The primary function of a geographic information system is determining the spatial relationships between features. The distance separating a hazardous waste disposal site and hospital, school, or housing development is an example of a spatial relationship.

ArcSDE presents several functions that determine, evaluate, and manipulate spatial relations between different geometry objects. Some functions return a value as a result of a spatial relationship. Other functions, such as predicates, return TRUE if a test passes and FALSE, otherwise, to determine if a specific relationship exists between a pair of geometries. The result returned by the distance function, the space separating two geometries, is a double precision number. Alternatively, functions like intersection return a geometry as the result of combining two geometries.

ArcSDE functions that compare or combine two shapes use Clementini operators. Eliseo Clementini and colleagues have formalized the possible relationships that can exist between shapes. The Clementini operators differentiate between a shape's boundary and interior. For example, a point is considered to be all interior and has no boundary. In the case of a line, its endpoints constitute its boundary, while the rest of the line is considered the interior. For polygons, the lines that form the edge are the boundary, while the rest of the polygon is the interior.

General coordinate validation rules

ArcSDE applies the following internal integer coordinate rules to geometry coordinates:

• Small polygon factor—Area geometry with area less than 0.5 integer units are deleted
• Sliver polygon factor—Long thin polygons (slivers) are deleted when the square of their longest side divided by the polygon area is less than or equal to 1 trillion
; for example, Longest side ** 2 / area <= 10**12
• Coordinate snapping factor—Coordinates separated by less than the square root of 2 integer units divided by 2 integer units (approximately 0.707 integer units) are snapped together; for example, Distance separating coordinates <= ( 2**-1 ) / 2

These rules can affect the geometries that are created by overlay functions, SE_shape_intersect*. These factors are more likely to be applied if the system units used to store your geometry are too small.

Minimum Distance

The minimum distance separating disjoint features could represent the shortest distance an aircraft must travel between two locations. The distance function reports the minimum distance separating two disjoint features. If the features aren't disjoint the function will report a zero minimum distance.

See also

Predicates

Intersection of Geometries

Difference of Geometries

Union of Geometries