When you overlay two topologies, you choose the method in which the two selected topologies interact. In some cases, the result varies according to which topology is the source and which is the overlay.
Intersect operations combine topologies and keep only the common geometry. Intersect acts like the Boolean AND operation. The results are the same whichever topology is chosen as the first or second. Object data is combined for the two operations.
Union operations combine polygons with polygons and keep all geometry. Union acts like the Boolean OR operation and can be used only with polygons. For example, you can combine parcels with soils information for property assessment. Use Union to maintain both sets of geometry together and pull them apart as needed.
Identity operations work like Union on the source topology and like Intersect on the overlay topology. Use Identity to combine nodes, links, or polygons with polygons and keep all the input geometry. Identity creates one topology with one link where the link is crossed by the overlay topology.
Clip operations use the overlay polygon topology as a boundary. The parts of the source polygons outside the overlay polygons are clipped and discarded. You can use this option to show polygons within a boundary polygon, such as a city or state boundary.
You can copy selected object data and external database data from the source and overlay topologies to a new object data table in the resulting topology. You specify the name of the new object data table that will store the data in the resulting topology. The object data table name should be a new name. Data fields in the resulting topology look like this:
|TOPOID||Polygon Identification Number|
|TOPONAME1_ID||Object ID in source topology (TOPONAME1 is name of Source)|
|TOPONAME2_ID||Object ID in overlay topology (TOPONAME2 is name of Overlay)|
|TOPONAME1_PERCENTAREA||Percentage area of parent polygon in source topology|
|TOPONAME2_PERCENTAREA||Percentage area of parent polygon in overlay topology|