2.7.2 Polygon with a Hole

Figure 2-4 illustrates a polygon consisting of two elements: an exterior polygon ring and an interior polygon ring. The inner element in this example is treated as a void (a hole).

Figure 2-4 Polygon with a Hole

Description of Figure 2-4 follows
Description of "Figure 2-4 Polygon with a Hole"

In the SDO_GEOMETRY definition of the geometry illustrated in Figure 2-4:

  • SDO_GTYPE = 2003. The 2 indicates two-dimensional, and the 3 indicates a polygon.

  • SDO_SRID = NULL.

  • SDO_POINT = NULL.

  • SDO_ELEM_INFO = (1,1003,1, 19,2003,1). There are two triplet elements: 1,1003,1 and 19,2003,1.

    1003 indicates that the element is an exterior polygon ring; 2003 indicates that the element is an interior polygon ring.

    19 indicates that the second element (the interior polygon ring) ordinate specification starts at the 19th number in the SDO_ORDINATES array (that is, 7, meaning that the first point is 7,5).

  • SDO_ORDINATES = (2,4, 4,3, 10,3, 13,5, 13,9, 11,13, 5,13, 2,11, 2,4, 7,5, 7,10, 10,10, 10,5, 7,5).

  • The area (SDO_GEOM.SDO_AREA function) of the polygon is the area of the exterior polygon minus the area of the interior polygon. In this example, the area is 84 (99 - 15).

  • The perimeter (SDO_GEOM.SDO_LENGTH function) of the polygon is the perimeter of the exterior polygon plus the perimeter of the interior polygon. In this example, the perimeter is 52.9193065 (36.9193065 + 16).

Example 2-7 SQL Statement to Insert a Polygon with a Hole

Example 2-7 shows a SQL statement that inserts the geometry illustrated in Figure 2-4 into the database.

INSERT INTO cola_markets VALUES(
  10,
  'polygon_with_hole',
  SDO_GEOMETRY(
    2003,  -- two-dimensional polygon
    NULL,
    NULL,
    SDO_ELEM_INFO_ARRAY(1,1003,1, 19,2003,1), -- polygon with hole
    SDO_ORDINATE_ARRAY(2,4, 4,3, 10,3, 13,5, 13,9, 11,13, 5,13, 2,11, 2,4,
        7,5, 7,10, 10,10, 10,5, 7,5)
  )
);

An example of such a "polygon with a hole" might be a land mass (such as a country or an island) with a lake inside it. Of course, an actual land mass might have many such interior polygons: each one would require a triplet element in SDO_ELEM_INFO, plus the necessary ordinate specification.

Exterior and interior rings cannot be nested. For example, if a country has a lake and there is an island in the lake (and perhaps a lake on the island), a separate polygon must be defined for the island; the island cannot be defined as an interior polygon ring within the interior polygon ring of the lake.

In a multipolygon (polygon collection), rings must be grouped by polygon, and the first ring of each polygon must be the exterior ring. For example, consider a polygon collection that contains two polygons (A and B):

  • Polygon A (one interior "hole"): exterior ring A0, interior ring A1

  • Polygon B (two interior "holes"): exterior ring B0, interior ring B1, interior ring B2

The elements in SDO_ELEM_INFO and SDO_ORDINATES must be in one of the following orders (depending on whether you specify Polygon A or Polygon B first):

  • A0, A1; B0, B1, B2

  • B0, B1, B2; A0, A1