiment a little more with web publishing, and found that the projects I was working on were worthy of a mention on the net. I revamped my website yet again, and began to put 3D renders of some work on there, and a few descriptions of what I was up to and some of the techniques I was working on. But I had no traffic. Manual search engine submission didn’t seem to work, links from some friends (equally obscure) websites, didn’t bring many visitors. It made me wonder what the point of it was, especially as I was hand coding everything, and was a considerable time investment.

‘Web logs’

I began to hear about ‘web logs’ at about that time, and had come across some ‘diary’ websites offering free accounts. I didn’t really take much notice of them as it didn’t seem like a system that could be used for much else than, well, a diary. Being fairly technically minded, I also looked down on them as being very limited systems, with uniform looks. I think that I actually closed my mind to them, and didn’t really watch what was going to happen to them.

Evolution and XML

In about 2002, I began to take notice of ‘web logs’ again – in that the name had now evolved to “blog” in the net-savvy mainstream (I’m certain it had been known as this much much before then, in certain circles). RSS and blogs appeared hand-in-hand, as a way of aggregating multiple sites together. I could see it going places. I bookmarked several blogs, and while I couldn’t find a decent RSS reader back then, I read them regularly in a browser, whenever I remembered. Some blogs went for ages without any posts, so I checked them less often, and forgot about some of them after my computer died.

Over the edge

After the death of my computer, which pushed me over the edge into the world of Apple computers (quite possibly the best computing decision I have ever made), I decided that it was time to pull myself together and get a ‘proper’ website. But how would I do that? How would I create a site where I could, once and for all, concentrate on the content, rather than mess about with Dreamweaver of hand-coding every time I wanted to put something online?

My mind drifted back to the blogs I had read, and back to the diary websites of the 1990s. Blogger.com was up and running with already large numbers of blogs (in 2003), and hadn’t (I think) at that point been bought by Google. Something interesting was happening to the web.

Democratisation of the web

It dawned on me that anyone could now set up their own website. Not in the Geocities ‘site builder’ sense, but powerful websites, blogs, that allowed interaction. Suddenly (relatively speaking), anyone could write about anything, and have their say about what someone else says, whilst linking the two together (pingbacks/trackbacks). Complete strangers could leave comments. The democratisation of the web was happening.

The power of blogging

I’m writing this entry using WordPress, a superb open source blogging platform. I only discovered WordPress last year, when it was at version 1.2. I was astounded by the community that used and supported it, so much so that when researching its use, I decided to leap in at the deep end and just give it a go. Admittedly, installing it, whilst very simple for me, is still a long way from being easy for everyone. But I have been extremely impressed with its reliability, support, and functionality that has really woken me up to the power of blogging.

Many of the people reading this will know this already, but it is quite exciting when you consider where these technologies are going. The combination of XML/RSS and blogging was a natural one – a blog is a way of sharing information, of easy and quick web publishing, and RSS is a way of facilitating information in a simple manner. Through RSS, and using NetNewsWire (an RSS reader for the Mac), on a daily basis I check about 70 websites for things I might be interested in. I can do this in a very short time, since all it takes is scanning through headlines (just like I had been doing for years with Usenet, and even email lists). Such a simple idea, but so effective.

Update services

But it gets better. Through the use of ‘update services’ such as Ping-O-Matic and Technorati each time a post is made, of your blogging platform supports it (and most do), your content is spidered and indexed within minutes of it going live, and is available on their search engines for people to find and read. There is no guessing game as to when Google will come along and find it, and put your posting at result number 1.56 million of 2 billion – it could be spotted and read straight away.

Tagging

I have a Technorati account, and through that, I have signed up to monitor a number of keywords. They give me a personalised RSS feed for that tag. If someone ‘tags’ a blog posting with one of the keywords I am interested in, when my RSS reader updates the feed for that keyword, I will know that something I am potentially directly interested in has been published, can then read, and comment if I wish. That simple concept is a very important one, in the days when a Google search often yields millions of results (many of them being faked by website spammers).

Tagging is something which is going to grow in popularity, and undoubtedly it is going to be misused (and according to Technorati, on their “State of the Blogosphere” report, already is on a lesser scale), but finally we are getting closer to a more useable web, a step closer to Tim Berners-Lee’s ‘symantic web’ perhaps, where metadata is of vital importance, and his original idea of a “read-write web” where publishing is a two-way process.

