Archaeological Aerial Thermography and Near Infrared Photography


A short outline with examples, old and new, and a footnote on ultraviolet aerial photography (also archived here)


Introduction

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Archaeological Aerial Thermography

Captured with a Flir One 160x120 pixel thermal imager


John and the late Rosie Wells on Cairnpapple

2011

Flir PathFindIR 320x240 pixel imager


Archaeological Aerial Thermography


(not to be confused with near infrared (NIR) photography)


The aerial visualisation of archaeological residues can be realised, or enhanced, by shadow relief, crop marks, parch marks, the drying out of bare soil, discoloration in soil, visible seasonal changes in flora, ice/water/snow collecting in hollows, frost, melting snow, 3D modeling, spectral analysis in the visible spectrum, NIR delineation of stress in flora, ultraviolet photography and thermal imaging. Of these techniques, the practical implementation of thermal imaging appears to have been the most problematic.



In 1981, an influential paper was published which set the scene for overlooking plant transpiration in favour of the more easily modeled physics appropriate to bare soil. However, Kay (McManus) Smith carried out a detailed study over vegetation ('Airborne thermal imaging therefore provides a useful complementary prospection tool for archaeological and geological applications for surfaces covered by vegetation' - 2004). More recent papers to read include those by Casana et al. (2017) and Piga et al. ('The thermal anomalies present correspondences to the radar time slices obtained from 30 to 50 cm' - 2014). Correlations between aerial thermal and geophysical surveys have also been found by Hugh Thomas et al. (Zagora Infrared Photogrammetry Project) and Michal Pisz et al. (Tibiscum Project - in press).

It was not until 2006, that a more useful, practical approach was considered for sites covered with vegetation. As a physicist, thermal capacity, conductivity and inertia etc. were familiar concepts but the images produced over vegetation in this latter paper were inspirational. I corresponded with Uli before buying my first thermal imager.

One of Ulrich Kiesow's aerial thermograms from 2011 with visible spectrum for comparison

archaeoflug.de


The role of water in the thermal imaging of both vegetation and bare soil captured my imagination. My considerations are independent of classical cropmarks and parching, as they can be captured and processed digitally at a much higher resolution. The ones in Uli's paper are there as proof of concept.

Transpiration, by day and less by night, is a cooling factor which can influence the thermal imaging of vegetation and thereby the delineation of archaeological residues in the ground as a result of variations in moisture content at root level. The magnitude of transpiration is further modified by the air and soil temperature, sunlight, relative humidity and wind speed. However, the ground has also to be considered, not only in relation to transpiration but as an entity in its own right.

When it comes to the ground, its thermal properties have been considered over the decades ad infinitum. There are some eloquent mathematical analyses relating to thermal conductivity and thermal capacity etc. but they are of little value in practical archaeological aerial thermography.

Just as transpiration can cool plants, rain can cool the ground. It can do this during run off, run through (drainage), retention and pooling. So, the physical properties of the ground in relation to water are also influential. After the rain has stopped, cooling also continues through evaporation.

The period of evaporation can yield a stark thermal contrast between wet, damp and dry areas, which in turn can reveal underlying features:



Vertical view of a partially dried out, former brickworks kiln site in Armadale, West Lothian, following a shower of rain.

March 2017

(thermal infrared at night - original video).

Such thermal amplification* is not only dependent on the thermal properties of the ground but it is also dependent on the quantity and temperature of the rainfall and its distribution over time.

We therefore have a complex, time-dependent interplay between the thermal properties and drainage characteristics of a location with the further influence, as with transpiration, of wind speed, air temperature and relative humidity.

* Thermal Amplification - My shorthand for the process of increasing surface temperature differences (through wetting and evaporation etc.) on mixed materials in situations of non-thermal equilibrium and in initiating such temperature differences in situations of thermal equilibrium.

So, with so many balls to juggle, when do I prefer to fly my imager? Over vegetation (which for me is usually grass), during the growing season, I head out on a sunny afternoon, when the relative humidity is not high, and the wind speed is about 8-20mph. Light rain earlier in the day, or heavier rain in the days before, can be helpful but the vegetation should be dry at the time of imaging:



Stratford Court Playing Fields following a light shower of rain (TIR right) earlier in the day, Stroud, Gloucestershire.

