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In archaeology, the process of identifying which sites to excavate has traditionally been a slow and labour-intensive endeavour, involving terrestrial surveys that might take weeks or months to cover a region. However a growing number of archaeologists are turning to drone (or UAV/UAS) technology in order to survey greater areas, more efficiently, and at finer resolutions than satellite imagery can achieve.

One such early adopter is Julien Guery. He is the founder of JMMG Consulting and an independent scientific consultant specialising in geoarchaeology and geomorphology. “I help scientists who are unfamiliar with lidar, photogrammetry, laser scanning and UAVs to establish the reliable, easy- to-use protocols they need in order to acquire, process and use high-quality datasets,” he says.

In mid 2014, Guery reached an agreement with the director of the French Archaeological Mission to Adam (MAFA) to travel to Oman, accompanying Guillaume Gernez and his team to fly a partner organisation’s senseFly eBee drones over several sites.

A need for (more) speedIn the past, MAFA’s team prospected sites of interest at ground level, using differential GPS instruments. However this approach had proved slow going, thus Gernez’s enthusiasm for working with UAVs. “We agreed to bring in the drones because we could

see that these would allow us to easily map much larger areas,” Gernez explains.

“During the six years the Mission already worked in Oman, they had no other means of discovering new sites and artefacts except in-field surveys, covering one tenth of the studied area during this time. In the three weeks we were on site flying eBees, we nearly covered it all, mapping 60 square kilometres of additional areas of interest,” Guery adds.

During the past six years… the Mission had covered one tenth of the studied area. In the three weeks we were on-site flying eBees, we nearly covered it all

Had drones not been employed, Guery adds, conducting manned aircraft surveys in their place would have been problematic.

“The only non-drone solution would have been to use a plane for a classic aerial photogrammetric campaign or a lidar campaign,” he says, “however, this was impossible because of the cost and the implementation constraints in a country such as Oman, which borders Yemen and Saudi Arabia.”

Unearthing new technology - using drones & digital

photogrammetry to make archaeological breakthroughs

By flying mapping drones in Oman, consultant Julien Guery helped the French Archaeological Mission to Adam to survey much faster than it had previously and to identify archaeological structures that may otherwise have remained undiscovered

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CASE STUDY

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senseFly is a Parrot company. Copyright © senseFly 2016. All rights reserved. senseFly Ltd., 1033 Cheseaux-Lausanne, Switzerland.

In late 2014/early 2015, Julien Guery and a colleague surveyed 13 separate archaeological sites in Oman with two eBee UAVs. This map shows the location of two of these sites: the ancient cities of Adam and Bisya. (Credit: Jessica Giraud, MAFA.)

Getting preparedPrior to travelling to Oman in December 2014, Gernez first identified the sites that Guery and his team would need to map with the eBee drones. These included three areas within the ancient city of Adam itself, the surroundings of Jabal Mudhmar, Jabal Qara, and the sites of Bisya, Salut, Suffrat Dishshah and Jabal Al Aluyah. The total area to be mapped was approximately 60 km2, with 40 km2 of this at Jabal Qara where the Mission is studying the funerary landscape of the Early Bronze Age.

The target drone outputs were high-resolution orthophotos and digital surface models (DSMs) that the mission could then integrate into its GIS database. “These orthophotos were needed to replace satellite imagery as GIS background maps and they would also be used for highlighting artefacts that lower resolution satellite images couldn’t pick up,” Guery says.

One key to this project’s success, Guery explains, was the methodical, in-depth preparation that took place in France beforehand. “Since we would be in the desert without services like internet or electricity, this meant doing our flight planning and conducting plenty of simulations in advance using the drone’s eMotion software.”

Base maps needed to be saved in eMotion, flight plans programmed, elevation data imported and flights simulated. “However the resolution offered by the satellite base maps we loaded into eMotion wasn’t enough to plan all the very low-altitude flights we would need to fly, so we knew we would need to do some re-planning and tweaking on-site too,” says Guery.

Obtaining flight permissions was an equally intense process. “The Omani government doesn’t authorise drone use for ordinary people, so you have to justify any mission and confirm that you will leave the country with your drones,” Guery says. “We contacted the Ministry of Heritage and Culture via the Mission to explain our goals and to show them examples of drone results. Through this we gained an authorisation from the Ministry of the Interior and the National Survey Authority (NSA). They gave us permission to fly, on condition we sent

them flight plans for the regions we wanted to map, and that a member of the NSA accompanied us on the project. After the mission, we supplied copies of all the photos and our KML files.”

