This is a repost of my unsolved QGIS dilema. The image shows a single selected feature that consists of two polygons and the corresponding attribute table. Some nodes of the left poly have been added and moved to illustrate the boundary/ overlap problem. Before the nodes were moved and added the two polys were snapped together and I could not get rid of the line between them. Hovering over the nodes, pop ups identify the polys as either 0 or 1. How does one create a single polygon from the two?
As I understand your issue, you currently have a multipolygon feature which has an invalid geometry since one part of it overlaps another part. There is a 2-step procedure that should help you to merge your multipolygon into one simple polygon:
First, run the
v.clean method which is located in the processing toolbox, GRASS section. Just select your layer and let the default options on. This should split your multipolygon into several features.
Then, select these new features and press the Merge features button in the Advanced Editing Toolbar (which you may need to activate first). This will merge the features into one beautiful polygon.
I've just whiled away a few hours wrangling with just such a problem and couldn't solve it using any of the
v.clean options using 2.18.2 for Mac.
I got lucky however, with trying the Multipart to singleparts geometry tool by choosing the attribute known to be unique resulting in a new temporary vector layer with three adjoining but separate features.
Select the adjoining features in edit mode and use the
Edit>Merge Selected Features option to join the separate features into a single polygon.
Don't forget to save the temporary layer to disk using the
Layers>Save As option otherwise the new layer will be lost even when saving the project.