Visibility and intervisibility have been important aspects of spatial analysis in landscape archaeological studies, but remain hampered by computational intensity, small-scale study area, edge efects, and bareearth digital elevation models. This paper assesses intervisibility and prominence in a dataset of over 1000 burial mounds in the Middle Tundzha River watershed in Bulgaria. The aim is to obviate the pitfalls in regional assessment of visibility through vegetation simulation and MC modelling and to gauge when intervisibility and prominence truly mattered to past mound-builders.
1. Introduction
’Will to visibility’ has been noted in funerary monuments from the United Kingdom to
Mississippi Valley, confirming that communities constructed new landscape features with respect to
natural landmarks as well as pre-existing sites and monuments[
In this paper I use 1073 burial mounds from the Yambol Province in SE Bulgaria to assess the impact of edge efects and of bare-earth models on intervisibility in a regional context. Parallelizing custom spatial functions in R, I calculate visual connectivity among the burial mounds, comparing the results from bare-earth elevation model with two diferent vegetationcovered simulations. Next, I extract the topographic prominence in mounds at two diferent radii and assess it against background values through Monte Carlo modelling. My aims are to (a) explore the feasibility of rigorous testing of intervisibility and prominence at regional scale in R, and (b) to assess how much visual connectivity mattered to mound-builders across Yambol through time.
The present dataset is one of the outputs of the Tundzha Regional Archaeological Project
(TRAP) which has operated in the Yambol Province since 2008[
For mound prominence I use Llobera’s (2001, 1007) definition of “topographic prominence as a
function of height diferential between an individual and his/her surroundings as apprehended
from the individual’s point of view”[14]. Replacing an individual with individual mound height,
I translate this calculation into R. The resulting value for each location is a percentage that
expresses the proportion of locations that lie at or below the mound top’s location within a
given radius. I calculate prominence at two diferent radii of 250 and 2000 m respectively to
gauge landscape afordances for various scales within the Yambol Province. An additional test
of burial mound visual dominance can be achieved by comparing the prominence distribution
of mounds with that of the entire region. Such bootstrapping was undertaken previously for a
subset of Bronze Age mounds in Yambol[
To assess intervisibility, I calculate line of sight (LoS) from each of the 1073 mounds inside the
Yambol Province boundary to all other neighbours within the region. A positive LoS means
that the view from point A to point B is unobstructed, meaning there is no higher elevation
between the two mound tops (see Figure 2). I translate this approach into R as the comparison
of elevation at location A to the elevations encountered on a straight line to location B, derived
from a raster profile. In order to reduce edge efects in 341 mounds located within a 5 km bufer
of the regional administrative border, whose view is constrained by the arbitrary boundary, I
also calculate their LoS to additional 1206 mounds within 25 km bufer outside the Yambol
Province border. The locations of the latter 1206 mounds have also been digitized from the
Soviet military topographic maps but remain unverified[
I mosaic together three ASTER DEM tiles[
Topographic prominence calculations using the bare-earth DEM confirmed that mounds were built in fairly prominent locations throughout the Yambol Province. As table 1 shows, more mounds appear more visually dominant within the 2000 m than in the reduced 250 m radius.
The drop in prominence in smaller radius calculation runs contrary to Llobera’s expectation of prominence decreasing as we calculate over a greater aggregate area. One explanation could be that the scale of area covered by 250 and 2000 m bufers is negligible compared to the scale of the region which routinely afords expansive vistas over 40 km radius. Maybe mound builders positioned their monuments carefully with an idea to longer-distance rather than short-distance visual control. A more realistic explanation, however, is that my calculation is spurious: percentage of lower elevation within a given radius is not a replacement for line of sight calculation, but is a simplistic way chosen to make the computational analysis feasible for a vast area (2000 m radius means a moving window of 132 x 132 raster cells enters in the calculation for each discrete mound point). To avoid misrepresenting reality, I evaluate bootstrapped data for prominence within 250 m radius alone.
