GIS: The Essentials - Sampling Spatial Units for Agricultural Surveys - page 54

3.3 Spatial Analysis of GIS Data

The analysis represents the core of any GIS system. A GIS analysis allows us to
visualize the patterns and relationships of geographical data. Current methods can
be very simple such as a map of the theme under investigation, or more complex
models such as those that simulate the real world by combining many layers of data.
There exist an extensive number of methods for analyzing geo-referenced data. In
particular, these techniques are grouped into two classes: spatial analysis and spatial
statistics. Spatial analysis studies real-world processes, and typically uses a GIS. It
provides information about the real world and the current situation in terms of
specific areas and features, and changes in situations or trends. Spatial statistics
concerns methods for understanding data distributed in a space, where position and
distance have meaning. See Sect. 1.4 for an introduction to spatial statistical
methods. Here, we only provide a brief overview of spatial analysis.
Spatial analysis is typically based on three elements of spatial information:
location, attribute, and topology (see Sect. 3.2 ). A key element is represented by
a representation of the location of any object, expressed using an appropriate
coordinate system.
The most basic coordinate system is the geographic coordinate system, where
the location of a point on the earth is determined using longitude and latitude. This
system is based on a grid of meridians and parallels: meridians connect the north
and south poles (i.e., longitude lines), while parallels form circular lines (i.e.,
latitude lines) around the Earth, parallel with the equator. The longitude of a
point is defined as the angular distance east or west on the earth

s surface, measured
between its meridian and the prime meridian at Greenwich (England). It is
expressed in degrees, or by some corresponding difference in time. The latitude
of a point is defined as the angular distance north or south from the equator,
measured on the meridian of the point. The longitude is measured 0-180 east
from the prime meridian or 0-180 west, where 180 longitude is the International
Date Line. Latitude is expressed in 0-90 north and 0-90 south of the equator.
The Universal Transverse Mercator (UTM) geographic coordinate system uses a
two-dimensional Cartesian coordinate system to provide point locations on the
Earth. It is a horizontal position representation, and not a single map projection.
The UTM system is used by many national mapping agencies, and it is based on a
transverse Mercator (conformal and cylindrical) projection with zones from north
to south. UTM divides the globe into 60 strips with a width of 6 longitude, starting
at 180 longitude (west). The Gauss-Kr¨ger grid system is used in numerous
European countries. It is based on the transverse Mercator projection and the Bessel
ellipsoid. The strips are 3 wide, leading to 120 strips. Contiguous zones have a
small overlapping area.
When an appropriate coordinate system has been defined, each point can be
represented and a basic spatial analysis can be conducted. Typically, a GIS stores
information in layers or tables that hold data about a particular kind of feature.

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