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Why are polygon features grouped into multi-line string features by the ArcSDE sdegroup command?
The sdegroup command merges features by combining their geometry into multi-part shapes. Polygon and multi-part polygon features are grouped into multi-line string features to avoid the loss of topology that occurs if polygon or multi-part polygon features are grouped into multi-part polygon features.
Topology is lost because multi-part polygons cannot share a common boundary. Polygon elements in a multi-part polygon may not intersect at a one dimensional line or two dimensional area geometry. They may only intersect at a zero dimensional, or point geometry. Therefore, when ArcSDE detects the invalid geometry of a line separating the two polygons, it corrects it by dissolving the line. This means the common boundaries of grouped polygons and multi-part polygons are dissolved.
Therefore polygons must be grouped into multi-line strings to preserve shared border topology.