Photogrammetric Recording of Subvertical Pits

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Up to now in my blog I have been trying to outline the uses of Photogrammetry in the two main areas of archaeology, recording and interpretation. Some things I have discussed were specific to preserving as much data as possible of an archaeological feature or object, by creating a virtual copy of it. Other posts were concerned with what can be done once that model has been made, to further our understanding.

This post is mostly about recording a specific type of feature, but it opens up some possibilities to help interpret them as well.

On some occasions during archaeological excavations we happen to stumble across some particular pits that present particular difficulty when planning. In these cases the issue is that the sides of the pit are not gradual or even vertical, but they actually overcut the side, giving it a bulging shape. During an excavation at Ham Hill, Somerset, one of the pits there had this particular shape due to it’s use. It was probably used to store grain, and the presence of a smaller hole at the top meant that the preservation would have been better.

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The plans draw of the pit were excellent, but even so it is difficult to convey the true shape of the feature using only 2D resources. I therefore created a model of it, by taking photographs like I normally would for a regular feature, with the addition of a few more from within the feature itself, by dropping the camera within it. The results are as follow:

 

Not only can you view the feature from the top, but it is even possible to see it from the sides, and rotate it that way, making it ever so clear how the feature was even now it is gone.

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In addition to that, the bulge is much clearer, and it is easier to draw conclusions on its use. As a permanent record it is excellent, as not only do we not loose any information, but we can also gain more than what we could see being limited by the simple top view.

It also opens up new possibilities. As of yet I have not experimented much with Maya 3D, however I have had a brief overview of how the program works and what it is capable of. One of my colleagues once showed me how to reconstruct a pot from the profile, and then proceeded to calculate the capacity of the finished pot. Theoretically speaking it should be possible to import the finished model of the pit in Maya, and then use the same algorithm to calculate how much grain the pit could have had at a time, which could help understand the density of the population of the area, as well as a lot of other interesting questions.

And the technology doesn’t stop here. This may be a very specific example, but the same ideas can be applied to many different kinds of features. Those with particular bases can be easily recorded by making a model of them, or stone structures can be perfectly copied digitally rather than only drawn by hand. There is still a lot of applications to discover.

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VisualSFM: Pros and Cons

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I’ve been working with Photogrammetry for some years now, and although I use a great variety of programs to edit the 3D models, from Mehslab to Blender, when it comes to the actual creation of the models I have only ever used 123D Catch. This is partially due to the fact I now feel very comfortable using this program, having learnt what it requires and how to achieve the perfect model, but also due to great quality and simplicity that 123D Catch offers.

Only recently have I ventured out of my comfort zone to explore other Photogrammetry programs, to see if any can compare or even replace 123D Catch.

I had a spell with Agisoft Photoscan, with interesting but limited results, so now I have turned my attention to another piece of freeware called VisualSFM. At present I have only tried out a few things, so my opinions will certainly change in the future, but here are the pros and cons I have found up to now (compared to 123D Catch):

PROS

  • It’s multiplatform: only a few weeks ago another user commented on one of my posts asking me for alternatives to 123D Catch as he couldn’t use it on his Mac. VisualSFM is not restricted to Windows/Ipad App, but supports all major systems, making it a great tool for all those that don’t have a PC but still want to try out Photogrammetry.
  • It allows control of the process: one of the good things of 123D Catch is that it is easy to use, but this is to the expense of more expert users. Uploading images and getting results with a single click is great, but it’s difficult to understand what is actually happening in between. VisualSFM instead guides you through all the steps, so if anything goes wrong you can pinpoint the problem, or you can understand which photos are better for research purposes.

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  • It works offline: many times I have found myself slowed down by a weak internet connection. With VisualSFM this is not necessary, which also means I can create models on site without the need of Wifi. Makes the whole process more efficient and means I spend less of my free time working on models.
  • The camera placement is more accurate: this one is still in testing, but up to now I have had no problems with cameras being placed in locations they are not meant to be in. With 123D Catch often a single photo stitches to the wrong place and causes the entire model to malfunction. With VisualSFM this doesn’t appear to be the case.

CONS

  • Less points: I compared a few models made with the same photographs by the two programs. The results suggest that VIsualSFM points may be placed more accurately, there is far less of them, making the overall model itself less accurate. In the pictures top is VisualSFM, bottom is 123D Catch.

