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Principles and Applications of Aerial Photography Desk based research is not just about reading papers for vital pieces of information, it is not just about tables, graphs, facts and figures. For many, primary data is all around us; aerial photography, for example, is an important source of information for researchers in landscape studies. This includes disciplines such as Landscape Archaeology (the study of how humans used landscapes in the past), (https://twitter.co (https://www (https:// (htt Navigate to... (http://www.environmentalscience.org/)

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Principles andApplications of AerialPhotographyDesk based research isnot just about readingpapers for vital pieces ofinformation, it is not justabout tables, graphs,facts and figures. Formany, primary data is allaround us; aerialphotography, forexample, is an importantsource of informationfor researchers inlandscape studies. Thisincludes disciplines suchas Landscape Archaeology (the study of how humans used landscapes in the past),

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Human Geography (how modern humans utilise the landscape) and climate science (todetermine land use and conditions; to track - for example - the growth and retreat ofseasonal ice and water levels or invasive flora species).

Anybody can learn how to interpret aerial photographs, and undergraduates inarchaeology and geography will study them in the first year of their degree. It is usually atmaster's level that students will study aerial photographs in great quantity, and are oftenexpected to produce academic reports or projects that utilise them in details that gobeyond merely interpreting the content of the photograph. Finally, they remain vital tocartographers in producing modern maps despite the prevalence of electronic methodsand satellite imagery in compiling our maps today (1), largely to take measurements whencompiling those maps. Aerial photographs are vital to any study of local environmentalconditions and they are used in many different ways, depending on the type ofphotograph used, the angle the photographs are taken at, and the elevation of the vehicleused to take them.

What Is Aerial Photography and How Does It Work?Aerial photography is - as it sounds - the process of taking photographs from the air, butthere is more to it than simply using a light aircraft or helicopter and flying up to takephotographs. There are many elements to an aerial survey that must be considered toensure that the data is useful enough to extrapolate whatever is being investigated. It isoften difficult to see elements of the landscape on the ground, features can easily bemissed, and what might seem like an insignificant bump from ground level can becomemore significant in a wider context (2); some landscape types are difficult to access onfoot so aerial photographs are vital to study and map them.

They have been used as a method of landscape studies for over a century (3), especially inarchaeology and researchers have learnt much about the world around us; itsapplications today are broad and coupled with the growing technology of GIS(http://www.environmentalscience.org/principles-applications-gis) (geographicinformation systems), the potential means that the method will not become obsolete anytime soon. Aerial photographs are taken in two basic forms and both have different usesand applications: oblique and vertical. Even today in an age of high quality digital imaging,black and white images are preferred - partly because they are cheaper but also partlybecause the contrast of black, white and greys makes it easier to pick out features (7).

ObliqueThese images are usually taken at an angle, typically 45 degrees but as they are oftentaken manually, they can be whatever angle gives the best view of the feature orlandscape. The oblique image is primarily used in archaeology to take a wider context of afeature and the area around it, and also to give depth. Nearly always taken at a much

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lower elevation than the vertical image and in few numbers, its application is fairly limitedand often taken for a specific purpose (8). There is a problem in perspective because thefarther away a feature is, the smaller it will appear: nearer objects of comparable sizeappear larger than those that are farther away so it is often best to take a selection or touse a frame of reference on the ground for perspective purposes. These images are takenfrom small fixed-wing aircraft and helicopters (3) and are perfectly suited for monitoringerosion of features and monuments throughout the year and over the course of manydecades (4).

Oblique Photographs: When Best to Take Them?The time of year is vital and many see winter as the perfect season to take aerialphotographs. There are many reasons for this, not least of all that it is easier to seefeatures in fields that do not have crops and will not be ploughed for several moremonths. Surviving features beneath the surface will often show up darker due to theshallower levels of soil. Snowy and frosty conditions perfectly emphasise ridges andfeatures and they can be photographed with a clarity not seen at any other time of theyear. The low level to which the sun rises casts much longer shadows, making visibility ofabove ground features much easier to spot. The perfect example here is relict medievalridge and furrow features (9).

