Geography Reference
In-Depth Information
transformed into an orthophotograph or orthophoto. An orthophoto is a geo-referenced
aerial photograph that has been geometrically corrected using a process called orthorec-
tification, such that the scale of the aerial photograph is uniform and the photograph is
planimetrically correct. This means correcting the photograph for inaccuracies caused by
distortion and displacement (Paine and Kiser, 2003). Distortion is any shift in the position of
a feature on a photograph that alters the perspective characteristics of the feature. Common
causes of distortion include lens distortion and aircraft movement. Displacement is any shift
in the position of a feature on a photograph that does not alter the perspective characteristics
of the photograph. Common causes of displacement include camera tilt and the effect of
topographic relief (Lo, 1976). The latter can affect the scale across the photograph since
features on higher elevations will appear bigger on photographs than the same features on
lower elevations. This displacement needs to be corrected to ensure that the scale of the
photograph is uniform, although this correction can produce visible radial displacements
of buildings, trees, bridges and other structures on the orthophoto, particular of large-scale
urban areas. This can cause mis-match problems if the orthophoto is used in a GIS and
overlaid with other feature data, such as digital building outlines.
Orthophotos have typically been used for photogrammetry and photo-interpretation.
Photogrammetry is the art and science of obtaining reliable quantitative measurements
from photographs such as distances, angles, areas, volumes, elevations, slopes and the sizes
and shapes of objects (Konecny, 2003). Photo-interpretation is the determination of the
nature of objects on a photograph and the judgement of their significance. It requires
an elementary knowledge of photogrammetry since, for example, the size of an object is
frequently an important consideration in its identification. Similarly, photogrammetry is
always closely related to photo-interpretation since there is always a need to know what one
is measuring (Lo, 1976). Nonetheless, work on aerial photographs is no substitute for field
work when interpreting features on the photograph. The end result of both photogrammetry
and photo-interpretation is frequently a thematic map (Paine and Kiser, 2003).
7.1.3 Google Earth and other internet portals
Google Earth was launched in June 2005 and sources hundreds of thousands of individ-
ual satellite images and aerial photographs from over 100 sources. Two principal sources
of satellite images have been from the Landsat programme and Digital Globe's Quick-
Bird programme (Nourbakhsh, 2006). The photographs have been mosaiced together and
wrapped around a three-dimensional virtual globe that can be downloaded from the inter-
net (http://earth.google.com). The technology was developed by Keyhole, a digital mapping
company that was purchased by Google in October 2004. To many commentators (e.g.
Goodchild, Chapter 2) Google Earth represents the first incarnation of 'Digital Earth' con-
ceptualized in a speech by Al Gore (Gore, 1998). Virtual tours can be made with users
zooming in from space towards a desired destination. Users can zoom in either by scrolling
with the mouse or by typing in a place name or postcode into a search box and the view can
be tilted and rotated as required. At the time of writing Google Earth also provides additional
data layers including three-dimensional buildings in 38 US cities, and road networks in the
US, Canada and western Europe which can be overlain on top of the images. Users can add
their own data to Google Earth using KML (Keyhole Markup Language) an XML-based
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