(visible and thermal infrared during the afternoon - the goal was on wheels!)

April 2016

Prolonged heavy rain resets the landscape for differential drying and differential transpiration to begin. Otherwise I fly at dusk, or in the early hours of darkness, with both wet and dry areas on the ground when flying over vegetation-free sites.

Our current thermal and near infrared kite kit

A Flir One (160x120 pixel) thermal imager attached to a phone and a Sony Nex-5N camera (on a Picavet suspension plus a GentLED-AUTO interval trigger) with the hot mirror removed but with no replacement window. The phone is deployed on the selfie stick which is used fully extended and has lockable sections.



John leaving footprints in the grass in 2011


As with other parts of the spectrum, aerial thermography should not follow a ground-based geophysical survey.



The Toots Long Barrow

Selsley Common

Gloucestershire


I have chosen this long barrow as an example as it is thermally complex. It is of variable height and width with two hollows, one enclosed and one transverse. The covering of grass varies from dense to sparse with areas of exposed soil and embedded limestone. So, with this feature, all thermal variables come into play both physical and biological.


Heidi Walker stood on the tump flying the kite with the selfie stick plus phone attached to the line

24th March 2019

Near infrared (720nm filter) in sunlight with John Wells in the foreground flying the kite (West to the left)

26th March 2019

The red and near infrared parts of the spectrum are good for producing high contrast shadow relief images in sunlight. However, when looking for differences in reflected near infrared light (e.g. as a result of stress-induced absorption in grass and crops etc.) it is best to capture images in overcast conditions.

For a NIR review see here


Infrared aerial archaeology

Vertical near infrared (850nm filter) in overcast conditions
Note the (excavation?) feature cutting into the tump in all the above images (not the transection)

17th July 2019


Thermal  infrared in sunlight

The soil was still moist in places because of the cool days and near freezing nights. The air temperature was 13C and the relative humidity ~55%. There was a very gentle breeze of about 10mph varying from the north to from the west, 3.30 to 4.30pm.

26th March 2019


Thermal  infrared in darkness



This image is rotated anticlockwise ~50 compared with the one above.The linear feature on the left is the path at the back of the long barrow in the above images. The temperature was 8C, having dropped from 14C at 2pm. The wind was gusting between ~8-22mph causing considerable instability. The relative humidity was ~79%, between 8-45pm and 9-45pm. The central right-angled feature was stable (not flickering).

30th March 2019


Cirencester Roman Amphitheatre in Darkness


September 2017

Here is a location covered in grass with the amphitheatre in darkness. We have been thermally visualising features at night which are covered with grass since 2011. Such observations are often at the limit of sensitivity of the thermal imager with the features often flickering in and out of view, as the field of view slightly changes. The Flir One is operated with the temperature span unlocked. Below is the same view in the visible spectrum.


Another example is the ditch around Cairnpapple in the image at the top of the page. A better example for Cairnpapple is the image below with a feature (the wide band running left to right) which, although not visible in recent times in the visible spectrum, does appear to be present in old aerial photos (1953, cambridgeairphotos.com) and is probably a product of the original 1947/8 excavation . Once again, this feature flickered in and out of view.


Cairnpapple at Night


August 2017

As with the amphitheatre, with the soil covered in grass, are we seeing these minimal differences as a result of a physical or biological phenomenon? Normally transpiration does not take place in darkness, although there are many exceptions. It seems unlikely that the effect is due to thermally visible interstitial ground, but not impossible, and the wind eliminates any localised heating patterns above the surface of the ground. In some instances, the variation in the dampness of the grass may be a factor, especially in ditches and hollows.



Selected Reports, Comments, Notes and Presentations.


As I no longer travel to meetings, I now include Linkedin comments on key points as a means of allowing open feedback.

Archive

Delineation of parch marks in the near infrared (720nm) by fellow WLAT Trustee Jim Knowles.

