2.12 Join polygons - LUPMISManual

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2. GIS data entry

2.12 Join Polygons

Level of expertise required for this Chapter: Intermediate; general Map Maker training

In many cases it is necessary to ‘join’ polygons, i.e. to unite two neighbouring polygons into one. In live layer you use the Join-tool from the toolbar at left, click on one unit and drag to the other unit.

All characteristics (id, style, label, attributed data) of the ‘first’ polygon (where you started the dragging) will be copied to the new unit.

Remember, this works only if the two units are neighbours with their precise boundaries coincide (no ‘sliver’, no overlap).

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What if their boundaries are not precisely the same?

In such a case, you first make the boundaries match each other with the Snap-tool from the toolbar left, before you join them. Drag the snap marker from the polygon to be adjusted to the main polygon. Try to minimize the distance of snapping (shown). This is a dangerous command, because it makes changes to the outline of the unit.

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What if the overlap is too large, i.e. too large for ‘snapping’ ?

In this case, you have to ‘cut’ outside the overlap area (See example below, from 1 to 2, but outside the green area).

Delete the cut-off part (example, blue) and then ‘snap’ them together, as explained above.

You can now join them.

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Alternatively, you can use the Intersect-tool from the toolbar left: Main menu > Utilities > Vector utilities > Intersect / Unite files > Intersect lines and polygons window: Add file > Select file type > Select folder and file > Tick ‘Find intersections’ > Tick ‘Tidy boundaries’ > Execute > Save result as … > Select folder > Specify new file name > Save

Back to the live layer, you can join all three units.

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What if they are not the same unit? They will be joined. The new entire polygon will take the characteristics of the polygon, from where you started the join dragging.

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For more experienced users, there are more joining tools in the Live layer actions window for the entire live layer:

  • Join adjacent polygons of the same style

  • Join adjacent polygons of the same ID

  • Join adjacent polygons of the same display label

  • Join adjacent polygons with the same data value


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