That's not to say that the warmer months and longer hours of light are not conducive toaerial photography. If there are stone remains beneath a surface, crops will growshallower as they cannot put down as much root and features will show up as cropmarks. Late evening conditions also cast longer shadows and the differing light levelsbetween morning, afternoon and evening can add depth when comparing multipleimages of the same feature(s) over the course of a day (9).

VerticalTaking a photograph straight down over a landscape is the more familiar form of aerialphotograph. It is a plan view so there is no perspective to distort the image. This alsomeans that it is difficult to read the lay of the land such as changes in height - thoughthere is a work around to create 3D image through stereoscopic views, using a device toexamine two at once. This usually gives a good impression of the variation in the elevationof land (8). They are taken at regular heights for consistency so it is easier to comparecontexts of a landscape taken on the same day, or many years apart to examinedevelopment. Rarely used in archaeological applications except perhaps sometimes tofind interesting earthworks and other sites that are easily missed on the ground (8), theycover a much wider area (6) and focus on topography rather than specific details (4).

Vertical Photographs: When Best to Take Them?

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As a rule, vertical aerial photographs are easier to interpret than oblique photographsbecause of the standardised ways in which they are taken - with set scales and at a singlenon-arbitrary angle (10). The same advantages generally apply to vertical as they do foroblique, but you will lack the perspective, the depth and the 3D effect even with theweather conditions mentioned above. At higher levels, you may miss crop and soil marks.If it is an overview you require, then vertical photography is the best way to go.

History of Aerial Photography and SurveyThe first aerial photograph was oblique and taken of a French village in the late 19century. The man who took it - photographer Gaspar Felix Tournachon - patented theconcept of using aerial photographs to compile maps (5); it was to prove much moreeffective than the time-consuming ground surveys that had then been the morecommonly-used method of the national mapping organisations that developedthroughout the 19 century (such as the UK's Ordnance Survey). George R. Lawrence tookaerial photographs of San Francisco in 1906 following the devastating earthquake, but itwas not until World War I - when potentially military applications were foreseen - that asystematic process of taking aerial photographs would become key to the development ofthe method.

Archaeologist OGS Crawford pioneered the use of aerial photography for this purpose(11), having seen its potential for studying the English landscape. Both the allies and theGermans regularly took photographs of each other's lines and resources in order to keepup to date with the enemy movements (5). Having experienced the success of thismethod of observation, Britain once again used aerial photography during World War II,employing teams of archaeologists to interpret masses and masses of photographs takenfor aerial reconnaissance purposes (11, page 105). After the war, researchers welcomedthe beginning of the modern movement of landscape studies, natural processes,archaeological features and treating the landscape as a feature and a monument in itself(12, page 8). With the arrival of satellite imagery developed through national andinternational space agencies, military aerial photography reconnaissance became lessimportant though not entirely eliminated.

The Cold War and the development of colour photography meant that militaryapplications continued and it was during this period that wider environmentalapplications developed too. Infra-red photography became crucial to vegetation mapping(20) and also to tracking and identifying diseased plants and trees (18). The function oftaking landscape photographs at different colours of the spectrum opened up a widerange of applications across the broadest possible scope of the environment. Bettercameras developed and both the USA and USSR were able to plan reconnaissance tripsover key sites from thousands of feet up in the air. It was then that satellitereconnaissance began to take over.

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Since then, aerial photography has been used extensively in archaeological studies andlater for such wider environmental studies as mapping forests (20) and changes invegetation over time (15), tracking changes in river direction, and depth and planningconservation work of river systems (16), and changes to the landscape after naturalprocesses such as landslides (14). Its applications are limitless with multiple functions ingeology, geography and wider landscape, rural and urban studies. It is a cheap andeffective remote sensing method. Even today with widely available satellite (13) imageryand public mapping such as Google Earth, aerial photography remains vital to landscapeand other environmental studies. It adapts as technology and human need adapts.

