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Well Known Text

Well known text (WKT) is a text mark up language for representing vector geometry objects. Most geometric functions accept a string containing well formed WKT syntax. These are known as WTK WKT strings. For example, "POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))".

WKT is commonly generated and used by Geographic Information Systems (GIS) or geographical databases. 

For more information on this topic see: Well Known Text

Geometry

Many geometry functions return a "Geometry" object this is the internal optimised representation of the results which can be passed directly into other geometry functions.

Where the final result of an expression is a geometry object then this will automatically be converted into a String once evaluation is complete.

To convert a Geometry object into a Well Known Text* string, so that it can be passed to other non-geometry functions toString() must be called.