Characteristics of aerial photographs

Depending on availability and the intended purpose, various types of aerial photographs may be used by archaeologists. Their characteristics, which are due to many factors including the ways they have been taken and the time of day, have a direct impact on 'fitness-for-purpose' and application.

So-called vertical photographs are taken using aircraft or satellite whose cameras or sensors point directly at the ground. These are generally taken for non-archaeological purposes, including planning, cartography and military intelligence, but may record much valuable information for the archaeologist. A truly vertical view is rare and generally only achieved in the centre of an image with increasing obliqueness to the edges. Vertical photographs are usually taken during surveys to cover areas of ground, and have an overlap between adjacent frames of 60% to allow them to be viewed stereoscopically. Aircraft and cameras for this work can be expensive and are usually supplied by specialist companies, although successful vertical surveys can be made using basic techniques (e.g. http://www.wix.com/thearco/aerial-photography). Orthophotographs, where the photographs have been processed to remove both inherent distortions and those caused by varying topography, provide a 'map-like' spatially accurate image.

Oblique photographs are usually taken by an airborne observer using a handheld camera pointing at the ground at angles of up to 60 degrees from horizontal (i.e. with wing pointing towards the ground), but more usually between 30 and 45 degrees. Photographs are generally of specific targets though areas of ground can be covered completely, and multiple images taken while orbiting a target are usual practice. Oblique aerial photographs are the usual means of recording during reconnaissance.

Stereoscopic viewing of suitable aerial photographs enables the brain to create a three-dimensional view of the ground. This allows the interpreter to see height differences that are otherwise flattened in vertical or near vertical photographs as so 'read' the landscape and archaeology more effectively. Vertical photographs taken with an overlap between adjacent frames of 60% allow stereo-viewing, and oblique photographs too can be viewed in this way if they are taken with suitable overlap.

The format of available photographs may include prints (contact and enlargements), negatives (sometimes on microfilm) and digital ('born-digital' and scanned). Older photographs will be black and white, and colour photography has only been universally routine since the introduction of digital cameras in the last decade, especially affordable digital SLR cameras that are the mainstay of present-day aerial reconnaissance. Negative size varies from 35mm film to 18x9 inch format, and together with flying height can have a direct impact on detail recorded (see scale below). Sensors in digital cameras also vary in size and thus affect the resolution in similar sized enlargements, either when viewed on screen or as prints.

Scale of photographs varies enormously but, regardless of the camera used, the original negative or digital image is proportional to the flying height and focal length of the lens. Prints from vertical photographs are often made at contact scale (i.e. at the scale they were taken) whereas those from smaller-format oblique cameras are usually enlarged or viewed on screen at various degrees of zoom. Vertical photographs are routinely available at scales varying from 1:3 000 to 1:60 000 and smaller (e.g. satellite), while low-level platforms (e.g. kite and UAV) can achieve much larger scales. Oblique photographs taken using a digital SLR using a 'normal' focal length lens (50mm) from about 700m above ground level will produce images with a maximum contact (or sensor) scale of about 1:1400. This scale will increase when the image is enlarged on screen or as a print. In general terms, large scale aerial photographs make it possible to see greater detail and smaller features, while small scale photographs are often more difficult to interpret because of low resolution and consequent lack of detail. For example, a feature 5m across will measure 1mm, or about the size of a blunt pencil point, at 1:5 000, while at 1:10 000 the same feature will be 0.5mm, and at smaller scales (e.g. 1:25 000, 1:100 000) the feature will be vanishingly small.

Judging the fitness-for-purpose of photographs is dependent on a user's experience and knowledge of the context in which they are to be applied, but some general principles can be considered. There are many factors and variables that affect whether archaeological information will be recorded on an aerial photograph and users will soon learn that one photograph is never enough. These factors include season and time of day, farming practices and the type of soil or bedrock. It must also be noted that what works in one country will not necessarily work in another, so changes in environment and climatic zone also play their parts in hiding or revealing buried archaeological remains. An appreciation of the types of features that may be expected and the means by which they are visible is vital. Thus, scale of imagery will impact on the size of features that can realistically be expected to be visible, while the time of year will condition whether plough-levelled sites have the potential to be visible as cropmarks or soilmarks, or whether earthworks and other expressions of topography will be visible with clarity. Varying vegetation cover through the seasons and the changing angle of the sun through the day or year, amongst other factors, will impact archaeological visibility, and so the utility of photographs taken under specific conditions.

Introduction to aerial photographs

Aerial photgraphy - bibliography