Geography Reference
In-Depth Information
within them are determined randomly. One way of matching spatial variation to sam-
ple density is of ered by the body of approaches known as geostatistics. With such an
approach, the variogram (see Section 9.7) is used to characterize the spatial variation
given some provisional survey and this information can be used to ascertain an optimal
sample spacing.
Whether a user obtains the sample themselves or is reliant on data provided to
them, it is necessary to be aware of the nature of the sample and to consider any poten-
tial issues that might arise in the use of the data as a function of the sample design.
An introduction to spatial sampling is provided by Delmelle (2009).
2.8.2 Secondary data sources
In studies utilizing historic maps, or paper-based sources of which digital versions are
not readily available, conversion from these paper-based sources to digital versions is
necessary. Scanning provides a rapid means of deriving a raster image from paper
maps, although it is rarely used now for spatial data generation. Where acquisition of
information on particular features is desired then digitization of mapped features is
ot en conducted. h e most common approach is probably to scan the map and trace
the edges of the features of interest to generate a new vector data layer; this is called
'heads-up' digitizing. Increasingly, spatial analyses are based exclusively on data that
were collected directly in digital format and the ot en tedious process of tracing fea-
tures from maps through digitizing is unnecessary for many users of GIS. An intro-
duction to historical GIS, which includes discussion of conversion of historic paper
maps into digital format, is the topic by Gregory and Ell (2007).
2.8.3 Remote sensing
Huge amounts of spatial data are generated through various technologies that fall
within the umbrella term 'remote sensing. h is may refer to airborne or spaceborne
sensors (or simply cameras) or technologies for measuring subsurface characteristics
such as electrical resistivity. Conventional aerial photography has been an important
source of data historically. Figure 2.6 shows an example of an orthophoto in an area
of Maryland in the USA. h e data were obtained from http://terraserver-usa.com/
and more information about digital orthophotos is available at: http://online.wr.usgs.
gov/ngpo/doq/. Orthophotos are remotely sensed images that have been georectii ed
(see Section 2.5).
h ere are now many dif erent kinds of airborne and spaceborne sensors that detect
radiation from dif erent parts of the electromagnetic spectrum. Sensors may be divided
into two groups:
passive sensors, which sense naturally available energy
active sensors, which supply their own source of energy to illuminate
selected features. Examples are radar and light detection and ranging (LiDAR).
Radar emits pulses of microwave energy and LiDAR emits pulses of
laser light.
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