Segments Versus Polygons
Geographic objects represented by vector data might or might
not be formatted as polygons. Imagine two variables, latcoast
and loncoast
,
that correspond to a sequence of points that caricature the coast
of the island of Great Britain. If this data returns to its starting
point, then a polygon describing Great Britain exists. This data might
be plotted as a patch or as a line, and it might be logically employed
in calculations as either.
Now suppose that you want to represent the Anglo-Scottish border, proceeding from the west
coast at Solway Firth to the east coast at Berwick-upon-Tweed. This data can only be
properly defined as a line, defined by two or more points, which you can represent with
two more variables, latborder
and lonborder
. When
plotted together, the two pairs of variables can form a map. The patch of Great Britain
plus the line showing the Scottish border might look like two patches or regions, but
there is no object that represents England and no object that represents Scotland,
either in the workspace or on the map.
In order to represent both regions properly, the Great Britain
polygon needs to be split at the two points where the border meets
it, and a copy of latborder
and lonborder
concatenated
to both lines (placing one in reverse order). The resulting two polygons
can be represented separately (e.g., in four variables named latengland
, lonengland
, latscotland
,
and lonscotland
) or in two variables that define
two polygons each, delineated by NaNs (e.g., latuk
, lonuk
).
The distinction between line and polygon data might not appear to be important, but it can
make a difference when you are performing geographic analysis and thematic mapping. For
example, polygon data can be treated as line data and displayed with functions such as
linem
, but line data cannot be handled as polygons unless it is
restructured to make all objects close on themselves.