The mission’s data collection window was also limited, with Guery only on site for three weeks, including the several days it took to get the drones into the country. “This left just eight operational days, so to hit our targets we flew our two eBees simultaneously, up to ten times a day, and then processed the data overnight using two PCs.”

Excavating iron age buildings in the East Mudhmar region. (Credit: Guillaume Gernez, MAFA.)

Examples of a partially excavated bronze age graveyard in Adam, as captured by the eBee’s RGB camera. (Credit: Guillaume Gernez, MAFA.)

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senseFly is a Parrot company. Copyright © senseFly 2016. All rights reserved. senseFly Ltd., 1033 Cheseaux-Lausanne, Switzerland.

Coverage vs detailThe major flight planning challenge that Guery faced is a common dilemma for professional UAV operators: finding the right balance between flight coverage and the required ground resolution. In Guery’s case, he needed to map some large areas, such as mountainous Jabal Qara, but also achieve ground sampling distances of down to 2 cm per pixel, which meant low altitude flying and therefore less coverage per flight. “To minimise the number of flights, we were very specific with the ground resolutions we set,” he says.

In the drone’s eMotion software Guery set a resolution of 2 cm per pixel for designated ‘small areas’, ranging from one hectare to three square kilometres. “Here the team would be looking to document small archaeological remains such as Neolithic stone alignments, or require a detailed view of the constructions at excavated sites,” he says. A 2 cm/pixel resolution meant a flight altitude, above take-off, of just 50 metres (164 ft).

By contrast, larger sites such as Jabal Qara could be covered with relatively fewer flights due to setting slightly lower ground resolutions, such as 6 cm/pixel, for a flight altitude of 250 metres (820 ft). “In these areas we were looking to detect new archaeological sites or remains, mostly ancient graves from the Hafeet period [3200-2700 BC] measuring several metres in diameter. This resolution was enough to detect microtopographic anomalies on the resulting 3D model and then determine their nature on the orthophotos,” Guery says. “Even at this slightly lower resolution we could still see all kinds of detail that wouldn’t have been visible on satellite imagery.”

At this resolution we could still see all kinds of detail that wouldn’t have been visible on satellite imagery

Flight planning using the drones’ eMotion software. (Credit: Guillaume Gernez, MAFA.)

Pushing the limitsThe two technical challenges that Guery faced were ground proximity warnings and signal loss.

The former was a result of the 2 cm/pixel flights’ low altitudes. Combined with the mountainous terrain, these would sometimes lead eMotion to issue ground proximity warnings—activated automatically at a perceived altitude of 30 metres—following which the drone would carry out pre-set avoidance manoeuvres. “We didn’t experience any ground collisions because of the eBee’s ground sensor and failsafes, but these actions would disrupt the flight, so we would need then to modify the flight plan. This in turn meant reduced flight autonomies,” Guery explains.

As for signal loss, this was occasionally an issue due to limited site access. “Sometimes, because of ground accessibility issues, it was impossible to go inside the area we had to map, so we’d be forced to stay at the bottom or periphery of the mountain relief and fly further,” Guery says. “This meant sometimes the drone was four kilometres away and we lost its radio signal, so we had to choose: let the drone’s default safety procedures do their work, such as returning home if the signal was lost, or return to the mission and trust our battery autonomies to complete the mission,” Guery says. “The quality of the security protocols embedded in the eBee drones made the difference and saved the day more than once.”

Technical challenges

Large areas vs. detailed resolutions

Pre-programming flight plans based on low-res satellite imagery

Ground proximity alerts during data acquisition

Occasional signal loss due to accessibility issues

High daytime temperatures (cool down of equipment required)

Dust & sand penetrating computers

Availability of power supply to ensure drone & computer autonomy

In the very definition of an intense workload, Guery and a colleague conducted 47 flights, over eight days, at 13 sites. Flights ranged in duration from 10 to 45 minutes with an average flight time of 35 minutes, while the weather conditions in Oman, during winter, were ideal for flying UAVs: low wind (less than 6 m per second); no humidity; and daytime temperatures ranging from 25° C to 35° C (77º F – 95º F).

Each flight’s data was then quality checked and processed overnight, generating an orthomosaic and DSM of each site using the eBee’s Postflight Terra 3D software.

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senseFly is a Parrot company. Copyright © senseFly 2016. All rights reserved. senseFly Ltd., 1033 Cheseaux-Lausanne, Switzerland.