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  • Still haven’t finished a model: once the point cloud is created, VisualSFM has finished its job and it becomes Meshlab’s responsibility to actually recreate the surfaces and reattach the textures. Up to now I have not managed to do this. I’ve talked about Meshlab before, but in short it crashes and malfunctions all the time. It took me days to recreate the surface the first time as the program refused to do it, and attaching the texture is still something I can’t seem to manage.
  • Needs user control: with 123D Catch you can leave the program running and return to a finished product. With VisualSFM you have to constantly interact with the program, meaning you can’t multitask.

Overall, it’s got potential. It’s not going to replace 123D Catch any time soon, but I am still going to try different things out to see how it reacts and find any advantages. For a full description of how to create models using VisualSFM visit here: http://www.academia.edu/3649828/Generating_a_Photogrammetric_model_using_VisualSFM_and_post-processing_with_Meshlab

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First Photogrammetry Article Published

New Photogrammetry Article

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I’m very glad to present you with my first (but not last) published article on the topic of Photogrammetry in Archaeology! The December edition of The Post Hole, that has recently been released, features a paper on “The use of Photogrammetric models for the recording of archaeological features”, which I wrote during the summer, and which I’m sure you will find of some interest.

It deals specifically with archaeoloogical features on site, and it looks at accuracy, methodology and uses, especially when it comes to recording. The aim of it is to show that far from being technology for technology’s sake, Photogrammetry can contribute greatly to our understanding of an archaeological site, as well as reinforce and improve traditional methods of recording such as section drawing and plans.

The article is based on a few sites I worked on, and that have featured on this website before, such as Ham Hill and Caerau.

This is however just scratching the surface of a technology that is now appearing more and more frequently in publications, and that will eventually become a fundamental part of archaeological recording.

Photogrammetry in Archaeology: Examples

I decided to make a video to show you some of the work I’ve been doing with 123D Catch and archaeological features. There’s not much to say about this one as the video is self explanatory, so just thanks for watching.

Using Photogrammetry with Archaeological Archives: Must Farm

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About a year ago I volunteered at the Cambridge Archaeological Unit for three weeks, during which I had some time to carry out some experiments with Photogrammetry with the help of some of the people there. One of the projects I carried out involved using the photograph archive they had to create 3D models with 123D Catch, to see if it was possible to create models from photographs not taken with this purpose in mind.

Looking through the archive one site in particular caught my eye, as the photographs perfect for this use: Must Farm, Cambridgeshire. The site itself is extremely interesting, and has won a Site of the Year award for the level of preservation. It is mostly a Bronze Age site, and throughout the years it was excavated it revealed a series of intact wooden boats, as well as a living structure which had collapsed into the river, waterlogging the timber frames and all that was within, including weirs and pots with food residue. For more information visit the official website http://www.mustfarm.com/.

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The photographs I found were from the 2006 excavation, and they consisted of series of shots from similar angles of same objects. The number of images per feature was around 8, depending on what it was. The most common things photographed were waterlogged wood beams, pottery spreads, sections and weirs.

Generally, the number of images and the fact they were all taken from a very similar angle would mean making a model is impossible. But through different test I have found that there is an exception to this rule when the object is particularly flat. If you are photographing a wall, there is no need to go round it to create a model, all you need is to take the images from the front and then change the angle slightly. The idea is also at the root of Stereophotography, in which two images at slightly different angles give the illusion to our eyes that they are in 3D. Similarly modern 3D films use a similar idea, with incredible results.

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Running the images through 123D Catch provided the proof of this theory, as in out of 40 or so potential models, around 70% were extremely good. The models had all the detail of a 3d model made with intentional photographs. Some details could have been a bit better, like some models of timber sticking out of the ground, for which only the front is available as would be expected, and the side of protruding objects which are blurry, but overall the results are amazing for what they are.

Most of the models are now available at http://robbarratt.sketchfab.me/folders/must-farm

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If we consider the amount of archaeological photographs that are taken at every site, surely amongst them there is enough to create at least some fantastic models.

Program Overview: 123D Catch

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123D Catch is my program of choice when it comes to Photogrammetry, and although it comes with some limitations, I still believe it is the most efficient piece of software when it comes to everyday archaeological modelling. It combines speed, accuracy, an easy interface and it is free to use, even commercially.

Speed: Especially in commercial archaeology speed is everything. You have a job to finish and a limited amount of time to do it, so the aim is to cut down the time spent recording while still preserving the same amount of information. 123D Catch is extremely quick provided it has a good internet connection. A basic model with 20 phoitos can be processed in ten minutes, and a more complex one in 15. Progrograms like Agisoft Photoscan take much more time, around 45 minutes for simple models, and many hours for larger ones. In addition to this 123D Catch models are processed o nan outside server, meaning you can run many diffferent models at the same time without slowing down your computer, while Photoscan really puts a strain on it. Othe rmethods of Photogrammetry, like those described by Ducke, Reeves and Score (2010), take days to process entirely. Similarly, a good laser scan can take a long time to process, and many stations may have to be set up.

