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

Anybody can learn how to interpret aerial photographs, and undergraduates in archaeology and geography will study them in the first year of their degree. It is usually at master's level that students will study aerial photographs in great quantity, and are often expected to produce academic reports or projects that utilise them in details that go beyond merely interpreting the content of the photograph. Finally, they remain vital to cartographers in producing modern maps despite the prevalence of electronic methods and satellite imagery in compiling our maps today (1), largely to take measurements when compiling those maps. Aerial photographs are vital to any study of local environmental conditions and they are used in many different ways, depending on the type of photograph used, the angle the photographs are taken at, and the elevation of the vehicle used 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, but there is more to it than simply using a light aircraft or helicopter and flying up to take photographs. There are many elements to an aerial survey that must be considered to ensure that the data is useful enough to extrapolate whatever is being investigated. It is often difficult to see elements of the landscape on the ground, features can easily be missed, and what might seem like an insignificant bump from ground level can become more significant in a wider context (2); some landscape types are difficult to access on foot 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 in archaeology and researchers have learnt much about the world around us; its applications today are broad and coupled with the growing technology of GIS (geographic information systems), the potential means that the method will not become obsolete any time soon. Aerial photographs are taken in two basic forms and both have different uses and 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 partly because the contrast of black, white and greys makes it easier to pick out features (7).

Oblique

These images are usually taken at an angle, typically 45 degrees but as they are often taken manually, they can be whatever angle gives the best view of the feature or landscape. The oblique image is primarily used in archaeology to take a wider context of a feature and the area around it, and also to give depth. Nearly always taken at a much lower elevation than the vertical image and in few numbers, its application is fairly limited and often taken for a specific purpose (8). There is a problem in perspective because the farther away a feature is, the smaller it will appear: nearer objects of comparable size appear larger than those that are farther away so it is often best to take a selection or to use a frame of reference on the ground for perspective purposes. These images are taken from small fixed-wing aircraft and helicopters (3) and are perfectly suited for monitoring erosion of features and monuments throughout the year and over the course of many decades (4).

Oblique Photographs: When Best to Take Them?

The time of year is vital and many see winter as the perfect season to take aerial photographs. There are many reasons for this, not least of all that it is easier to see features in fields that do not have crops and will not be ploughed for several more months. Surviving features beneath the surface will often show up darker due to the shallower levels of soil. Snowy and frosty conditions perfectly emphasise ridges and features and they can be photographed with a clarity not seen at any other time of the year. The low level to which the sun rises casts much longer shadows, making visibility of above ground features much easier to spot. The perfect example here is relict medieval ridge and furrow features (9).

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

Vertical

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

Vertical Photographs: When Best to Take Them?

As a rule, vertical aerial photographs are easier to interpret than oblique photographs because of the standardised ways in which they are taken - with set scales and at a single non-arbitrary angle (10). The same advantages generally apply to vertical as they do for oblique, but you will lack the perspective, the depth and the 3D effect even with the weather 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 Survey

The first aerial photograph was oblique and taken of a French village in the late 19th century. The man who took it - photographer Gaspar Felix Tournachon - patented the concept of using aerial photographs to compile maps (5); it was to prove much more effective than the time-consuming ground surveys that had then been the more commonly-used method of the national mapping organisations that developed throughout the 19th century (such as the UK's Ordnance Survey). George R. Lawrence took aerial photographs of San Francisco in 1906 following the devastating earthquake, but it was not until World War I - when potentially military applications were foreseen - that a systematic process of taking aerial photographs would become key to the development of the 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 the Germans regularly took photographs of each other's lines and resources in order to keep up to date with the enemy movements (5). Having experienced the success of this method of observation, Britain once again used aerial photography during World War II, employing teams of archaeologists to interpret masses and masses of photographs taken for aerial reconnaissance purposes (11, page 105). After the war, researchers welcomed the 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 and international space agencies, military aerial photography reconnaissance became less important though not entirely eliminated.

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

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

Applications of Aerial Photography

In Archaeology

As discussed earlier, in archaeology aerial photography is ideal for locating lost monuments 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 only be seen under certain conditions. They are usually discovered through any of the following (8).

Crop Marks and Parch Marks: Seen in summer, crop marks are signs of a subterranean feature that show up as irregularities in the pattern of crops. Growth of the crop might be stunted due to extant remains such as stone foundations, or they might be higher than the surrounding crop due to underlying water systems such as dried up drainage channels or long-gone artificial water features such as fishponds. Parch marks occur in areas of particularly dry summer. In some conditions, the crop may simply be a different colour. Parch marks differ in that they are discolourations in the crop as a result of prolonged drought. Areas where ground water dries up quickly and areas where there may be more groundwater will show up clearly. Caution is advised when interpreting both crop marks and parch marks as the anomalies may be archaeological, geological, or due to variations in soil and ground water courses. Modern pipes may also flag a false positive for features of interest.

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

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

In Urban Studies

Urban development and the history of urbanism is a growing niche of landscape studies which has a wide range of uses through history and archaeology, the history of cartography, the history of commerce, sociology and even for modern urban planning. Town developers need to study the impact of expansion and development of urban centres on the landscape and the impact on the environment (19). New facilities (for example a new sports stadium) will require a rethink of the infrastructure and the impact that the new facility will have on people living in the area - will we need to build more houses? Upgrade the roads? Will this affect protected areas? Aerial photography taken at low levels is vital to examining the existing infrastructure (9).

In Climate Change

We all know about the effects of climate change on global temperatures. These global changes are reflected everywhere, and societies and communities are seeing changes to their local environment. If it isn't river beds drying up, droughts getting longer, wetter seasons getting wetter and the reduction of inland lakes drying up completely, one of the most practical applications is tracking of invasive species into water bodies (17) that just a few years ago would not have provided an adequate environment for those species. Researchers keep vital records in changes over seasons and years to track local effects of climate change and risks to local ecosystems. Localised aerial photographs will highlight the die-off of certain vegetation, or the increase of invasive species.

In Other Earth Sciences

They can also be used to study the process of natural changes, such as variations in soil and geology over time as well as changes to the underlying ground that leads to disasters such as landslides (19). Not quite as useful to geologists due to the relative expense and difficulty in interpretation compared to archaeological applications, aerial survey nevertheless has uses and benefits and the historical record for changes to the natural landscape is vital to understanding how the landscape may change in future. Annual rainfall, whether lower or higher than normal, can have far-reaching consequences and it is this where geology's interests in aerial photography are most important.

Though increasingly taken over by satellite images and digital mapping of GIS in recent years, photogeology still has some practical applications for finding mineral and fuel deposits, mapping areas and tracking geological changes and water management as well as general geological research that other applications cannot contribute to (21). A great example of this is water drainage ahead of proposed new urban developments - flood plain risks and subsidence.

References

Matthew Mason

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