Archaeological Kite Aerial Thermography (KAT)

West Lothian Archaeological Trust

e-mail John

Kite Aerial Remote Sensing

Thermography

Cairnpapple  West Lothian

2017

How We Started

2011

Currently (2011), commercial low-level aerial thermography is usually carried out using helicopters eg PDG Helicopters  fly a gyro-stabilised Flir Polytech Kelvin 350III.

This camera contains a high definition 640480 pixel detector that allows temperature readings either in real time or from a stored image. It has a 0.04C sensitivity and 1C accuracy. The primary advantage of this platform is that large areas can be surveyed with precision.

For smaller areas, light UAVs can be employed using the small Flir Tau imager range.

Our simple kite system (2011) is built around a waterproof 320 x 240 pixel microbolometer with a 19 mm wide angle lens, giving a field of view of 36 x 27 degrees.

Flir PathFindIR 320 x 240 pixel thermal imager.

This provides a resolution of a little less than the 352 x 288 pixel, conventional Pencam image of Cairnpapple, taken by James Gentles in 2003.  The sensitivity of the camera is 0.1C but it is not calibrated for temperature readout. The advantage of our system is that it is relatively cheap and light. However, the total weight of the prototype rig (including camera) is 1.5kg!

Cade and John with the sandwich box housed thermal camera and KAP rig

 

Wood Park, Armadale

2011

From: John Wells et al., Kite Aerial Thermography, International Society for Archaeological Prospection, Newsletter, 29, 9-10, November 2011.

 

Site of Ogilface Castle

West Lothian

Looking eastwards  2008 Near infra-red  2008
Vertical near infra-red  2008 Thermal infra-red  2012

 

 

Configuration 2

Flir PathFindIR auto-ranging thermal IR camera unit (1,900 inc.VAT (2011), incorporating a window heater - 6w/2w on/off), 12v/4800mAh lithium-ion battery (15 from e-Bay), 12v/60w voltage stabiliser (30 - this is power over-rated by an order of magnitude) and PVR (57). The original, digital recorder was replaced by a PVR, 23 September 2011.

The rig with the new PVR (the battery is external for safety)

The price of this camera is outside the remit of our normally cost-conscious work. However, we expect the price (and weight) of handheld, thermal imagers to continue to fall in the near future.

For simplicity, we considered the available handheld, thermal IR units, but none offered this resolution, robustness and field of view, at this price (August 2011).

Is such an approach a useful and viable option for archaeological KAP, as the near infra-red has already proved to be? That is the question we hope to answer. The two main considerations are (1) The slow speed of capture of individual frames and (2) The optimal environmental conditions for camera deployment.

VirtualDub is available as a free download for stabilising videos using the plugin Deshaker filter (Download), but we have not used it so far.

A HQ FF4.0 kite has been purchased to cope with the rig over a wider range of wind speeds. We also have a Power Sled 81, but we are more trusting of the consistency of performance of HQ flow forms in strong winds. Although we have not lost a camera since we started KAP in 2007, infrequently, sleds can shear out of the sky.  Normally, this is not a problem, but it would be a disaster and dangerous with a 2k, 1.5kg payload.

 

A second PathFindIR camera was ordered in April 2012 for work on the DART Project

Scaffolding for mounting the thermal imagers at

Harnhill Manor Farm (Gloucestershire)

Royal Agricultural University

June 2012

 

John preparing the PathFindIR for 24 hour continuous thermography

 

Thermal imagers for calibrated stills (left) and PathFindIR for 24 hour video (right)

 

Dave Stott (Leeds Uni.), John Wells (WLA Trust), Graham Ferrier (Hull Uni.), Ant Beck (Leeds Uni.) and Tom Smith (Imperial College)

At this point, Dave and Ant had been taking still thermal images at intervals from the top of scaffolding for 2x24 hours.

 

 

Configuration 3

July 2012

To reduce weight, the 60w input voltage stabiliser (235g) has been replaced with one rated at ~10w (22g). Specifications:

Input Voltage: 11.5 to 35.0 Volts

Output Voltage: 10 Volts +/-5%

Line regulation: 10 mV (typical)

Load Regulation: 12 mV (typical)

Operating temperature: 0 to  125C

Storage temperature:  - 65 to 150C

The unit (CCVS3 from e-bay) has thermal overload and short circuit protection.

Moisture-proof enclosure.

Several off-the-shelf voltage stabilisers are available for around 10 each. These are solid state replacements for electro-mechanical devices used on older cars to regulate the voltage to the control panel. The 4800mAh battery (176g) has also been replaced with a smaller one rated at 1800mAh (80g). The PVR (149g) has been replaced with a smaller, lighter model (82g). The total weight of the rig is now 1.16kg.

Configuration 3  July 2012

 

Update

 

Our Latest Thermal Imager

2015

160x120 pixel Flir One

Lighter and cheaper (200 in 2015) than the PathFindIR (over 2,000)

Much has changed since 2011. Relatively low-cost, lightweight thermal imagers (like the GoPro sized Flir Duo) have made aerial thermal imaging more accessible, especially for those using sUAVs as an aerial platform.

Kites remain cost-effective and are amenable to different experimental payloads/configurations. Flight times are not battery dependent.

 

Flir One attached to a Leagoo Elite 1 phone

This imager is a vast improvement on the PathFindIR for kite work, despite the lower resolution.

 

Sensitivity of the Flir One

(160x120 pixel)

A warming hand Wet paper

video video

 

We fly the Flir One on a selfie stick suspended from a kite line.

Kite with selfie stick suspended from the line

Selsey Common (Gloucestershire)

 

Application in Archaeological Remote Sensing

 

How do different temperatures arise on the ground?

1. Different materials heat up and cool down at different rates. Such differences can be thermally imaged when the environmental temperature changes over a short time scale. This has been a standard approach for delineating archaeological residues, especially in arid conditions.

2. You can also get temperature differences in vegetation (http://www.univie.ac.at/aarg/aerarch/papers/Thermal.pdf) when there is differential transpiration. Think of it like sweating. For example, plants growing above a wall footing have less access to moisture, transpire less and are consequently hotter than surrounding plants. This process can reveal crop marks throughout the year when the relative humidity is not too high.

3. We have found differential drying to be the most predictable approach. So we wait for rain and then later head out to a site, often when a local road has shown significant signs of drying. Longer times are usually need around our home base in West Lothian than around our southern base in Gloucestershire.
Visual drying of the ground is well established for revealing archaeological residues. Looking at small temperature changes produced by drying effectively amplifies this phenomenon. Sometimes, drying can be partly related to elevation as  in the handheld thermogram of the Roman amphitheatre in Cirencester (Gloucestershire) where previous excavation work is visible.

Cirencester Amphitheatre

Gloucestershire

England

Timing is crucial, as can be environmental light or darkness.

With aerial thermography, in all but the driest of climates, you do not plan to go out to a site on a given date, the site and weather conditions tell you on the day (even hour) when it is best to fly.

So, with some thermal images, we may know what variable gives rise to the main temperature differences, but in many cases it can be a combination of variables.

 

Stratford Court Playing Fields

Gloucestershire

England

April 2016

In the visible spectrum the field was a uniform green

 

The two images below were captured above a former Brickworks site

Etna Brickworks

West Lothian

Scotland

April 2017

Video

April 2017

 

Roscommon Castle

Roscommon

Ireland

April 2016

 

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