The resulting prominence density chart in Figure 3 shows that the Yambol landscape data (represented by the dark grey band enclosed by light gray confidence interval) forms an arc with the main mode at 30-60% prominence and a secondary tight peak at 80%. The burial mound curve rises to a tight global maximum at 70% and escapes the bounds of randomness in the 60 to 85% prominence band as well as the 10 to 30% band. From this visual investigation alone, we can see that the mound locations difer from a random sample taken from the landscape of the Yambol Province both in the low and high values. The next question is how significant is this result, really? T-test and Mann-Whitney test in Table 2 show the prominence simulation results are robust and statistically significant except in the case of upper confidence interval. The p-value for upper confidence interval is above the threshold of 0.05 indicating that there is some uncertainty here. Given the robust lower and median values, I would lean towards a significant diference between the prominence of burial mounds and the overall landscape for most but not all of the confidence interval.
When modelling intervisibility, the results for the bare-earth model (BOM) deliver on expectations: when there is no vegetation, most mounds in Yambol are highly intervisible. The group with the lowest intervisibility - having only 1 to 10 visible counterparts - is small at 143 features out of 1073 (13%). Out of the 1073 mounds, 508 (47%) can see 10 to 100 counterparts and 422 (40%) mounds see over 100 other mounds. The most visually dominant 60 mounds can see over 250 other mounds. Their lines of sight extend across the entire region, as far as 60 km away, and probably further if the study area were extended.
The mound with the highest intervisibility: 9412, west of the Yambol City, has an
unobstructed line of sight to 491 mounds across the region, visible in Figure 5A. Yet, we can
reasonably doubt the real possibility of recognizing anything over 15 km distance, except perhaps fire
beacons at night or smoke stack from a burning village during daytime. Anything else - even
a 7 m high mound - would be reduced to an insignificant and unrecognizable dot especially
under less than ideal atmospheric conditions [
In addition to considerations of scale and limits of human vision, vegetation changes the
situation dramatically (see Figure 4). In the first simulation with static 10m tall vegetation,
intervisibility drops by 35-90%. Over 400 of 1073 (40%) mounds lose 90% of their field view due
to a patch of trees. Intervisibility groups get radically adjusted. Most mounds now fall in the
lower intervisibility group, seeing 1-10 mounds. Their count having risen three-fold over the
BOM. The middle rank drops by over 20%, but is still strong at 370 (36%) mounds with 10 to
100 intervisible counterparts. Only 103 mounds (10%) remain in the visually dominant rank
seeing over 100 other mounds. This considerable drop in intervisibility makes sense in light of
the 50% tree coverage and is consistent with Skov-Petersen’s [
In the second scenario with variable 20m (10m mean) vegetation, the decline in visibility is also present, but less pronounced. Intervisibility group membership shufÒes again. Compared to BOM, the lowest rank doubles to 41% with 441 mounds, the middle rank remains almost the same at 516 mounds (48%) and the highest rank of mounds attenuates to 116 (11% of total). Even though some trees in second scenario are higher than in the first, the overall vegetation variability makes this overlay more ’permeable’. This contributes to a less dramatic drop in the middle intervisibility rank, shufÒing the class membership gradually.
Now that we have addressed the groups, what efect does vegetation have on individual mound ranking, especially the formerly leading ones?
The downward adjustments contribute to reshufÒing among the leading mounds. Given the one time run, these results are merely illustrative of the general tendency and probability. In the present iteration of 20m variable vegetation model, the former total winner no. 9412 moves into a second place with 249 intervisibile mounds, a decline of 46% over BOM. It is superseded by 9044, a 6m tall mound near Botevo, with 291 visible mounds. Only four of the ten leading mounds retain their dominant position: 9412, 9411, 9044 and 8700. The remaining six sink in status with their line of sight obscured by trees, while others take their place. The specific winners depend on vegetation configuration, but they are always selected from the topranking group. In the end a rule emerges: unless the mound location happens to be covered by vegetation, once in a commanding position, always highly intervisible.