2011


2008

John and Rosie Wells' first infrared poster presentation at a conference in 2008

(Later ones at Fort William and Dunkeld are not listed)

Rosie and John Wells, Infra-red and kite aerial photography, Scotland's Rural Past Conference, Dunkeld, 2008. (Poster)



Cairnpapple in August 2009


2009

Rosie and John Wells, Visible and near infra-red kite aerial photography of Ogilface sites in West Lothian, Community Archaeology in Scotland (Conference at Queen Margaret University), East Lothian Council and Archaeology Scotland, 82-85, May 2009.



The site of Ogilface Castle, Armadale, West Lothian.


The site of Ogilface Castle, Armadale (720nm NIR filter), looking northwards, November 2007.

Rosie and John Wells, Aerial Photography by Kite, The Archaeologist, 50-51, No74, Winter 2009.


2010

Rosie and John Wells, The History of Torphichen Preceptory and the Order of the Knights Hospitaller in Scotland, Newsletter of the St John Historical Society, Clerkenwell, 7-14, June 2010.


The remains of Torphichen Preceptory with Castlethorn Hillfort top left


2011

John Wells, Rosie Wells and Jim Knowles, Kite Aerial Photography - Back to basics, Remote Sensing and Photogrammetry Society, ArchSIG Newsletter, 8-9, Spring, 2011.

John Wells, Rosie Wells and Jim Knowles, Gormyre Hill: Geophysical and kite aerial photographic survey and 3D modelling, Discovery and Excavation in Scotland, Journal of Archaeology Scotland, 12, 184, 2011.


Gormyre Hill after snowfall  ^North

February 2009

John Wells, Rosie Wells, Cade Wells and Jim Knowles, Kite Aerial Thermography, International Society for Archaeological Prospection, Newsletter 29, 9-10, November 2011.


Our first thermal infrared kite rig in 2011

Flir PathFindIR 360 x 240 pixel, auto-ranging thermal IR camera unit, incorporating a window heater (6w/2w on/off), 12v/4800mAh, lithium-ion battery, 12v/60w voltage stabiliser and PVR. The original digital recorder was replaced with a PVR (23 September 2011). This rig required very steady winds.


Cade and John Wells


2012


A section from one of six West Lothian Council display boards on our work (2012).


John Wells and Rosie Wells, Castles From The Air, The Castles Study Group Bulletin, 14, 20-21 (Summer) 2012.



The site of Ogilface Castle in sunlight, Armadale  (720nm NIR filter, ^NNW).
January 2010



Ogilface Castle with high contrast overcast NIR (720nm) with features not visible in the visible spectrum.

                                1. Eastern half near-vertical (2008)      2. South-East quadrant oblique (2008)
                                3. Central vertical (2010)                      4. Western end vertical (2010)


Archaeological aerial thermography 2012

The site of Ogilface Castle, Armadale (thermal infrared, ^E), low-level oblique
March 2012

2013

Jim Knowles, Geophysical Survey Report, Blackness, Castle field, Earth Resistance Survey, Report No WLAG 001 (Revised), March 2013.



June 2010

Jim Knowles, Gormyre Camp, A Multidisciplinary Approach, Survey and Excavation Report, Report No WLAG 002, March 2013.


Trust archaeologist Jim Knowles out at Beecraigs

John Wells, West Lothian Archaeological Trust - Kite Aerial Photography, ArcLand Conference k2u2, Dublin, May, 2013.

West Lothian Archaeological Trust, Low Level Aerial Photography, A technique for everyone, Archaeology Scotland Magazine, Issue 16, 15-17, Spring 2013.

John Wells, Kite Aerial Photography: A low level aerial photographic technique for everyone. A one-day workshop of the R.S.P.Soc. UAV Special Interest Group at the University of Worcester, July 2013. (Keynote Presentation)

John Wells, DART Heritage Remote Sensing Horizon Scanning Workshop, Leeds University, September, 2013.


DART thermal imaging experiment at the Royal Agricultural University in June 2012 with:

Dave Stott (Leeds Uni.), John Wells (West Lothian Archeological Trust), Graham Ferrier (Hull Uni.),

Ant Beck (Leeds Uni.), Tom Smith (Imperial Coll.) and Rosie Wells (W.L.A.Trust) behind the camera.

