Madness part 2: processes of emerging inter-visibility

The second post in the madness series, describing the run-up to my PhD submission! Last time I wrote about why visibility networks might be an interesting method in archaeology. There was a hidden agenda in that post however: I am not just interested in visualising a visibility network, that has been done before by many archaeologists. My main interest is in understanding the decisions that went into the establishment of lines of sight. That is, the processes that led to the visibility network I study. This might sound rather ambitious, since many factors influenced the selection of the settlement locations I study in my PhD, and visibility networks are merely one factor derived from our limited knowledge of past settlement patterns. However, I argue it is necessary to understand such processes. Mainly because when archaeologists formulate assumptions about how lines of sight affected past human behaviour, these assumptions imply a sequence of events rather than a static state. Therefore, a method is needed that allows one to test the assumed processes, and I have some ideas on how to go about this 🙂

Visibility network between  Iron Age and Roman settlement in Southern Spain
Visibility network between Iron Age and Roman settlements in Southern Spain

Network representations of archaeological data are often used as static snapshots conflating an ever-changing dynamic past. By performing an exploratory network analysis we get an idea of their structure during a given period of time. Such an approach can be considered a type of exploratory data analysis. However, archaeologists use these data networks as representations of past phenomena. It is these past phenomena that archaeologists are ultimately interested in understanding, and most of past phenomena were not static but involved change through time. It is entirely plausible that at an earlier or later stage in time a given network would have had a different structure.

A commonly used technique for archaeologists to overcome this problem is to formulate theoretical assumptions about how the emergence or disappearance of a relationship between pairs of nodes in their data networks affected the change of past networks over time (from here-on referred to as dependence assumptions). Such dependence assumptions are frequently accompanied by (explicitly formulated or implied) expectations of the kinds of network patterns they give rise to. In other words, archaeologists frequently make theoretical statements about dynamic processes that cause change in past phenomena, and how these are represented in networks of archaeological data. Nevertheless, we rarely evaluate whether processes guided by our dependence assumptions can actually give rise to the networks we study, nor do we consider the effect multiple dependence assumptions can have on each other in such processes. Instead, archaeological network analysts have relied on the identification of the expected patterns in an observed network’s static structure when discussing the social processes that caused a network to change from one state to another.

The study of visibility networks in archaeology serves as a particularly good example of this problem. Archaeologists have used visibility networks as a method for studying the role particular visibility network patterns could have in structuring past human behaviour, for example through communication networks using fire or smoke signalling, or the visual control settlements exercise over surrounding settlements. Formulating dependence assumptions for visibility networks implies a sequence of events where new lines of sight will be established as a reaction to pre-existing lines of sight. For example, if we observe that a settlement is positioned in a visually prominent location from where many other settlements can be seen then we might formulate the hypothesis that this location was intentionally selected to enhance communication with or visual control over neighbouring settlements. A further example: if an effective signalling network was considered during settlement location selection then settlement locations inter-visible with other settlements would have been preferred. However, archaeological network analysts have so far studied these processes exclusively through an analysis of static network representations. By pointing out the patterns of interest, an exploratory network analysis can only take us so far to evaluate our dependence assumptions, leaving hypotheses surrounding the intentional creation of visibility patterns untested. A good example of this is Tilley’s (1994) study of a network of inter-visibility between barrows on Cranborne Chase, in which an observed network pattern is interpreted as the intentionally established outcome of an untested process: “One explanation for this pattern might be that sites that were particularly important in the prehistoric landscape and highly visible ‘attracted’ other barrows through time, and sites built later elsewhere were deliberately sited so as to be intervisible with one or more other barrows. In this manner the construction of barrows on Cranborne Chase gradually created a series of visual pathways and nodal points in the landscape” (Tilley 1994, 159).

Very few visibility studies have explored hypotheses about such processes explicitly (see Swanson 2003 for a notable exception). In my case study, however, the decisions to establish certain patterns of visibility among urban settlements are the focus of attention. Most crucially, I will try to evaluate to what degree this changed through time. The approach taken here is experimental. It will initially focus exclusively on the patterns of inter-visibility between settlements, exploring their observed structure as a static snapshot, and then addressing the following hypothetical question: if the visibility patterning that we have observed was the only reason for selecting the locations of sites, what then would be the process that is most likely to have led to the observed patterning? This question will be evaluated through a statistical approach that models the creation of visibility patterns in abstract space (i.e. by simulating the creation of points and lines without taking the landscape’s topography into account as a constraint). Finally, the results of this exploratory network analysis and statistical simulation approach will be re-contextualised within a wider archaeological discussion to shed light on aspects of the changing interactions between urban settlements in the study area through time, as reflected through visibility patterns.

Next time I will introduce the archaeology of this study area and show you some actual results 🙂

As always, I very much welcome your comments. They are very valuable to me in these last stages of my PhD.

References:
Swanson, S. (2003). Documenting prehistoric communication networks: A case study in the Paquimé polity. American antiquity, 68(4), 753–767.

