Comparing spatial objects for equality

[Ned]

Compariing spatial objects for equality is very subtle, because you have to define what is meant by equal.  In the case of polygons, you can have 2 different polygons that describe the same shape, but have a different start and end point.  Most people would say these are the same, but if you compare them exactly, they are in fact different.  For instance:

[ -104, 39], [ -105, 39 ], [ -104, 38 ], [ -104, 39 ]

[ -105, 39 ], [ -104, 38 ], [ -104, 39], [ -105, 39 ]

These 2 sets of points describe the same shape just from different starting points.  The same problem exists with lines - the same line specified backwards and forwards looks the same on a map, but would not be superficially equals.

The easiest way to test for equality is to test: does object A contain object B and also, does object B contain object A.  If they both contain each other, they must cover the exact same area on a map.

In a formula or filter tool, you can say:  ST_Contains([A],[B]) AND ST_Contains([B],[A)

In a SpatialMatch tool, you have to use a custom DE-9IM string to describe the match you want.  In this case, use the string "T*F**FFF*".  I won't try to document that string - see https://en.wikipedia.org/wiki/DE-9IM for a reference of how that works.

Attached is a module (for Alteryx 10) that demonstrates both techniques: