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Can one row in an attribute table be linked to many separate polygons? I did a series of selects, buffer, intersects on some shapefiles and my resulting table has for one row many polygons. Can this be correct or do I have some further operations to do?

I am doing a site selection assignment so I used the criteria given and did selects, buffers (eg certain distance from roads and rivers) etc and my final attribute table has x number of rows. When I select a row, that one row identifies four polygons on the map. I am asking if I should stop here or should I separate out the polygons into their own identifying row.

2 Answers 2

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Yes, it's called a multi-part shape and is perfectly normal. At the simplest, they can be created by selecting a bunch of different polygons and using the Merge tool. Whether they are desirable is a different story. You would need to describe in much more detail what your process is trying to accomplish and whether you want one row for each shape or not for us to provide any further direction (or explanation as to why you got the result you did).

And the answer to whether you should separate them is 'it depends' - there's still not quite enough information to give you specifics, but it's very easy to generate multiparts with the tools and process you describe. You may want to split them (two parcels selected in the same buffer but they're independent choices) or you may not (a park was cut up during the intersect into two separate shapes, but selection of the park is what matters, not the individual pieces).

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    The question says: one row linked to many separate polygons. I read it literally as "many separate polygons", not as "one multipolygon". I do not know enough about that ArcGIS workflow and perhaps it actually makes multipolygons. Generally one to many relations are OK but they are hard to handle with traditional GIS. You can find reading by searching "GML application schema" and European INSPIRE data definitions are full of such examples. You can start from Cadastral parcels.
    – user30184
    Commented Nov 1, 2015 at 22:00
  • It's the dissolve tool that creates multipart polygons, not the merge tool. Commented Nov 2, 2015 at 4:37
  • While a agree with @user30184 that is what the question says, she also indicates doing a series of operations revealing that something seemingly odd is going on and Chris's answer hints at the source of the problem. If you don't want this behavior a solution can be added here to resolve it. Commented Nov 2, 2015 at 15:02
  • @user30184 That is a possible interpretation, and a now deleted answer mentioned joins and relates, but the OP didn't. Based on my own experience with ArcGIS and the tools/process described, it seemed more a geometry/editing issue to me (yes, Arc does create multiparts under certain circumstances) than a join/table issue.
    – Chris W
    Commented Nov 2, 2015 at 20:07
  • @EmilBrundage There are a number of tools/processes that can generate multiparts. The Dissolve tool is one of them and can generate them but doesn't have to - it gives you a choice, unlike most of the operations that result in multiparts. The Merge tool can also create multiparts and is (in my opinion) the fastest way to demonstrate the concept - in an edit session select two disconnected polygons, and then choose Merge from the Editor drop-down menu. When prompted, choose which source poly to preserve the attributes of and click ok. You get a multipart.
    – Chris W
    Commented Nov 2, 2015 at 20:14
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If the multi-part Polygons are not desireable, you can get rid of them all using the Advanced Editing Toolbar (see Separating a multi-part feature).

1) First add the toolbar by going to Customize->Toolbars->Advanced Editing.
2) Enter an edit session to edit the polygon layer in question.
3) Select any (or all) record(s) that you wish to explode.
4) Click the "Explode Multi-part Features" button on the Advanced Editing Toolbar.

Advanced Editing Toolbar with "Explode Multi-part Feature" button highlighted

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    FYI there's also the Multipart to Singlepart tool. Big difference is that works on an entire layer/FC at once, not just selected, and creates a new FC. Explode just splits them apart.
    – Chris W
    Commented Nov 2, 2015 at 20:21
  • Excellent point. I have also used that. The benefit being also that you leave the original layer intact! Commented Nov 2, 2015 at 20:22

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