Likewise, accounting for the edge efects does not massively alter the order of the leading mounds. While many of the border region mounds grow in visual dominance, the absolute winners’ field of view grows too. To illustrate the point, 9412 dominates intervisibility with a line of sight to 409 other mounds inside the region in the bare-earth model. When we extend the vision beyond the region to limit edge efects, 9412 continues its lead with a line of sight to 491 other mounds (an increase of 20%). In the top ten mounds, eight retain their positions when we extend the view beyond the border of the Yambol Province. While the arbitrary limit on vision imposed by the regional boundary suggests that edge efects will be considerable for mounds in immediate proximity of border, it is the relief that is the decisive factor. Extending the view 25 km outside the region makes the intervisibility numbers go up by as much as 20%. However, they go up for many mounds in the region, not perturbing the regional order dramatically.
These results confirm two points: (1) landscape afordances in the Yambol Province drive the intervisibility and prominence results, with vegetation having considerable efect on individual mound field of view, and (2) ancient mound builders were aware of these landscape attributes and exploited them (or not) intentionally, building in just the right locations. Visibility mattered, but was not the only criterion. To underscore the point, locations exist in the region that ofer supreme visual dominance of 99%, such as the peak of Bakadzhitsite or the Dodoparon hill. Scaling these peaks, however, is clearly beyond the needs for ancestral worship or territorial signaling of the local communities.
In the comparison of intervisibility and prominence, two winners merit a bit more detailed
attention: mounds 9412 and 8007 occupy the highest intervisibility rank (see Figure 5). Both are
perched on an outcrop with an open view across the Middle Tundzha watershed. 8007 has over
95% prominence and 9412 has 76 to 89% across all vegetation models at 250 m radius. Mound
9412 sits close to northwest border on a slope that faces the rest of the region to southeast.
Mound 8007 sits near the geographic center of the region, with an unobstructed view of the
entire region. Culturally, the mounds perhaps also share a story. Mound 8007 belongs to the
40 excavated mounds in the region, having been investigated in a 2010 rescue campaign before
the opening of a nearby quarry. Cultural material, DNA, and C14 results indicate it was built
by the Steppe people during the Early Bronze Age [
This study explores the impact of vegetation and of edge efects on intervisibility and prominence rates among 1073 mounds in the Yambol Province, SE Bulgaria. Two diferent vegetation regimes are generated to facilitate a comparison of visual connectivity among Yambol mounds within bare- and vegetated-earth models. Line of sight results show that Yambol mounds located on isolated outcrops achieve supreme visual control over their surroundings. Adding vegetation of variable height between 1 and 20 m reduces the intervisibility less than adding vegetation of static 10 m height, even if mean vegetation height remains the same. Adding vegetation into digital elevation models always wreaks havoc on the individual mound intervisibility and reduces the visually-dominant group membership by 75%. Unless hid by a forest directly, however, the ultimate winners remain largely the same thanks to the inexorable topographic properties of the local landscape. Finally, edge efect correction (adding a 25 km bufer to the region) has a negligible efect on the intervisibility results, confirming that 3,355 sq km area is large enough for a regional-scale visibility study.
The present analyses were possible thanks to dedicated spatial libraries and parallelisation options ofered by the R programming language. In addition to making workflows efÏcient and reproducible, these libraries place compute-intense regional analysis within the reach of scholars without access to computing clusters. Others can now adapt and improve the present code, and pursue visibility explorations further, be it to investigate regional intervisibility networks or study the visual relationship between mounds and associated settlements and road networks.
The paper was possible thanks to 10 years of fieldwork funded in sequence by the ARC Linkage project 0989901, grant no. 19686 from the Endeavour Short-Term Mobility Programme of the Australian Ministry of Education, and the Aarhus University Forskningsfond Starting grant no. AUFF-E-2018-7-22 awarded to the ‘Social Complexity in the Ancient Mediterranean’ (SDAM) project. It involved scholars from three continents and dozens of volunteers as well as researchers from the Regional History Museum of Yambol, Sofia University of St. Kliment Ohridski, New Bulgarian University, Aarhus University, UNSW Australia, and Macquarie University. An early version of this paper was presented at the international conference “Research of Ancient Thrace between Traditionality and Modernity: Theoretical Aspects and Scientific Methodology” on 11-13 April 2024 in Sofia, Bulgaria. Code for this paper is available via GitHub.