Geography Reference
In-Depth Information
photographs to help support massive post-war reconstruction programmes. The beginning
of the space age in the late 1950s saw the development of aerial photography shifting from
aeroplanes to satellite platforms with commercial satellite imagery available from the 1970s
onwards.
With the wholesale emergence of GIS in the 1980s, commercial mapping companies in
the UK and the United States invested considerable resources into developing digital aerial
photograph databases that could be used to extract map detail (Jones, 2003). By 1995, the
first commercial imagery databases in the UK were being compiled and published by The
GeoInformation Group under the product label 'Cities Revealed', initially released for cen-
tral London but then for other urban areas as well. Britain's national mapping agency, the
Ordnance Survey (OS), also began a systematic programme of aerial photographic surveys
in 2002 as part of the development of their digital framework product OS Mastermap. In
addition, many local authorities in Britain, the United States and elsewhere have commis-
sioned aerial photographic surveys on a regular basis (Cassettari, 2004), although access to
these resources is often difficult and controlled (Chamberlain, 1992). Finally, in 2005 aerial
photography experienced a worldwide resurgence in interest with the advent of Google
Earth and other similar internet portals providing high-resolution images free of charge.
7.1.2 Aerial photography: a technical background
In order to understand and appreciate some of the debates on the uses of aerial photography
for visualization in planning and property research, it is first necessary to have a basic
comprehension of some its technical aspects. This section will only outline some of the
basic concepts and the reader is recommended to the work by Lo (1976), Konecny (2003)
and Paine and Kiser (2003). Aerial photographs can be categorized into two main types:
vertical and oblique. In a 'true' vertical photograph the axis of the camera is orthogonal to
the ground at the time of exposure. In an 'unintentionally tilted' vertical photograph the axis
is no more than 3
◦
from the vertical. Most vertical aerial photographs fall into this category.
In an oblique photograph the camera's axis is tilted between 30 and 60
◦
from the vertical.
If the horizon is visible, the photograph is a high oblique; if it is not visible then it is a low
oblique.
When taking vertical aerial photographs, flight paths are arranged in parallel strips called
swaths that have an overlap of 20-30 per cent. Along the flight path there is usually a forward
overlap of 60 per cent between photographs. The overlapping swaths permit the photographs
to be joined together and also allow stereomodels (three-dimensional models) to be viewed
using a stereoscope (Lo, 1976). The aircraft flies a straight-line course at constant speed and
at a pre-determined height - the height determining the eventual scale of the photograph.
As a rule flights are only made in clear, cloudless skies when sun's altitude is higher than 30
◦
but lower than 60
◦
, preferably during the middle of the day when the quality of sunlight
is better. Winter is preferred for topographical surveys as there is less foliage (called 'leaf
off ' aerial photography), although this can cause problems in areas prone to prolonged
snow-cover as this can hide important features as well as winter often being associated with
poor weather conditions and extensive cloud cover.
It is common for vertical aerial photographs to be geo-referenced to a conventional grid-
referencing system (either manually using control points of automatically by GPS) and
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