Geography Reference
In-Depth Information
Figure 5.1
Point-in-polygon test example.
coincides with a boundary. Such cases are dealt with through additional stages added
to the algorithm employed.
h e point-in-polygon operation can be extended to the idea of assessing if an island
polygon is contained within another polygon. Firstly a MER can be used to assess if
containment of the island polygon by another polygon is possible. Next, a series of
horizontal bands is drawn across the polygon that may contain the island polygon.
h e i rst and last locations on these lines that intersect the island polygon are then
treated as for a point (as shown above). If the number of intersections for each line is
odd then the polygon is enclosed completely (Burrough and McDonnell, 1998).
5.2.2
Overlay operators
h is section deals with methods that enable answering of questions such as 'How
many sites of special scientii c interest (SSSI) occur in areas with highly polluted soil?.
h is kind of question is well depicted using Boolean logic (see Section 2.11.1). If SSSIs
are termed 'A' and highly polluted soil is termed 'B' then the statement A
AND
B (also
given by
A
«
B
) would correspond to our question: all areas fuli lling both criteria are
selected. In Appendix D, an approach to the line intersection problem is outlined and
this approach can be employed to implement the methods detailed in this section.
Polygon overlay operations are dei ned and illustrated below. h ese approaches
assume that the features of interest (e.g. polygons representing SSSIs and those repre-
senting areas with highly polluted soil) are stored in separate layers. In turn the Union,
Intersect, and Identity operations are outlined. h ese operations are similar, dif ering
only in the spatial features that remain in the output data layer. In all cases, an input
layer and a second overlay layer are shown along with the overlay output.
Union overlay joins two sets of polygons and retains all areas from both layers (and
the attributes in both layers are kept), so it makes no practical dif erence which is the
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