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Each function that belongs to this group takes a geometry value
as its argument and returns some quantitative or qualitative
property of the geometry. Some functions restrict their argument
type. Such functions return NULL if the
argument is of an incorrect geometry type. For example,
Area() returns
NULL if the object type is neither
Polygon nor MultiPolygon.

User Comments
The GLength function can not be used for calculating the distance on a sphere ... like earth. correct me if i m wrong.
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