A side-by-side comparison of ASTER Global Digital Elevation Model (GDEM) satellite data of Oman’s Sufrat Dishshah Palaeolithic site (left), the morphology of which the Mission is studying, and the higher resolution DEM (right) that the team generated using drone data. This DEM enabled the team to recognise details such as meander lateral variations, palaeomeanders, and several levels of incision for the first time. (Credit: Tara Beuzen, MAFA.)

Protohistoric graveyards in Adam South, central Oman. These were identified for the first time, despite the presence of erosion, using the Mission’s drone-sourced digital surface model. (Credit: Guillaume Gernez, MAFA.)

Project workflow Determine areas to be mapped

Plan & simulate drone flights

Adjust ground resolutions/flight times as necessary

Download satellite data background maps & elevation data

Required to work offline in the desert

Transport drones to Oman

Map smallest, most accessible areas first

Perform three-day survey of Jabal Qara (the mission’s largest site)

Process data over several nights

Generate orthophotos & DSMs

Check data quality in GIS software

Deliver final data to mission director & National Survey Authority

Raw data with metadata, KML flight plans, orthophotos & DSMs

In all, the mission took two months; five weeks of preparation in France and three weeks in the field. “We managed to deliver all the project’s orthophotos, DSMs and images to the Mission on the last day in Oman,” Guery says. “This really shows the efficiency of the drones and their software, in that we covered over 60 square kilometres in eight days, with all our processing and product creation done in the field. It was just exhausting but thrilling too, with long days of flying and then processing all night!”

The biggest advantage of employing drones, says Guery, is the quality of the DSMs they generate. “The orthophotos are great and make it easy to see and record artefacts with a high level of detail, but the DSMs provide high-resolution topographic information that archeologists can’t currently access via any other method. As we demonstrated with the new graves we detected in Adam, we can detect artefacts we couldn’t see on the ground, either because these are too eroded or too large to be understood with the human eye, and which would be impossible to see on satellite images. With micro-topographic drone surveys we can discover new sites and new archeological structures, and understand more accurately what these sites were really like in the past.”

Studying morphology in Sufrat Dishshah, the DSM’s 3 cm resolution allowed the Mission’s staff to determine three phases in the life of the river there. “By creating this chronology we get a better understanding of how human occupation evolved around this river, information that simply wouldn’t have been available without photogrammetry,” Guery adds.

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senseFly is a Parrot company. Copyright © senseFly 2016. All rights reserved. senseFly Ltd., 1033 Cheseaux-Lausanne, Switzerland.

Future fantastic Both Guery and Gernez, the Mission’s director, see drone use as a key component of future archaeological progress. “Prior to using UAVs, this kind of data acquisition would not have been possible. The ground resolution, across the area we achieved it, is really unrivalled,” Guery claims.

“The data that Julien delivered would have represented several weeks or even months of ground surveys with a full team and its resolution would have been impossible to obtain,” adds Gernez, “It does not replace our investigations on the ground, such as artefact collection and analysing the precise architecture of tombs and other buildings, but it accelerates a lot the discovery of archaeological areas; it gives precise locations for every structure, plus a precise cartography and a complete DSM.”

The data would have represented several weeks or even months of ground surveys… it accelerates the discovery of archaeological areas, it gives precise locations for every structure, plus a precise cartography and a complete DSM

A 2 cm ground resolution orthophoto extracted from the first aerial survey of the Mission’s Bisya/Salut site.

A section of eBee-produced orthophoto of the ancient city of Adam, with important villages within the oasis highlighted in red.

A screenshot of the project’s 3D point cloud of Sufrat Dishshah, a 10 km2 Palaeolithic site.

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senseFly is a Parrot company. Copyright © senseFly 2016. All rights reserved. senseFly Ltd., 1033 Cheseaux-Lausanne, Switzerland.

PROJECT STATISTICS

A section of DSM showing a northern part of the Adam region.

About Julien GueryJulien Guery is an independent scientific consultant. The founder of JMMG Consulting (www.jmmg-consulting.com) he specialises in geoarchaeology, archaeology and geomorphology.

Watch the project video: www.facebook.com/CaptairDijon/videos/618323804939856/

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10

35 min

61 km2 (23.5 mi2)

0.13 pixels

Weather:

77 hoursprocessing data

Outputs:

2 - 6 cm/pixel

13,792

2 eBeedrones

47 flights

3 staff (2 flying/observing,

1 processing data)

4.0 cm/pixelmean GSD

2

batteries

photos

total coverage

mean re-projection error

low wind, dry 25°C - 35°C

Orthophotos, DEMs

ground resolutions

avg. flight time

months data delivery