manipulation it raises questions about the
distinctions between so-called ‘gifts of nature’
and social artefacts derived from nature.
The global commons idea is used to avoid
the problems of open access, and to stop the
privatization of the world’s environment
(Buck, 1998; Goldman, 1998). There are
important questions about who ‘owns’ the en-
vironment, and who should ‘own’ the environ-
ment. For example, is the Amazon rainforest
the property of indigenous communities, the
state of Mato Grosso or the country of Brazil,
to be logged for economic benefits if so de-
sired? Alternatively, is the Amazon rainforest
the ‘green lungs’ of the Earth – in other words,
part of the global commons? The concept of
the global commons is appealing in that it can
avoid the dangers of open access and private
ownership, but it also potentially represents a
new form of conquest where indigenous
peoples’ rights to self-determination and their
ability to survive are made subservient to what
are seen as the environmental agendas of
affluent Western countries.
The concept of a ‘global commons’ requires
international agreements to be enacted in na-
tional legislation. Whatmore (2002a, p. 107)
questions the global commons approach in
relation to genetic material, and notes that
legal arrangements for the proposed global
commons, which unravelled, must be under-
stood as emerging from ‘amidst, rather than
‘‘outside’’, the spatial practices of national
sovereignty and private property’. The neces-
sity and the ability to include air,water
and other environmental considerations
within a form of property rights is contentious.
Rose (1999a, p. 50) questions the necessity
of a ‘global commons’ approach, when many
of the so-called global problems ‘have com-
ponents that are much more localized’. She
challenges Garrett Hardin’s largest issue for
the global commons – that is, population
growth (seepopulation geography) – and
suggests that it is the impacts of population
growth (specific pressures on specific re-
sources) that should be managed at a local
level. pm
Global Positioning Systems (GPS) The
most widely known Global Positioning System
(GPS) consists of 24 satellites, orbiting the
Earth twice daily at a height of 200 km, used
to find locations and to geocode data
(see geocoding). It was developed by the
US Department of Defense and is managed by
theUSAirForce.Asimilarsystem,GLONASS
(Global Navigation Satellite System), is
operated by the Russian Federation. The
European Union and Space Agency are devel-
oping Galileo, to be deployed in 2010 as a ser-
vice independent of, but interoperable with
(see interoperability), both GPS and
GLONASS.
The basic idea of a space-borne positioning
service is that at least four of the system’s
satellites will be above the local horizon at
any given time or location on the Earth’s sur-
face. The GPS receiver identifies a signal from
those satellites, which contains information
about each satellite’s orbital position and
the time of the signal’s transmission. From
these data and by trilateration (triangulation),
the coordinate location and the elevation of
the receiver can be calculated. In practice,
built structures and vegetation canopies may
obscure the line of sight between GPS
receivers and satellites, reducing the accuracy
of the measured location. Three satellites
need to be visible to calculate location; four
if elevation is also required (Clarke, 2003;
Longley, Goodchild, Maguire and Rhind,
2005).
GPS readings typically are accurate to about
5–25 m: horizontal accuracy is generally
greater than vertical accuracy. A problem is
the distortion of the satellite’s signal as it passes
through the atmosphere. The effects of this can
be measured at fixed receivers where the loca-
tions given by the GPS are compared against
their known positions, communicating any
correction to calibrate GPS receivers nearby
(a process known as differential GPS).
An extension to this approach is to upload the
information back to satellites for automatic
correction (a Wide Area Augmentation
System). However, no positioning service is
absolute: it depends on the datum (model of
the Earth’s shape) that is used. For GPS, this is
the World Geodetic System 1984 (WGS84).
Further errors will arise as this is converted to
a local datum or coordinate system (notably
when the three-dimensional model is projected
on to a two-dimensional grid).
GPS is popular for in-car ‘satellite naviga-
tion’ and likely to be integrated more fully with
portable communication and computing tech-
nologies such as mobile telephones and PDAs
(Personal Digital Assistants), further fuelling
the growth of location-based services, includ-
ing ‘Where am I?’ or ‘Where’s my nearest
... ?’ queries. GPS can enable personal navi-
gation and discovery, and have clear social
benefits (in search and rescue and emergency
management, for example). However, because
people might be tracked and located, so GPSGregory / The Dictionary of Human Geography 9781405132879_4_G Final Proof page 307 2.4.2009 6:30pmGLOBAL POSITIONING SYSTEMS (GPS)