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In some settlement pattern studies, 
may be important. The term, aspect, refers to the compass direction towards
which any plot of land may face; it is the compass direction of the slope
of the land. Although aspect is not a limiting factor in this particular study,
you should look at an aspect map for the Northern Tucson Basin by selecting
.
As you can see, cells of land that face different directions are color-coded
(red = north, yellow = east, light blue = south, dark blue = west, white =
no slope/flat surface). GIS software applications calculate aspect maps from
DEMs in a similar manner to slope maps.
This tutorial (as well as the next two) will show you how ARC/INFO creates
new themes using existing themes and a variety of commands. The first command,
,
is used to combine the data from two or more themes to form a new theme. For
example, the theme, 
,
is the product of the union command. Select 
and 
and then to
see how union simply merges themes together to produce a composite theme that
contains all of the information (visual and attribute) from each of the donors.
The diagram below illustrates how the union command operates; the red area
represents the information that is kept in the new theme.
Tutorial #5. Intersect Command
The 
command is another function commonly called upon to create new themes from
two or more existing themes. Unlike the union command, which is additive because
the theme it produces has more information than do either of its parts separately,
the intersect command is reductive. GIS operators use the intersect command
to extract only those areas that are common to the features of two or more
themes. Thus, the only areas retained by the intersect command are those where
polygons from all themes overlap. The diagram below illustrates how the intersect
command operates; again, the red area represents the information that is kept
in the new theme.
To further illustrate the intersect command select 
.
is the intersection between
and 
.
Compare
to
to get a better idea of what the intersect command does.
is a view with both
and
themes superimposed. When these two images are placed side by side it is obvious
that the intersect command selected only the areas common to the polygons
of both themes to create .
In other words, only areas that are considered to be within habitation sites'
boundaries AND also within the boundary of the Zone 2 ecologically viable
area are retained by the intersect. All areas that are within the boundary
of only one or the other are not retained by the intersect. The resulting
theme also has the tabular data from both input themes.
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Aspect: the direction in which a slope or surface faces, especially in the context of exposure. Aspect can be calculated by many GIS, and is usually expressed in degrees relative to North. For example, North is 0 degrees, and South is 180.
Union: the outcome of the union command is an output data layer/theme that retains all the elements of all the input data layers/themes.
Intersect: an ARC/INFO overlay command. Intersect is the integration of two spatial data sets that preserves the data that fall within the area common to both in a new theme.