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I'm new in QGIS, I have two shapefiles Northern Ireland (NI) and Ireland and there is overlapping with theirs borders. enter image description here Is there same way that could use border of NI for example and give me output merged those two files with border of one NI on inside part? Or can anyone help me how to solve this because as you can see I have also those "white parts" in between those two shapefiles.


I took advice from @Joseph, so I use shapefile (Ireland and NI together from ArcGIS and also the layer of NI constituency (it isn't county) NI_shapefile.zip. I did first

Processing Toolbox > QGIS geoalgorithms > Vector general tools > Split vector layer

I got Group A (Ireland) and Group B (NI) from Ireland.shp Then I use

Processing Toolbox > QGIS geoalgorithms > Vector overlay tools > Intersection

using Group B (NI) and NI_shapefile (I change projection to be EPSG 4326) I get intersection layer. Now I wanted to check if everything is good and at first seemed like it isenter image description here but then I zoomed and there is still white parts between borders enter image description here I tried to get layer with difference and then to do union but then QGIS crashes, every time.


I merged difference from NI_shapefile and Group B(NI that I got from splitting the whole Ireland) and added(merged) to Ireland part and that looks like this, enter image description here enter image description here so I don't have white parts but I have like double lines in the boundary. Can I erase them with some function and use the outter boundary?

  • If you need to correct geometries, you would have to merge both shapefiles and have a look to the GRASS v.clean algorithm in the Processing Panel ("Toolbox) with some snap option 'Snap lines to vertex in threshold') : grass.osgeo.org/grass72/manuals/v.clean.html. If you don't find it you will need to activate the Processing Plugin. See here : docs.qgis.org/2.14/en/docs/user_manual/processing/toolbox.html – gisnside Aug 16 '16 at 12:20
  • Are you sure you used the correct layer to intersect with? I do not see any gaps in my version. – Joseph Aug 17 '16 at 10:49
  • @Joseph I think that problem is in layer with NI Consistency file because it seems it's moved a little bit, so I made layer with difference NI consistency and NI layer that I got with split from whole Ireland layer. – Una Aksentijevic Aug 17 '16 at 14:39
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Both your layers are not alligned correctly. Instead of manipulating the vertices of both layers to match up, I would suggest starting afresh and download the shapefile for Ireland from ArcGIS which contains two polygons for both Ireland and Northern Ireland:

Ireland & Northern Ireland

You could then split this layer into two using the Split vector layer tool:

Processing Toolbox > QGIS geoalgorithms > Vector general tools > Split vector layer

Note that Northern Ireland is not broken up into counties as shown in your image, so we could use the Intersect tool:

Processing Toolbox > QGIS geoalgorithms > Vector overlay tools > Intersection

Choose the Northern Ireland polygon you received from the Split vector layer tool as the Input layer and your Northern Ireland counties layer as Intersect layer. Your new layer should now have the same counties as your original layer but still alligned perfectly:

Result

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Depnepds on wich boundary you want keep original. If you want keep the NI boundaries then you can manually extend the Ireland polygon over the NI shapefile, then perform difference and merge the result with NI shapefile:

  1. Edit the Ireland layer - You have to "cover" the white spaces by polygon. Move vertexes over (inside) the NI polygons or create new topologically clean "background" polygon (with checked aviod intersection in advanced snapping otpions)

  2. Cut the overlaying parts - Use Difference algorithm. Input will be Ireland layer and Difference layer will be NI. Generated output is Ireland layer extended to NI boundaries.

  3. Merge vector layers or Union - Merge NI layer and generated output layer from step 2

Algorthms can be found in processing toolbox or in menu Vector --> Geoprocessing tools

Note: If you don't need use exact theese layers, consider using another datasource for boundaries. There are many sources of free data based on openstreetmap etc.

  • can you please explain me how to "cover" and move vertexes over the NI? – Una Aksentijevic Aug 16 '16 at 21:02
  • I meant just toggle editing Ireland layer and manually move vertexes or drawing new polygon over the gaps ("inside" polygons in NI). This polygon can overlap NI since you cut the useless part in step 2 - this extend the boundaries exact to NI polygon. How to edit layer geometry read in QGIS documentation – Oto Kaláb Aug 17 '16 at 6:27

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