7 Replies Latest reply on Oct 9, 2012 8:49 AM by MKennedy-esristaff

    Combining Polygons

    marcz009
      I'm working with a polygon feature class that has polygons of the countries in the world. After changing the projection I noticed that there was a border that appeared where the initial central meridian was located. Not only was there a "border" but I noticed that there was an actual gap between two polygons. One in Russia and another in Antarctica at 180 degrees West. I have split the polygons using the 'Explode Multipart Feature' tool so that I can edit their individual vertices. I'm now stumped on how I can combine the two polygons into one without a border.

      I do NOT mean I want to use the 'Merge' tool in the Edit menu so that they share attributes. I need to combine two polygons into one shape without a border or gap. Can anyone help me? I have attached a picture showing the gap.
        • Re: Combining Polygons
          recurvata
          Maybe I'm misunderstanding, but  Merge seems like the correct tool. Just overlap the borders, select both polygons, and then use the Merge tool on the Editor dropdown. Voila, one feature. Technically, I think you could get away with merging with a shared border, but overlapping is easier and makes sure you didn't miss a sliver.

          As to why you have two polygons from one after changing  the projection, I have no idea. Are you sure they weren't two very close polygons in the original?
          • Re: Combining Polygons
            marcz009
            Thanks for your advice. I've tried the overlapping and merge to no avail. There is still a gap even after that. Here is the prompt I received which may help me explain why there is a gap after the projection. I went from a Robinson to a Miller.

            "Performing the projection is the easy part of the exercise; doing the cleanup on the results is what takes a little time. At a minimum you will have to combine polygons�??because there will be a couple of unnecessary borders due to the shifting of the central meridian (fixes are needed for eastern Russia and Antarctica at 180°W). The easiest fix is to use the Dissolve tool (in ArcToolbox). Hint: you will have to edit polygon boundaries and you may have to Explode Multipart Features (Advanced Editing Toolbar) in order to do so."
            • Re: Combining Polygons
              marcz009
              Here are the before (Robinson) and after (Miller) which also may help. You can see the erroneous borders created in Russia and Antarctica after the projection. These are the actual gaps.
              • Re: Combining Polygons
                Wayne_Whitley
                Then if you've accepted you have to edit polygons with editor tools (as opposed to a geoprocessing solution), you simply have to 'extend' one (or both) polygons (via modifying; moving/adding vertices) to at least touch before executing the merge.  Of course, exercise care not to create an inaccurate depiction of reality.  Just wondering if it makes better sense to merge before projecting?
                • Re: Combining Polygons
                  recurvata
                  Then if you've accepted you have to edit polygons with editor tools (as opposed to a geoprocessing solution), you simply have to 'extend' one (or both) polygons (via modifying; moving/adding vertices) to at least touch before executing the merge.  Of course, exercise care not to create an inaccurate depiction of reality.  Just wondering if it makes better sense to merge before projecting?


                  I think what he's saying is that doing that, same as I suggested above, won't work because Arc is splitting the polygons anyway due to some discrepancy about the central meridian of the new projection. Maybe create a new polygon overlapping both polygons and merge all three? Never seen this happen, so not sure. I don't see how Dissolve would work without overlapping the polygons anyway, but I rarely use that tool, so maybe it would work.
                  • Re: Combining Polygons
                    Wayne_Whitley
                    Ah, I see now what you mean and yes, about the 3rd poly, that's been a fav fast editing trick adding a 3rd overlapping poly to fill the gap, particularly when you want to be careful modifying the existing shapes as little as possible.  In a dissolve execution if the polys do not overlap, a multipart feature is created (I think there is an option to disallow multipart but that doesn't solve the problem).
                    At the very least, if there is a problem editing the polygon at this location, could hack some sort of a solution - maybe Feature To Line, hack out the line segments creating the annoying gap, 'mend' the gap by adding 2 line segments essentially connecting the 2 halves, then execute Feature To Polygon (provided you have an ArcInfo license).  There are other workarounds too... I'd try recurvata's suggestion 1st, then report back - good luck!
                    • Re: Combining Polygons
                      MKennedy-esristaff
                      The original data is built with left/right extents at the equivalent of -180/+180 longitude in the Robinson projection. In-house we call the line that is the equivalent of 180 degrees from the central meridian, the "anti-meridian." That's where features have to be clipped and closed when data is projected to the coordinate system.

                      Russia does have a separate part in the western hemisphere. When data is reprojected, we do not try to merge features back together for performance reasons. Particularly at the +/180 degree line, the gap is bigger than you might expect. This helps keep vertices from hopping back and forth across the 180 line and makes it easier to create a clean, closed polygon.

                      Melita