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I have been trying to produce an outline of a stylised island using dissolve however there are internal "lines" that remain after I use dissolve? The hexagons that are being merged were created with the grid function so there shouldn't be any gaps present between them. I have tried "delete holes" however QGIS does recognise the lines as holes. QGIS is also showing the output as one large polygon too.

I'm a bit stumped as to why this is occurring. Alternatively is there is a different way to easily produce the outline of the polygons?

The below photos show the grid created input and the dissolve output with lines on it.

Input grid

Dissolve tool output showing lines

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It seems that the polygons are not contiguous (there are gaps between them). To solve the problem, you can you use Clean tool located under GRASS tools, which has the a clean tool Snap. You need to run QGIS with GRASS enabled.

Search for Clean tool in toolbox and open it:

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Select the polygon layer that you want to clean, then go to the clean tools and click on the ... 3 dots.

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Select Snap option

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You may need to adjust the snapping tolerance in order to get the desired output.

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Another quick solution could be to buffer the polygons by a very small amount such that it eliminates the overlap. this will obviously only be an applicable solution if accuracy of the buffer amount is not an issue for you.

I have sometimes done this approach, instead of using snapping, mainly because of performance but also if the source data has inherent errors, which causes the snapping to fail. i was spending far too much time trying to correct data errors, when in the end a simple buffer not only solved the data errors, but also resulted in nice overlap for a dissolve to work.

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