28 January 2009

Laser Scanning the Hung-e Azhdar Rock Relief

Why laser scan a rock ?

This extraordinary relief is carved on a huge limestone boulder at the cliff edge of a remote, not to say 'hidden' valley in the rugged mountains of northeastern Khuzistan [at the southwestern edge of the Iranian plateau, sharing a border with southern Iraq (= the big red blob on the map, below right)]. In ancient times, this was the heartland of Elymais, sometimes a small empire, more often a vassal to more powerful states.

At the time this relief was carved, Elymais was under Parthian rule.

Or was it?

Well, yes and no. As so often, scholars differ.

Look closely at the relief. You'll see that it is divided into two very different compositions -- and carved in entirely different styles. So, the question is: does this relief record a single historical event? Or are two different happenings accidentally fused together by craftsmen too lazy to smooth away an earlier unfinished relief?

What and When?

In the absence of inscriptions, the relief can only be studied through the images themselves. We'll start with the easier bit.

There's no doubt that the four standing figures on the right -- all pictured frontally in the Parthian manner -- belong to the Parthian period and were sculpted some time between the 1st and 3rd century AD. They wear the long tunics and baggy trousers of local Elymaid bigwigs. The tallest of the standing figures is clearly the main subject of the relief. He is the largest figure, topping the horseman on the left by almost a head. His prominent weapons and the gesture of the left hand on the sword hilt shows that he is a warrior prince, surely an independent ruler. And I think he must be one of the last Elymaid kings, for the mass of rounded, puffy curls on either side of his head also appears on Elymaid coinage at this time (left), before the kingdom is finally and forever vanquished. That would date him to the end of the 2nd or 3rd century AD

The King of Elymais (for so we must think of him) and his retinue seem to greet the man on the horse -- who is, in turn, followed by a single attendant. Both horseman and attendant are in profile view. These are the only figures among all of the Parthian rock reliefs pictured in profile.

Who is this rider? He wears a diadem tied at the nape of the neck with long fluttering ends, a symbol of kingship worn by Elymaid, Parthian, and Seleucid rulers. So we know he's a king -- but where is his homeland? The profile head with short hair and a naturalistic, bearded face suggest a Hellenistic Greek portrait so strongly that the entire relief has been dated in the 2nd century BC by comparison with coin portraits of Mithradates I (171-138 BC). Mithradates was the first 'Great King' of Parthia, who conquered Elymais in about 140 BC -- temporarily only, as it reappeared a few decades later as an independent kingdom (lasting on and off until its conquest by the first Sassanian king, Ardashir).

In close-up, this is how they compare.

I'm not convinced by the resemblance (although it's possible, of course, that after 300 years his exact image was lost). For example, the sculptor has carefully depicted a heavy necklace (a torc) around the rider's neck, a detail that never appears on Mithradates' coins but sometimes on coins of his successors. Perhaps the sculptor substituted the image of a later Parthian ruler for that of the 'Great King'. But what brings him to Elymais? The bird with a wreath and a palm branch flying towards the horseman (visible on the close-up, below left) is a clue. This is a Roman victory symbol (based on the Roman eagle) which becomes common on Parthian coins from the 1st and 2nd centuries AD. The bird could apply to either Mithradates I or II, both of whom were victorious in Elymais.

But, wait a moment. A second bird is flying toward the Elymaid king -- and he's pictured a head taller than his presumed conqueror. I suppose we could imagine that the horseman is not an historical character but meant to be more than human -- a deified or heroized early king. Whether human or super-human, he's badly out of proportion: his head is much too large. That may suggest that the head was copied from another source and clumsily attached to a ready-made body.*

And, then, there's his horse.

The prancing horse with arched neck and raised foreleg is a Greek image which spread throughout the Near East during the Seleucid period. This stallion (quite clearly male) has the short, clipped mane and high flowing tail of Greek horses. His bridle ornaments and fluttering ribbons, too, are very un-Parthian.

If the rider is heroized, his mount might be too. Could he be copied from Bucephalos, the famous horse of Alexander the Great? He does look a bit like a marble statue; doesn't he? In which case, we are certainly in the realm of myth or legend.

Anyway, why would an Elymaid ruler in the later Parthian period order a relief which shows him greeting or venerating a mounted figures also embellished with borrowed Roman victory motifs?

With the decline of Parthian power, Elymais was again independent and relatively powerful. The tall standing figure could be a nameless, late Elymaid king paying homage to his real or imagined ancestor in order to stress the legitimacy of the local dynasty's rule. That assumes, of course, that he had at hand suitably ancient images which -- with a bit of cut and paste -- served as a model. And that he was sophisticated enough to adorn this long-ago king with the correct trappings of his own era.

I don't think so.

Whichever way you look at it, the arguments are tortuous.

Enter lasers.

The whole story depends, of course, on the left and right sides of the relief being contemporary. Naturally, there are two opposing views about that. The carving in the two halves of the scene looks different. Some argue that the horseman is sculpted in higher relief than the standing men. This may only be an illusion based on a swelling in the rock surface which makes the horse seem in higher relief than the tallest standing figure. So, if the entire relief was indeed executed at one time, the stylistic differences have no chronological significance.

Lasers should settle the question, or so we hope. A study now underway will allow a detailed examination of the sculpted surface and precise measurement of the carving’s depth at different points of the scene, and should shed light on the carving techniques of all the figures.

The Iranian-Italian Joint Expedition in Khuzistan: 1st Campaign (March 4 - 12, 2008)

The sculpted surface of the boulder (above, left) was divided into 34 squared sectors. About 15,000 markers were placed on the surface to allow the scanner to recognize its position in a 3D model. Note that the scanner itself never touches the surface of the rock, so the process is non-destructive.

Vertices have been located on the ground by a GPS receiver and by traditional topographic method, while 70 ground control points, placed on the sculpted surface (below) are located to define a network. This network has 4 main vertices, while the ground control points orientate the frames acquired by the laser scanner and the digital photogrammetric camera.

The acquired frames were merged to create a 3D digital model of the rock relief consisting in 7.692.104 points, with an accuracy of 0.2 mm approximately [ I like that approximately!].

A specific software has been created to analyse these data. This software manages all the elaborated files: traces of tools on the sculpted surface, differences in the depth of the carving, and the natural conformation of the rock will be examined in order to verify whether there are also differences in the carving technique and evidence of re-sculpting in the two halves of the relief.

The first results (reported on 5 December 2008) look like this:

It's the best image ever. The contradictions in iconography and style are perfectly clear. Isn't science grand!

-- Do we have our answer?

-- No, not yet.

Tune in for the results of the second season -- coming next month.

* This idea does not hold water (in my opinion) since the attendant on foot behind the mounted figure -- although possibly unfinished -- also has an overly large head. It seems more a stylistic trait than a copying error.

References: I have made much use of T.W. Kawami, Monumental Art of the Parthian Period in Iran, Leiden, 1987.


  1. Great Project. Why not using laserscanners of Z+F - they are the most accurate in medium range applications?


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