Accuracy:  I’ve talked aout his before, so I won’t discuss it much here. Compared to other programs (and laser scanning), 123D Catch does seem to trail a bit behind when it comes to accuracy. The error margin seems to be between 1% and 0.17% (see table in earlier post and Chandler and Fryer 2011). Stereo closed range Photogrammetry has a range of 0.1% and 0.01% (Chandler and Fryer 2011) and laser scanning has a general standard deviation of 0.05-0.02% (Boehler & Marbs 2004; English Heritage 2011; Kersten et al. 2005). The main question to ask though is: do we need an error margin better than 1% for what we are using it for? Up to now I have had no problem with 123D Catch’s accuracy, so I feel that the answer to that question is no.

Easy interface: I’ve always felt strongly that this type of technology should not simply be used by professionals who spend ages learning how a program works, but that it should be accessible to everyone. Therefore it is important the software is simple to use, with a good interface. With 123D Catch all it takes to make a simple model is click the upload button and then the process button. When it comes to more complex features of course it’s not as simple, but for everyday use it is ideal. The Ducke, Reeves and Scores method is extremely complex, and requires to user to know how to code and to use four different programs. The Photoscan interface is simpler, but the navigation tools still could do with improvement.

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Freeware: Not much to say about this. It is free, so accessible. It provides the same results as expensive programs and anyone can own it.

Limitations: Of course there are also some limitations to the program. The main one I have found is that unlike Photoscan, it relies on an internet connection, which makes it difficult to use on a site. While digging on Ham Hill I had to pop down to the local pub every night and use their extremely slow internet connection to process models, which would take ages. 123D Catch also struggles with larger features, mainly due to the fact it doesn’t work well with more than 60 or so photographs. There are ways round it, like manual stitching, but it always requires more effort than should be necessary. Manual stitching itself is temperamental, and half of the times it takes a long time to find points in an image to then get an error message.

Overall I would still recommend it as the best program for Photogrammetry.

References:

Boehler, W. and Marbs, A. (2004). Investigating Laser Scanner Accuracy. Available: http://archive.cyark.orarchive.cyark.org/temp/i3mainzresults300305.pdf. Last accessed 9th May 2013.

Chandler, J. and Fryer, J. (2011). Accuracy of AutoDesk 123D Catch? Available: http://homepages.lboro.ac.uk/~cvjhc/otherfiles/accuracy%20of%20123dcatch.htm. Last accessed 9th May 2013.

Ducke, B., Score, D. and Reeves, J. (2010) Multiview 3D Reconstruction of the Archaeological Site at Weymouth from Image Series. Computers & Graphics, 35 . pp. 375-382.

English Heritage. (2011). 3D Laser Scanning for Heritage . Available: http://www.english-heritage.org.uk/publications/3d-laser-scanning-heritage2/3D_Laser_Scanning_final_low-res.pdf. Last accessed 9th May 2013.

Kersten, T., Sternberg, H. and Mechelke, K. (2005). Investigations into the Accuracy Behaviour of the Terrestrial Laser Scanning System Mensi GS100. Optical 3-D Measurement Techniques VII. 1 (1), p122-131.

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8 Reason Why We Should Be Using Photogrammetry in Archaeology

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If you are an archaeologist you should be using Photogrammetry because:

  1. It is easy to use: Unless you are dealing with something extremely large or extremely complex, Photogrammetry has an extremely high success rate. When it was still based on camera calibration, complex calculations and precise measuring was necessary, but with more modern programs often all that is needed is to take the photos and upload them. Decent models are easy to produce, and more complex ones are achievable without issue with some experience. Overall, anyone could potentially use it in small scale archaeology with no experience, and on large scale with limited training.
  2. It is quick: With a good internet connection I can probably model a single feature in under 10 minutes. And by single feature I mean anything from a posthole to a stone spread. In situ finds could be recorded in no time, cutting back on the need to plan everything by hand. A complex stone wall could be preserved for the archaeological record simply by taking a few dozen photographs, and sections can be recorded with much more realism than any hand drawn plan can achieve. A rough sketch of the section would of course help the interpretation, but the measuring time would considerably go down, as it would be possible to measure on the model using Meshlab.
  3. It is practical: Laser scanning is the current fashion in archaeology, but the problem with laser scanning is that you need to provide expensive equipment, you need to carry that equipment around and need to train specific people to use the machines and the software. Photogrammetry requires nothing more than a camera and a laptop, which are usually much more accessible on site. If a delicate object is found, that may not survive excavation, it is much easier to take some photographs with the site camera, to then edit later, than to bring in the equipment to laser scan it.
  4. It is accurate: As shown in one of my recent posts, the accuracy of 123D Catch is extremely good for the type of process. Although it cannot compete with that provided by laser scanning, an error margin of less than 1% means that any task required for interpretation can be carried out without having to worry of the results. The level of accuracy is ideal for presentation, for interpretation and for recording.
  5. It is photorealistic: No other technology gives you the photorealism that can be achieved by Photogrammetry. Due to the fact that at the base of the models we have the photos, and that the finished product contains .mtl files that record the exact position of the photographs, the surfaces of the features can be recorded as they are in real life. The models seem realistic because they are not a simple collection of points, but a combination of points and images.Image
  6. It is entertaining: Archaeology is not simply about recording the past, it is also about getting the information out there, to the general public. It is important that anyone interested in an archaeological has the opportunity to learn about the site itself. Academic texts are amazing when carrying out research, but for the average archaeological enthusiast, who lives in a now mostly digital world, texts can be seen as confusing. Photogrammetry provides a visual component to the archaeological record, making it possible for people to see from their own living room the archaeology, as if they were actually present at the site.
  7. It is constantly improving: At the moment there are some problems and flaws with the programs that may cause concerns to more traditional archaeologists. These problems however are only temporary. With such a great interest in the digital world, teams of developers are constantly trying to update and improve all software, and if at present programs like 123D Catch are not perfect, they can only get better. Also, 10 years ago this level of accuracy in Photogrammetry was unheard of, yet today it has got to this point. In another 10 years how much will the programs change for the better?
  8. It is not as simple as it looks, in a good way: There are different levels to Photogrammetry. The basic level is the simple recording of features and artefacts for the only reasons of recording and presenting. There is however a second level, which uses the models created to analyse archaeology, like I show in my previous post about finding inscriptions in coins. There is a third level, which alters the way the programs are used, by changing a part of the process to get greater results. An example is the attempt I did on reconstructing the Sphinx using tourists’ photographs, or the idea of using series of photographs in archaeological archives to reconstruct features long gone. Finally the fourth level is the more interesting one. It’s using many Photogrammetric models to create a single model, i.e. recreating pots by putting fragments together digitally or entire sites by gluing together individual features. So it is not only pretty models of features, it is much more.Image

Modelling Large Scale Features with 123D Catch

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In the previous entries we have seen the use of Photogrammetry in archaeology for the recording of features and artefacts. With models of this kind the procedure is pretty simple: you take 20 or so photos from different angles and then run them through 123D Catch to get the end result. The angles themselves generally should be every 45 degrees in a circle around the object and the same from a different hight, but because of the small scale there is quite a bit of leave way on precision of these positions.

The same cannot be said when dealing with a larger feature or an entire site, which for Photogrammetry generally refers to anything larger than 2 metres or so. In these cases it not only a question of angles and of how precise these angles are, it is also a question of making sure that every single point of the surface is recorded on at least three photographs. In smaller stuff this happens easily, as each photograph contains nearly the entirety of the feature. But on larger features the only way to achieve this is to take the images from a distance, which reduces the quality.

Many tests I have conducted have suggested that the best way to achieve a large scale model is to photograph the first spiral around the feature at a distance, in order to set the basis for the model, and then at a closer angle to  get the detail. This will increase the number of photographs needed, so the trick is to find a balance between the number of photographs and the need to photograph all points.

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If the model still has missing data the other approach is manual stitching. Manual stitching can be easy and straightforward or complex and problematic, so sometimes it is just easier to take the images again. If this is not possible 123D Catch does allow to glue unstiched photos together, and even to look through the photos that are already stitched to see if any mistakes have been made (this has saved me a number of times).

The main thing with large features is to try many different approaches until one works. Persistence as usual is key for great models.

Here are some examples:

https://sketchfab.com/show/gYd5v278pG0RGmle4XLTTVGOXAc

https://sketchfab.com/show/lzkuybFnqpQx6xptArTUCk0Wq2b

https://sketchfab.com/show/ghSMxAytcyxtghYhSe9Tfu68WcH

https://sketchfab.com/show/f19e2c6044b4417fbf1b8bdf9e8206eb

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