Archaeological Aerial Thermography and Near Infrared Photography
Captured with a Flir One 160x120 pixel thermal imager
Flir PathFindIR 320x240 pixel imager
Archaeological Aerial Thermography
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).
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 with
visible spectrum for comparison
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.
(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.
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, in April 2016.
(visible and thermal infrared during the afternoon - the goal was on wheels!)
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.
* 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.
Our current thermal and near infrared kite kit
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.
Images produced with the above kite kit
(and also the images at the top of this page, except Cairnpapple)
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
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
(usually best in overcast conditions for NIR contrast)
Thermal infrared in sunlight
The soil was still moist in places because of the cool days and near freezing nights. The air temperature was 13°C 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
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 8°C, having dropped from 14°C 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
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. Another example is the ditch around Cairnpapple in the image at the top of the page. 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. (visible spectrum)
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
Cairnpapple at Night
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.
(Later ones at Fort William and Dunkeld are not listed)
and John Wells, Infra-red and kite aerial photography,
Scotland's Rural Past Conference, Dunkeld,
Cairnpapple in August 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 (720nm NIR filter), looking northwards, November 2007.
Rosie and John Wells, Aerial Photography by Kite, The Archaeologist, 50-51, No74, Winter 2009.
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.
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 (February 2009)
John Wells, Rosie Wells, Cade Wells and Jim
Aerial Thermography, International Society for Archaeological
Newsletter 29, 9-10, November
Our first thermal kite rig
Flir PathFindIR 320 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
John Wells and Rosie Wells, Castles From The Air, The Castles Study Group Bulletin, 14, 20-21 (Summer) 2012.
The site of Ogilface Castle, Armadale (720nm NIR filter), January 2010.
The site of Ogilface Castle, Armadale (thermal infrared), low-level oblique looking eastwards, March 2012.
Jim Knowles, Geophysical Survey Report, Blackness, Castle field, Earth Resistance Survey, Report No WLAG 001 (Revised), March 2013.
Blackness Castle June 2010
Jim Knowles, Gormyre Camp, A Multidisciplinary Approach, Survey and Excavation Report, Report No WLAG 002, March 2013.
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:
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.
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
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)
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)
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)
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.
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)
John Wells, The Cheapest Way to Begin Kite Aerial Photography (KAP) and Thermography (KAT), Linkedin, 2 August, 2017.
John Wells, Kite Aerial Thermography and Rainfall for Delineating Archaeological Residues, Linkedin, 6 August 2017.
John Wells, Aerial Thermal Imaging of Archaeological
Residues is Not All About Thermal Inertia, Linkedin, 12 December
John Wells, We Need to Change Our Mindset When it Comes to Archaeological Thermography, Linkedin, 12 July, 2019.
ARCHAEOLOGY IS BOTH HOT AND COOL
Archaeology is both hot and cool
Blank canvas follows heavy rain
But many simply fail to see