Madness part 1: visibility networks

ALL_SEEING_EYEI recently wrote I would keep you posted on my two months of madness in the run up to completing my PhD. Turns out I have very little time to write blog posts now … who would have guessed?!? But just to get things started, here is the first one. Let’s talk about the networks we can create using our eyes, let’s talk about visibility networks! I want to encourage everyone to comment and discuss these posts, I would really benefit from your input as I wrap up this four-year long struggle with the PhD beast.

One of my PhD’s case studies is on visibility networks. What are those, you ask? Well, they don’t really exist. That is to say, they are useful abstractions of possible past social phenomena. I use networks to represent whether past individuals standing on one point in a landscape, like a settlement, could see some other point in the landscape I am interested in, such as another settlement. This figure shows you how such a network could be created: an individual with a certain height standing on site A can see site B which is positioned somewhere else in a landscape, only if the view is not obstructed by hills or mountains. You can then represent this individual and the point he/she observes as points (or nodes in network terminology) and the line of sight from the observer to the observed point as a directed line (or arc in network terminology). Do this for tens or hundreds of observation locations and a complex network of lines of sight emerges.

JAMT_Brughmans-etal_fig5NEW

Many people have asked me why this is useful. What do visibility networks add to existing approaches to studying past landscapes and settlement patterns, such as viewsheds in GIS for example? I like to believe I have a pretty good answer to this. Sometimes archaeologists are interested in understanding a past phenomenon that concerns the potential interactions between two entities, in which cases networks offer the best representation and analysis technique. To give an example, if we are interested in studying a past communication network that used fire or smoke signals to share information from one settlement to another (Like in Disney’s Mulan or in The Lord of the Rings), then evaluating the visibility of an entire landscape is overkill. All you need are the points and the lines. We do not have to analyse whether every square meter of a landscape was visible, but just that one point of interest. So selecting the best conceptualisation and abstraction of the past phenomenon you are interested in understanding can save you quite a lot of computing time. And it allows you to focus on representing and exploring your hypothesis, and not get distracted by other questions (if focus is what you want of course).

Mulan by Walt Disney
Mulan by Walt Disney

Moreover, we can do so much more once we have abstracted and represented our information about such a past communication system as a network. We can use network analysis techniques to determine the structure of this network, to compare it to other communication networks, to evaluate how efficient it was at sharing information, which settlements were key in sharing or blocking information, and so on. All of this offers a fresh new look on our data and provides results that can feed into our archaeological discussions and imaginations. Of course, the numbers a network analysis spits out are never the final word. They should always be re-contextualised in a wider archaeological research context rather than being taken at face value, or as an extra piece of  “primary information”.

I am definitely not the first archaeologist to have come up with the idea of visibility networks. Although it is not a very common topic, it has been done every once in a while in the past four decades, as you can see from the bibliogrpahy below this post. Many archaeologists focused their efforts on understanding signalling networks as described above (e.g. Shemming and Briggs, Swanson, Ruestes Bitrià). Another common phenomenon is the study of visual control, a popular topic in the study of Iron Age Spain (e.g. Grau Mira). There we see large fortified settlements on hilltops often called oppida, surrounded by smaller rural settlements. The oppida are often inter-visible with the rural settlements, whilst the rural settlements are less commonly inter-visible with one another. Archaeologists have suggested that this allowed for the oppida to visually control the smaller settlements, that it tells us something about social interactions between these communities, and possibly even about settlement hierarchies.

In following blog posts I will be giving you some more information about my efforts to explore such hypotheses of visual communication and control in Iron Age and Roman Southern Spain. Stay tuned!

Any thoughts or comments? Don’t hesitate to get in touch!

Archaeological studies of visibility networks:

Grau Mira, I. (2005). Romanization in Eastern Spain: a GIS approach to Late Iberian Iron Age landscape. In J.-F. Berger, F. Bertoncello, F. Braemer, D. Gourguen, & M. Gazenbeek (Eds.), Temps et espaces de l’homme en société, analyses et modèles spatiaux en archéologie. XXVième rencontres internatioales d’archéologie et d’histoire d’Antibes (pp. 325–334). Antibes: Éditions APDCA.

Grau Mira, I. (2004). La construcción del paisaje ibérico: aproximación SIG al territorio protohistórico de la Marina Alta. SAGVNTVN (P.L.A.V.), 36, 61–75.

Grau Mira, I. (2003). Settlement Dynamics and Social Organization in Eastern Iberia during the Iron Age (Eighth-Second Centuries BC). Oxford Journal of Archaeology, 22(3), 261–279. doi:10.1111/1468-0092.00187

Ruestes Bitrià, C. (2008). A Multi-technique GIS Visibility Analysis for Studying Visual Control of an Iron Age Landscape. Internet Archaeology, 23, http://intarch.ac.uk/journal/issue23/4/index.html.

Shemming, J., & Briggs, K. (2013). Anglo-saxon communication networks. http://keithbriggs.info/AS_networks.html [accessed 4-10-2013]

Swanson, S. (2003). Documenting prehistoric communication networks: A case study in the Paquimé polity. American antiquity, 68(4), 753–767.

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