Polygon object type is intended for description of the areas limited by the closed contour. The contour of the object is a polygon. Examples of such objects are the territories of woods, lakes and settlements.
A simple polygon object has one contour with number of points from 4 or more. For example: a triangular closed contour contains 3 points with different coordinates, the coordinates of the last point (4) coincide with the first, ensuring closure of the contour.
A simple polygon object:
More complex objects contain additional closed contours: internal and external. Internal contours define the areas excluded from the area of the first contour of object. An example of such object is a lake with an island. In this case the lake should have a contour of the basic object (lake) and an internal contour (island).
Polygon object with internal contour:
External contours define the areas added to the area of the first contour of object. Polygon objects having external contours are called multipolygons (see Figure 8). An example of such object is the territory from two land plots belonging to one owner, separated by a land plot of other owner.
Multipolygon (green orchard):