Geographic objects represented by vector data might or might
not be formatted as polygons. Imagine two variables,
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
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 axes.
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
to both lines (placing one in reverse order). The resulting two polygons
can be represented separately (e.g., in four variables named
lonscotland) or in two variables that define
two polygons each, delineated by NaNs (e.g.,
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
but line data cannot be handled as polygons unless it is restructured
to make all objects close on themselves, as described in Link Line Segments with Common Endpoints into Polygons.