2012


Scaffolding for mounting the thermal imagers


The two thermal imagers as mounted

A Testo 875 imager for stills and calibration (left) and for 24 hour continuous video a Flir PathFindIR

CC-BY David Stott




 The location of one of the experimental trenches dug the previous year (NIR 720nm filter)
2012

John Wells, Jim Knowles and Rosie Wells, Kite Aerial Photography,  Independent Archaeology, Newsletter of the Council for Independent Archaeology, ISSN 2046-3227, 76, 10-14, November 2013.  (Colour reprint of article from Newsletter No 71)


2014

John Wells, The Scottish National Aerial Photography Scheme (SNAPS)*, News Article, History Scotland. Vol 14, No2, 14, March/April, 2014.


Rosie Wells with her main KAP kit on the left and other cameras (and wall posters) shortly before she died.
*SNAPS was set up in memory of Rosie. The thermal rig is in the middle of the table (2012).

John Wells, Jim Knowles, Ron Dingwall and Cade Wells, The Scottish National Aerial Photography Scheme (SNAPS), Rathcroghan, Archaeology Above and Below 2014, Tulsk, Ireland, April 2014. (Abstract)


2015

John Wells, Kite Aerial Photography in the Near and Thermal Infra-Red, Rathcroghan, Archaeology Above and Below 2015, Tulsk, Ireland. April 2015. (Abstract)

Jim Knowles, Kite Aerial Photography: How to Get Started, Its Uses and Safety, Rathcroghan, Archaeology Above and Below 2015, Tulsk, Ireland. April 2015. (Abstract)


2016

John Wells, The Scottish National Aerial Photography Scheme 2013-2016, Rathcroghan, Archaeology Above and Below 2016, Tulsk, Ireland. April 2016. (Abstract)


Jim Knowles, Kite Aerial Photography as used in Professional Archaeology, Rathcroghan, Archaeology Above and Below 2016, Tulsk, Ireland. April 2016.

2017

John Wells, An Introduction to Archaeological Kite Aerial Thermography, Rathcroghan, Archaeology Above and Below 2017, Tulsk, Ireland. April 2017. (Abstract)

Jim Knowles, West Lothian: Above and below, a different perspective, Rathcroghan, Archaeology Above and Below 2017, Tulsk, Ireland. April 2017. (Abstract)


Short Notes on Linkedin

The Cheapest Way to Begin Kite Aerial Photography (KAP) and Thermography (KAT), Linkedin, 2 August, 2017.

Kite Aerial Thermography and Rainfall for Delineating Archaeological Residues, Linkedin, 6 August 2017.

Aerial Thermal Imaging of Archaeological Residues is Not All About Thermal Inertia, Linkedin, 12 December 2018.

We Need to Change Our Mindset When it Comes to Archaeological Thermography, Linkedin, 12 July, 2019.


Archaeology and Near Ultraviolet (NUV) Photography with a Sony (APS-C) SEL 16mm, Pancake Lens, Linkedin, 30 July 2019.





Near ultraviolet aerial photo of a former brickworks site taken with an unmodified compact camera.














2012


For a UV review see here


An example of UV film photography by Christopher Brooke taken from this archived page.


Sutton Hoo excavations in 1985 (visible spectrum)

Dr Christopher Brooke, Department of History, University of Nottingham.



Sutton Hoo excavations in 1985 (false-colour ultraviolet 340-400 nm)
Christopher applied this approach to buildings and more recently he has been using thermal imaging (PDF download 2018).

Dr Christopher Brooke, Department of History, University of Nottingham.

____________________________________________________________________________

 

ARCHAEOLOGY IS BOTH HOT AND COOL

Archaeology is both hot and cool
With imaging's temporal tool
It works by day and by night
On residues it sheds a light

Blank canvas follows heavy rain
Evaporation yields a thermal gain
Not just with soil
Plants have a roll
Transpirationally
Sweat a cooling low

But many simply fail to see
Until they image thermally

John