For Union_analysis result: Will FID attribute for these gap features all be -1?

[CarolLiu]

I used union_analysis on 2 polygon features. The resulting table did show -1 for some of FID attributes. But when I checked the details on ArcMap, I found out that FID attributes for a lot of gap features were not -1 (instead 18, 22, etc). Does anybody know why? Thanks.

[DanPatterson_Retired]

From the help Union—Help | ArcGIS for Desktop did you specify gaps or no gaps? they produce different outputs depending upon the union and what happens when gaps are identified.  Perhaps a figure would help if you have an example that could be used for commentary

[CarolLiu]

I just tried both (setting gaps/no-gaps). It did not make any difference. 3 screenshots have been enclosed down. Scenario: FireBoxMap layer is polygon (figure #1); FireBoxMap was converted to line; FireBoxMap_PolygonToLine_Buf layer was generated from FireBoxMap using 50 ft as buffer distance (figure #2); union_gap layer came from Union_Analysis of (FireBoxMap layer and FireBoxMap_PolygonToLine_Buf layer) with Gap Allowed (figure #3). The identify info shows the gap (highlight with green on the map). As seen, FID_FireBoxMap is 32, while FID_FireBoxMap_PolygonToLine_Buf is 52.

[DanPatterson_Retired]

ok... still confused, I see a green pencilled in area, but I don't see the record selected in the table..

[CarolLiu]

I just did another screenshot. The same feature with highlight in Table.

[CarolLiu]

I think I figured it out now. Many thanks, Dan!!!