Applications of Aerial Photography

In ArchaeologyAs discussed earlier, in archaeology aerial photography is ideal for locating lostmonuments and tracking features, especially those that are not visible at ground level,those that are under the soil and cannot be seen on a field walk and those that can onlybe seen under certain conditions. They are usually discovered through any of thefollowing (8).

Crop Marks and Parch Marks: Seen in summer, crop marks are signs of a subterraneanfeature that show up as irregularities in the pattern of crops. Growth of the crop might bestunted due to extant remains such as stone foundations, or they might be higher thanthe surrounding crop due to underlying water systems such as dried up drainagechannels or long-gone artificial water features such as fishponds. Parch marks occur inareas of particularly dry summer. In some conditions, the crop may simply be a differentcolour. Parch marks differ in that they are discolourations in the crop as a result ofprolonged drought. Areas where ground water dries up quickly and areas where theremay be more groundwater will show up clearly. Caution is advised when interpreting bothcrop marks and parch marks as the anomalies may be archaeological, geological, or dueto variations in soil and ground water courses. Modern pipes may also flag a false positivefor features of interest.

Soil Marks: Best studied in winter when no crops are growing or grasses have large diedoff, both rainy and dry conditions are conducive to picking out buried features. Typicallyshowing up as darker areas, they can indicate underlying stonework, the outline ofprehistoric features such as barrows and cursus monuments, and ditches. The sameissues above apply - they could be natural or modern features.

Low Profile Monuments: From the ground they may seem like natural bumps in theground or be so slight as to be barely perceptible. From the air, their appearance is farmore revealing. On their own they may or may not look like anything important but ifaccompanied with the above, can appear more significant.

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In Urban StudiesUrban development and the history of urbanism is a growing niche of landscape studieswhich has a wide range of uses through history and archaeology, the history ofcartography, the history of commerce, sociology and even for modern urban planning.Town developers need to study the impact of expansion and development of urbancentres on the landscape and the impact on the environment (19). New facilities (forexample a new sports stadium) will require a rethink of the infrastructure and the impactthat the new facility will have on people living in the area - will we need to build morehouses? Upgrade the roads? Will this affect protected areas? Aerial photography taken atlow levels is vital to examining the existing infrastructure (9).

In Climate ChangeWe all know about the effects of climate change(http://www.environmentalscience.org/history-climate-change) on global temperatures.These global changes are reflected everywhere, and societies and communities are seeingchanges to their local environment. If it isn't river beds drying up, droughts(http://www.environmentalscience.org/drought-issues-facing-united-states) gettinglonger, wetter seasons getting wetter and the reduction of inland lakes drying upcompletely, one of the most practical applications is tracking of invasive species into waterbodies (17) that just a few years ago would not have provided an adequate environmentfor those species. Researchers keep vital records in changes over seasons and years totrack local effects of climate change and risks to local ecosystems. Localised aerialphotographs will highlight the die-off of certain vegetation, or the increase of invasivespecies.

In Other Earth SciencesThey can also be used to study the process of natural changes, such as variations in soiland geology over time as well as changes to the underlying ground that leads to disasterssuch as landslides (19). Not quite as useful to geologists due to the relative expense anddifficulty in interpretation compared to archaeological applications, aerial surveynevertheless has uses and benefits and the historical record for changes to the naturallandscape is vital to understanding how the landscape may change in future. Annualrainfall, whether lower or higher than normal, can have far-reaching consequences and itis this where geology's interests in aerial photography are most important.

Though increasingly taken over by satellite images and digital mapping of GIS in recentyears, photogeology still has some practical applications for finding mineral and fueldeposits, mapping areas and tracking geological changes and water management as well

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as general geological research that other applications cannot contribute to (21). A greatexample of this is water drainage ahead of proposed new urban developments - floodplain risks and subsidence.

References

http://www.colorado.edu/geography/gcraft/notes/remote/remote_f.html(http://www.colorado.edu/geography/gcraft/notes/remote/remote_f.html)https://www.english-heritage.org.uk/professional/research/landscapes-and-areas/aerial-survey/archaeology/ (https://www.english-heritage.org.uk/professional/research/landscapes-and-areas/aerial-survey/archaeology/)http://www.historic-cornwall.org.uk/flyingpast/images/PDF_downloads/Aerial%20Survey.pdf(http://www.historic-cornwall.org.uk/flyingpast/images/PDF_downloads/Aerial%20Survey.pdf)https://www.english-heritage.org.uk/professional/research/landscapes-and-areas/aerial-survey/archaeology/aerial-reconnaissance/ (https://www.english-heritage.org.uk/professional/research/landscapes-and-areas/aerial-survey/archaeology/aerial-reconnaissance/)http://www.papainternational.org/history.asp(http://www.papainternational.org/history.asp)http://www.papainternational.org/aerial_photography_today.asp(http://www.papainternational.org/aerial_photography_today.asp)http://gis-lab.info/docs/books/aerial-mapping/cr1557_05.pdf (http://gis-lab.info/docs/books/aerial-mapping/cr1557_05.pdf)http://www.bajr.org/documents/aerialsurvey.pdf(http://www.bajr.org/documents/aerialsurvey.pdf)http://www.rcahmw.gov.uk/LO/ENG/Heritage+of+Wales/Gallery/Aerial+Photography/(http://www.rcahmw.gov.uk/LO/ENG/Heritage+of+Wales/Gallery/Aerial+Photography/)http://www.geog.ucsb.edu/~jeff/115a/lectures/geometry_of_aerial_photographs_notes.html(http://www.geog.ucsb.edu/~jeff/115a/lectures/geometry_of_aerial_photographs_notes.html)Bowden, M. 1999: Unravelling the Landscape. Stroud: TempusAston, M. 2003: Interpreting the Landscape from the Air. Stroud: Tempushttp://landsat.gsfc.nasa.gov/?p=5139 (http://landsat.gsfc.nasa.gov/?p=5139)http://www.isprs.org/proceedings/XXXV/congress/comm4/papers/395.pdf(http://www.isprs.org/proceedings/XXXV/congress/comm4/papers/395.pdf)http://www.cabdirect.org/abstracts/19716605728.html;jsessionid=343A50927C6C8C806A0CB1F58560732C(http://www.cabdirect.org/abstracts/19716605728.html;%20jsessionid=343A50927C6C8C806A0CB1F58560732C)http://onlinelibrary.wiley.com/doi/10.1002/wat2.1037/full(http://onlinelibrary.wiley.com/doi/10.1002/wat2.1037/full)

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https://www.tum.de/en/about-tum/news/press-releases/short/article/30993/(https://www.tum.de/en/about-tum/news/press-releases/short/article/30993/)http://www.oneonta.edu/faculty/baumanpr/geosat2/RS%20History%20I/RS-History-Part-1.htm(http://www.oneonta.edu/faculty/baumanpr/geosat2/RS%20History%20I/RS-History-Part-1.htm)http://www.isprs.org/proceedings/XXVII/congress/part7/196_XXVII-part7-sup.pdf(http://www.isprs.org/proceedings/XXVII/congress/part7/196_XXVII-part7-sup.pdf)http://www.forestry.gov.uk/pdf/QJFarticle.pdf/$FILE/QJFarticle.pdf(http://www.forestry.gov.uk/pdf/QJFarticle.pdf/$FILE/QJFarticle.pdf)http://pubs.usgs.gov/pp/0373/report.pdf (http://pubs.usgs.gov/pp/0373/report.pdf)

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Matthew Mason

MG Mason has a BA in Archaeology and MA in Landscape Archaeology, both fromthe University of Exeter. A personal interest in environmental science grewalongside his formal studies and eventually formed part of his post-graduate degreewhere he studied both natural and human changes to the environment ofsouthwest England; his particular interests are in aerial photography. He hasexperience in GIS (digital mapping) but currently works as a freelance writer as theeconomic downturn means he has struggled to get relevant work. He presently livesin southwest England.

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