GIS: The Essentials - Sampling Spatial Units for Agricultural Surveys - page 52

features required for spatial analysis: adjacency, containment, connectivity, and
intersection. Adjacency is a type of spatial relationship in which two or more
objects are neighbors. Containment refers to the possibility that a single feature is
contained within the boundaries of another feature. Connectivity denotes whether
two segments are connected (see below for a definition of segment). Intersection
concerns the existence of a common area shared by two different areas. A GIS
provides the tools to deal with all three elements of spatial data.
Some GISs use the raster structure, especially if they handle satellite data, while
others (for example, ESRI ARC/info) use a vector and raster data approach.
The raster is a regular matrix of values (see Fig. 3.2a ), and is defined in terms of
picture elements (i.e., pixels). A map is described using pixels, which are
referenced by a row and column position. Raster data represent a continuous
surface, and are simple to use. In particular, continuous fields such as elevation,
temperature, satellite images, and scanned maps can be described using a raster
structure. The main disadvantages of this representation are data redundancy and
the difficulty in interpreting topological relationships. Additionally, spatial analysis
is less accurate using a raster approach than with other techniques.
The vector approach (see Fig. 3.2b ) uses vectors to describe spatial features. In
mathematical terms, a vector is defined using a starting point, a direction, and a
length. The coordinates ( x , y ) represent the starting point, while an angle represents
direction. The vector data model is used to describe areas, lines, and points (see
below for definitions of these objects). The topology is clearly identified when using
the vector structure, and it results in a more accurate spatial analysis than raster
data. Vector data are particularly suitable for describing roads, utility networks, and
property boundaries. Most GISs allow conversions between vector and raster
formats, and vice-versa.
Spatial objects must be defined in a clear way so that they can be properly
analyzed. Generally speaking, spatial data can be in the form of points, lines (i.e.,
segments), and areas (i.e., polygons).
A point refers to a single location. It is usually considered to be a zero-
dimensional object or have a negligible size when compared with the whole
study area (see Fig. 3.3a ). A point in GIS is represented by a pair of coordinates
( x , y ). For example, we can consider the farms in a study area as a map of points.
The entity and the node represent important characteristics of spatial points. The
entity is used to describe the location of point features, while the node is the
location of a point that has supplementary topological characteristics such as
intersections or the end points of lines.
A line is generally defined as a one-dimensional object. It has a position,
direction, and length (see Fig. 3.3b ). A line contains at least two pairs of coordinates
(say ( x 1 , y 1 ), ( x 2 , y 2 )). In other words, a line should connect a minimum of two
points; the start and end points of a line are referred as nodes, while any change of
direction on a line is a vertex. Other features of lines are edges and directed links.
The edge is a connection between two nodes, and the directed links are links with a
specified direction.