问题
I found in here this document. I read it but I keep wondering how to define a Polygon with 3 rings in WKT?
回答1:
You can use either the POLYGON or the MULTIPOLYGON type, but make sure the outer container ring is listed first followed by the inner hole rings. The orientations of the inner rings are not important since holes are explicit in the syntax.
X & Y are space separated, coordinates are comma separated, and ring extents are limited by parentheses and separated by commas. Polygons (outer ring plus any inner rings) are also limited by parentheses.
Finally, inner rings cannot cross each other, nor can they cross the outer ring.
Examples:
POLYGON ((10 10, 110 10, 110 110, 10 110), (20 20, 20 30, 30 30, 30 20), (40 20, 40 30, 50 30, 50 20))
MULTIPOLYGON (((10 10, 110 10, 110 110, 10 110), (20 20, 20 30, 30 30, 30 20), (40 20, 40 30, 50 30, 50 20)))
来源：https://stackoverflow.com/questions/8139739/wkt-how-do-you-define-polygons-with-3-rings-2-holes