Tag Archives: Photogrammetry

The life of a post-hole

with Kim Darmark

Anl_211

Digging post-holes can be rather tedious, non-rewarding work. Cut through, photograph, document the section, take samples and on to the next one. To a certain extent, this has been true at Kvarnbo as well, but a surprising number of the post-hole remains have, on the contrary, been challenging and exciting to investigate. This is due partly to the fact that so many of them have a life history which is possible to reconstruct. It is often possible to follow the events that have taken place through the different constructional elements making up the feature. In quite a few cases, it is possible to see the edges of the original pit, filled with a lighter primary fill, which was deposited in the pit at the moment of its construction. This fill is often lined with stones, creating a solid base for a beam to rest on. Both naturally rounded stones from the surrounding and rugged, fire-cracked blocks of stone have been chosen for this purpose. The primary fill contrasts markedly from a darker secondary fill, which enters into the pit at a considerably later date, when the old supporting beam is removed from the pit. The darkening of the fill represents many years of intensive use of the site, during which large amounts of organic remains have been tossed on site, creating refuse layers, usually referred to as cultural layers. This dark soil only infills the small chamber that is produced by the stones in the stone packing, and is often also rich in finds, which constitutes the other interesting aspect of post-hole investigation at Kvarnbo. Since we sift the soil through sieving net using water, we find every little find, even tiny ribs, scales and vertebrae from fish, which has ended up in the post-hole. The usual finds are bones and pottery, but a few less common items have been found as well, such as pearls, and small copper loops. Almost every feature that has been investigated this season has contained at least a few finds, some of them large amounts, and gives testimony to the richness of the cultural layer once present at the site.

Press this link to see the 3D model of feature 211 during investigation 😉

Traditional Strategy and New Techniques

by Frands Herschend

Strip (off the plough soil), map (the patterns you see) and sample (material from mapped structures) – is an archaeological field strategy applied to settlement remains in agricultural land, where crops have been growing for centuries. Ultimately, the method has become prolific, because of the gradual shift in the use of the cultural landscape. This shift made arable land more important and organized not least by means of roads. Consequently, farms were moved out of the farmland when possible. In Iron Age, on the other hand, arable land was less important, while grassland and meadows contributed substantially to subsistence. Not surprisingly, a rational Iron Age farm situation was in the centre of the farm’s agricultural area. Few roads were needed.

In Kvarnbo, change has been model, and today, the farms at Johannisberg are situated next to and above the arable land in which the Iron Age Kvarnbo hall stood on its small drained hillock.

When the excavations were planned, the strip, map and sample method was the obvious choice. But there are different way of stripping, mapping and sampling, and at Kvarnbo we have introduced a new mapping method in order to develop the general method. The testbed was successful during the test excavations in 2014, and in 2016, this mapping method was developed to become a routine.

Except for a handful of GPS reference points defining some of the test pits before the excavations started (see here), nine main reference points were defined after the top soil was stripped off in the area of over 1000 m2, and complemented with 183 reference points inside of that area. Supported by these, all (georeferenced) exact measurements, plans and sections are based on photographic 3D models. Also, a large scale plan was made with a drone during a 10 minute photo session and its orthographic projection, printed as an overview, has enabled the team to orientate itself on the site.

New techniques

Archaeological documentation goes hand in hand with interpretation. Description dominate fieldwork without excluding interpretation, and in the field, ocular observation is the general mode of perception: either you see something of you don’t. Later on, during the report writing process, interpretation and lab results dominate in order to answer the question: what cultural phenomena have we excavated? The problem in field archaeology is not what one observes, the problem is that which cannot be seen.

Overlapping features in the trench of 2014

In this section from the trial excavations of 2014, one can see archaeological features from Late Iron Age (A12 and A13) been transected by more modern plough furrow (A18).

For instance, looking at plough layer and plough furrows or any other depression, the point is to describe some sort of “ploughing biography” of the field as a part of the its involvement in history. It is easy to see the dark furrows when they cut into the yellow underground, but difficult to detect them at the bottom of the plough layer, and impossible higher up, although they may well exist in soil 20 centimeters or more below the surface. In fact, only by means of close observation of soil sections can one distinguish between ploughed and not ploughed soil.

Since we want to sample the contents of the different fills in postholes, we make a preliminary section of the first centimeters of the soil until, based on this section, we can dig away the plough soil, that is, the contamination of the prehistoric fill. As a result, we can, for example, conclude that the burnt clay, which represents burnt walls and was abound in the test pits, doesn’t exist in the postholes of the hall (because it was never burnt down).

So far, we have sectioned, described and interpreted c. 230 features of 273 on the c. 1000 m2 of our site. This is time consuming, but the ensuing 3D documentation is fast. On average, therefore, a team of four professional archaeologists and 2-3 amateurs can section, describe, interpret, sift and model 20-25 features per day, and sample soil for chemical and macrofossil analyses. 3D-modelling brings a new better and cheaper standard to strip, map and sample methods.

