Description

A common problem when generating polygons from Well Known Text (WKT) is listing the rings within the polygon in the correct orientation. The vertices of the outer ring must be listed counter-clockwise, while the vertices of the inner rings must be listed in a clockwise direction.

If you use the POLYGONFROMTEXT() function to create GEOGRAPHY values from WKT strings, the rings can be individually correct but, if they are not oriented properly, the resulting polygon will not match your intended geographic region. As a consequence, using the polygon in VoltDB geospatial functions, such as CONTAINS() and DISTANCE(), will produce unexpected answers. You can use ISVALID() to test if the polygon is valid, but ISVALID() simply tests correctness, it does not fix simple errors, such as ring orientation.

MAKEVALIDPOLYGON() both tests the polygon and corrects any errors in ring orientation. The argument to the MAKEVALIDPOLYGON() function is a GEOGRAPHY object representing a polygon. The output is another GEOGRAPHY object, identical to the input if the input is valid, or with the orientation of the rings corrected if they are listed in the wrong direction. If there are any other issues with the polygon that cannot be corrected (such as an incomplete ring or crossed lines), the function throws an error.

Example

The following example uses the MAKEVALIDPOLYGON() function to correct any potential orientation issues with the location column in the country table.

UPDATE country SET boundaries = MAKEVALIDPOLYGON(boundaries);