GIS: The Essentials - Sampling Spatial Units for Agricultural Surveys - page 58

Overlay operations are part of most spatial analysis processes, and generally
form the center of GIS projects. Overlay is defined as the process of combining two
different thematic maps of the same area and overlaying them, one on top of the
other, to form a new map layer. It can also be described as the ability to integrate
data from multiple different sources.
In overlay analysis, logical operations are used to study the spatial conditions of
different layers. Overlay is generally performed using arithmetic, Boolean, and
relational operators, and can be executed in both the vector and raster domains.
The arithmetic operators (*, /, , +) allow for the addition, subtraction, multi-
plication, and division of two maps, or numbers, or a combination of the two. The
Boolean operators (AND, NOT, OR, and XOR) use Boolean logic (TRUE or
FALSE) on the input values. Output values of TRUE are written as 1, and
FALSE as 0. Relational operators (

) evaluate specific
relational conditions. If the condition is TRUE, the output is assigned 1; if the
condition is FALSE, the output is assigned 0.
In the context of multiple layers, the Boolean operators define the basic opera-
tion of union and intersection. The union operation refers to the Boolean OR
operation. It creates a new output coverage from two layers. The Boolean AND
operation represents the intersection. With this process, more layers are merged,
and only the intersect coverage is maintained in the final output.
During vector overlay, map features and associated attributes are integrated to
produce a new composite map. Vector overlay can be performed on different types
of map features: polygon-on-polygon, line-on-polygon, and point-on-polygon. A
polygon-on-polygon overlay is a process that merges spatially coincident polygons
from two coverages and their attributes, to create a third coverage that contains new
polygons and describes new relationships. Line-on-polygon is a spatial operation in
which lines in one coverage are overlaid with the polygons of another coverage, to
determine which lines, or portions of lines, are contained within the defined poly-
gons. For example, using this function we can define the total kilometers of
highways within the boundaries of a certain region. Finally, point-on-polygon
overlay is a spatial operation where points from one feature dataset are
superimposed on the polygons of another, to determine which points are enclosed
in the polygons. For example, this function can be used to determine if an address
(i.e., a point) is located within a certain zip code area (i.e., polygon). The overlay
process merges the attribute data associated with each feature type. The resulting
table will contain both the attribute data and spatial information.
In raster overlay, the pixel or grid cell values in each map are combined using
arithmetic and Boolean operators to produce a new value in the composite map. If
two grids are aligned and have the same grid cell size, then it is relatively easy to
implement overlay operations. The maps can be treated as arithmetic variables. For
this reason the method is often described as map algebra. A new layer of values is
produced from each pair of coincident cells. The values of these cells can be added,
subtracted, divided or multiplied. We can compute the maximum value, mean
value, a logical expression, and so on. The output cell simply takes on a value
equal to the result of the calculation. The simplicity of raster overlay makes the

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