Geography Reference
In-Depth Information
Figure 2.6
Orthophoto of an area approximately 4 km west of Airedele, Maryland, USA. Area:
2.4 km by 1.6 km. Spatial resolution: 4 m. Image courtesy of the United States Geological Survey.
Sensors vary in terms of the spatial resolution (dei ned in Section 2.2.1) and the spec-
tral resolution of the output imagery. h e term 'spectral resolution' refers to which
parts of the electromagnetic spectrum are measured. h e number of available bands
(i.e. parts of the spectrum) and the specii c parts of the spectrum represented deter-
mine the uses to which the imagery can be put and these are likely to be selected with
particular purposes in mind since dif erent parts of the spectrum will highlight dif er-
ent characteristics of the Earth's surface. h e majority of remote sensing systems col-
lect information in one or more visible, infrared, or microwave parts of the spectrum
(Lillesand
et al.
, 2007). h e infrared parts of the spectrum are large compared to the
visible part of the spectrum. h e far infrared part of the spectrum, for example, is
sensed in the acquisition of thermal imagery. Lillesand
et al.
(2007) provide a detailed
account of remote sensing principles and practice.
Remote sensing is ot en used to derive topographic models, i.e. digital models of
the surface of the Earth (or, indeed, elsewhere). h e term 'digital elevation model'
(DEM) refers to such a model and the most common form is a raster grid with cell
values representing elevations above some arbitrary datum such as mean sea level.
Some key technologies for constructing DEMs (including airborne LiDAR and radar-
based systems) are summarized by Lloyd (2004). In the case of airborne LiDAR, out-
puts are 'point clouds' from which surfaces can be generated.
2.8.4
Ground survey
h ere are several widely used means of obtaining information on the spatial position
of features through ground survey. Traditional survey techniques include tape and
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