 

The Mighty Hole

Soil stripping is finished! We have a trench of about 1000 square meters, of which roughly half is already manually cleaned and hidden under the tarp. There are three large mounds of soil surrounding the (mighty) trench (which are pretty good for taking overview shots 😉 ). The subsoil in the trench is quite varied; from a smooth yellow and clayish silt in the south to a  difficult and stony moraine in the middle and diabolically hard clay in the north. But archaeological features are everywhere! These are especially visible against the yellow silt in the southern side of the trench, while revealing them in the other areas requires extensive manual cleaning. Furthermore, traces of plowing are constantly present, easy to follow and generally oriented to the north-south… We have hundreds of features marked out using yellow wooden pegs. Of these a good amount will probably turn out to be just natural depressions, but many are clearly related to prehistoric use of the area. Joining these together to an understandable pattern is a challenge in which a bird-eye-view is crucial. This will be obtained through photographic 3D modelling and is the reason why we are eagerly awaiting the clouds to hide the ever-shining shine above our site 😀  schaktning

Site reporting

After having been digging and writing tens-and-tens of technical site reports for the past 10 month, how much fun do you think it is to write another site report?? Even if it is a site report on the archaeological investigations at “my own site” of Kvarnbohall? Well, let me be frank, not so much fun… or, maybe, just marginally fun. Site reports are, after all, quite boring to write as it is mostly about technical data. Although, site reports are, at the same time, also, one of the most important kinds of archaeological writing. Anyway, I am back to working with the excavation data from the investigations at Sa 14.9. When it comes to this particular site reporting, I managed to do a lot during the autumn already, but, apparently, I then started from the simpler end and have to deal with all the complex issues now… Todays “headache”, for example, is the section below – the cut was placed through two features, feature 3 = the wall and feature 4 = post-hole, but the section revealed at least 4 different features, which I had, in fact, documented during the excavations already, but digitalizing the data was, nevertheless, not easy; despite the fact that I had my 3D models and a quite a number of photographs and my own excavation diary entries to aid the work… 11 sections digitalised… 10 sections to go… sigh?

CAA-SE workshop

CAA_SELast week, the photogrammetric work during the excavations at Kvarnbohall was presented by me, Frands and Daniel at CAA-SE workshop in Stockholm. CAA-SE as in Computer Applications & Quantitative Methods in Archaeology – Swedish Chapter. Half an hour felt hardly enough in order to be very specific, but I do think that our presentation went well as we managed to cover everything we wanted to talk about. In general, it felt like unlike the other cases presented, our photogrammetric work at Kvarnbohall was much more daring as deriving measurements from 3D models was already initially part of the method employment (i.e. total station was practically ditched) and we were not using the method parallel to traditional archaeological documentation. The visually attractive representation of site, trenches and features (something that seemed to be emphasized by others at the workshop)  is obviously very important as well, but it is the possibility of taking accurate measurements from the photographs  that seduced me in regard to photogrammetry. Well, I appreciate this hybridized documentation technique as a whole!

(but I should seriously start working with other aspects of the site-report as well…. (although, I am not yet tired of exploring the models created – there is something very meditative to view the post-holes – you can try it yourself 😉 by following this link) ).

A stone :)

ÅM 726_1069As I have totally fallen for photogrammetry (being probably a bit obsessed with it right now) and I’m extremely satisified with all of the 3D models created so far over the features excavated at the Late Iron Age longhouse site in Kvarnbo, I just had to experiment with some artefacts as well. Now, the artefact I happened to chose for my little experiment is not from Kvarnbo and it is not even dated to the Iron Age, but it is a stone (ÅM 726:1069) from the Neolitic site of Glamilders on Åland (you can have a look at the site report (in Swedish) following this link). However, it is probably one of the most fascinating stones from the Neolitic period in the North 😉 The thing is that there is a figure carved on the stone that was probably used for grinding, but no-one really knows what does the figure represents – is it a frog? …a turtle? …a beaver? …a seal? …a woman giving birth? …some very happy person throwing hands on the air? 😀 In any case, I just wanted to see if and how this relif shows on a 3D model and I think it worked out pretty well for my first experiment with 3D modelling an artefact (which is why I wanted to share this picture)!

My very own photogrammetric revolution

stenenAnl 10 o 11Lately, I have spent so many hours in front of my computer that I have no such thing as free time anymore and I hardly know what is happening outside, especially, as this sitting in front of the computer can only be done after my regular work at the Museum of Åland. My obsession is connected to photogrammetry 🙂 As I have written here before, during the excavations at the longhouse site in Kvarnbo, we chose photogrammetry as a main documentation method and totally skipped time-consuming hand-drawn recording (yep! totally!). Photogrammetry is a process that produces spatially accurate images from ordinary 2D photographs – with these images being georectified one can produce photographic plans of the site or a feature and its stratigraphy and one can take accurate measurements directly from the photo. And, the 3D models created from the same photographs create a visually very rich final product that you can go back to again and again. As there were quite a lot of features discovered and I am pretty new to the method, you can imagine that much of the time is spent just testing around, but the more time I spend to this, the more I am convinced that photogrammetry is the thing for future fieldwork. Especially, as it is also a very good basis for creating the usual site plans and feature drawings. The models created have been exported to ArcGIS platform for digitalization done from the photogrammetry and – Voilà! – the plan or a section drawing as you are used to see in archaeology is there as well! With one major improvement = you can always go back to the uninterpreted view.Anl 9