Social Bookmarking

Going a little beyond blogging, but still in the ‘global online community’ sense of mind, we have social bookmarking systems such as del.icio.us. With del.icio.us you can create a free account, and add to it website addresses that interest you, via a simple bookmark you can add to your bookmark toolbar. You visit a website, and if it’s interesting, simply click the “Remember This” bookmark, and you will be logged in to your del.icio.us account, where you can add a description to the URI, and add tags (more metadata!).

This way, your bookmarks are separate from your own computer, and you can access them from wherever you are. But there’s more, as ever! You get an RSS feed. So people can subscribe to your links if they think you’re such an interesting person (!). But, more useful perhaps, is the ability to subscribe to a particular tag via RSS. For example, I subscribe to the tag “archaeology” and I can see what archaeological websites people are visiting and bookmarking as worthy. It helps me a great deal from a personal perspective, and it helps me in a professional manner, since I run a large archaeological website myself, and it helps me to stay informed.

Live Links

How many times have you visited a website where the links page has been dull, uninteresting, and unchanged (possibly for years)? I myself am guilty as charged. Some of the sites I run haven’t had their links pages updated for a very long time. When I get around to it, I can use my del.icio.us RSS feed to have a dynamic list of links, constantly changing as I add interesting links to the system. I can even categorise them through the tagged RSS feeds. Again – a very powerful and useful system.

WordPress has several plugins that allow you to integrate del.icio.us links into the sidebar, for instance, so ease of use is on its way.

RSS goes yet further – Flickr

I have recently, in my newly revitalised use of the web, signed up for a Flickr account, where I can upload and share pictures using a brilliantly designed system. I can tag photos with keywords, and do similar searches to the del.icio.us system – I can subscribe to an RSS feed of any member’s public photos, and have my RSS reader alert me when they have added new ones to their collection. I can subscribe to an RSS feed for a particular user’s tag (e.g. when Camerar/ uploads a new picture of a butterfly), or for photos tagged with a certain term (e.g. ‘butterflies‘) across everybody’s photos on Flickr.

Flickr allows me to, when logged in, click a “Blog This” button on any photo, which will, when configured, allow me to display that photo resized for my blog, along with my posting about it.

Podcasting

To add a further arrow to the already burgeoning quiver, blogs can now deliver audio. It’s such a simple idea – simple record some audio, export it as an mp3, and attach it an an enclosure from your blogging system. People can then subscribe using XML, and have their machine automatically download the latest shows.

Blogs now have a voice, and with the spead of computers and computer literacy, there are is a rapidly growing audience. Commonplace programs such as iTunes have a podcasting function built in to them, so if you know how to buy music from the iTunes store, or even rip a CD, you can subscribe to a podcast.

People are even attaching video to their blogs, and with the emergence of portable video players which are smaller than notebook PCs, there’s even a growing audience there.

Power of Blog?

Whilst this article hasn’t just been about blogging specifically, all of the technologies have been related to it in some way. Personally, I have found that since I have started to read blogs, I feel better connected to the world, and what is happening. I no longer rely on what large publishing companies tell me is happening – I can hear it directly from the people. I can read about issues from people with different biases, and make up my own mind. If I disagree with someone’s opinion, or agree, or want to add something, I can. I can do it on my own blog and trackback their post, or leave a comment on theirs. Some podcasters even allow you to submit an audio comment.

Tagging and update services mean that my website is actually read by people, unlike 5 years ago and beyond when it was not. Tagging allows me to find things I am interested in very efficiently, without having to wade through the encroaching tide of Google results, which is slowly being choked by fake sites.

Flickr allows me to share photos, comment on them, use them, and see what’s happening in the world from the eyes of ordinary people rather than official photographers.

The ‘power of blog’ is in fact the people behind the blogs. It is the community that is growing with it, fostering communication, free information exchange; ordinary people can influence the world, and become well connected in myriad ways.

The blogging platforms themselves help programmers to meet, for example, and find out about each others work and ideas, self-perpetuating the improvement and innovation of the systems themselves.

For good and for bad, blogging is here to stay. The need to communicate is very human, and the true ‘power of blog’ is its humanity, and the links and bonds that it is making.

Tom Goskar