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SAP Sybase SQL Anywhere 16.0 » SQL Anywhere Server - Spatial Data Support » Spatial data » Advanced spatial topics


How spatial comparisons work

There are two methods you can use to test whether a geometry is equal to another geometry: ST_Equals, and ST_OrderingEquals. These methods perform the comparison differently, and return a different result.

  • ST_Equals   The order in which points are specified does not matter, and point comparison takes tolerance into account. Geometries are considered equal if they occupy the same space, within tolerance. For example, if two linestrings occupy the same space, yet one is defined with more points, they are still considered equal.

  • ST_OrderingEquals   With ST_OrderingEquals, the two geometries must contain the same hierarchy of objects with the exact same points in the same order to be considered equal under ST_OrderingEquals. That is, the two geometries need to be exactly the same.

To illustrate the difference in results when comparisons are made using ST_Equals versus ST_OrderingEquals, consider the following lines. ST_Equals considers them all equal (assuming line C is within tolerance). However, ST_OrderingEquals does not consider any of them equal.

Line A and B have the same two end points, but they are specified in different order. Line C looks like line A, but at a slight angle. Line D has three points defined for the line, but consumes the same space as lines A and B.
 How SQL Anywhere performs comparisons of geometries