Agriculture Reference
In-Depth Information
These layers can be combined with each other to create an outline picture of the
real world. This simple concept is invaluable for solving many real-world problems
such as tracking navigation, managing governmental services, and modeling global
atmospheric circulation.
A layer is a collection of specific elements (such as trees, roads, and buildings)
that can be viewed together with other layers for a complete description of the area,
or separately to give more specific information of that particular element. A layer
can contain both spatial and attribute data.
Spatial analysis methods can be broadly divided into single-layer and multi-
layer operations.
Single-layer operations are applied to only one data layer at a time, and are also
known as horizontal operations. They provide the essential data preparation tools
for spatial analysis (Chou 1997 ). Generally, single-layer operations can be split into
three groups: feature manipulation, feature selection, and feature classification
(K¨hl et al. 2006 ). Feature manipulation changes the spatial features of a data
layer. Feature selection identifies features using spatial manipulation or logical
expressions. Feature classification classifies features into groups.
Feature manipulation techniques include boundary and proximity analysis. We
can change the boundaries of objects using boundary operations, and define new
boundaries. The most important boundary operations are: clipping, erasing,
appending, dissolving, splitting, and elimination.
In a clipping procedure, a polygon-shaped region is extracted from a map and
only the attributes and features that lie within the specified area are output. In other
words, the boundaries of the polygon are placed onto the whole region; all other
areas are discarded (see Fig. 3.4 ).
In contrast to the clip procedure, erase creates a new output by deleting the
features inside a polygon from the input data. This process preserves all the portions
falling outside the erase polygon (see Fig. 3.5 ).
The split procedure is used when the original map is separated into subdivisions
(see Fig. 3.6 ). Each new output contains only those portions of the input features
overlapped by the split polygons.
Append combines an unlimited number of adjacent maps into a single map,
creating a single coverage (see Fig. 3.7 ).
Dissolve procedures are typically used after adjacent maps are appended. This
process creates a new coverage by merging adjacent polygons, lines, or regions that
have the same value for a specified item (see Fig. 3.8 ). In this way unwanted new
boundaries and links are eliminated.
The eliminate operation is commonly used to remove unwanted sliver polygons.
Sliver polygons are very small polygons along the boundary of normal polygons. In
many cases, they are invisible at normal scales (see Fig. 3.9 ).
Feature manipulation also includes proximity analysis, which is based on the
distance from selected features. A typical operation in this class is buffering. The
buffer operation creates a zone of specified width around a point, line, or polygon. It
is also referred to as a zone of specified distance around coverage features. There
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