QGIS3 - Join by location (falsely) adds neighbouring polygons












3















I have two layers:

Layer 1 - containing municipalities of country X in the year 2014 (around 600)

Layer 2 - containing municipalities of country X in the year 2018 (around 500)



Between 2014-2018 several municipalities have merged. E.g. municipality A, B and C in the 2014 layer no longer exist in 2018 but are now part of municipality D.



I want to know for each 2014 municipality what its corresponding 2018 municipality is. So for municipality A, B and C the outcome should be D.



enter image description here



enter image description here



I tried to achieve this by performing Join by Location (using intersect). I would expect the joined layer to have 600 features (i.e. the number of 2014 municipalities) with a new attribute containing the corresponding 2018 municipality. However, the joined layer I get has over 2400 features. Municipality A is present 4 times, one entry has municipality D as new attribute (as expected) but the other 3 entries have the neighbouring municipalities of A as new attribute (E, F and I). I.e. QGIS seems to think that municipality A does not only intersect with D but also with all neighbouring municipalities of A.



Am I doing something wrong here?



Both layers have the same CRS.



Edit: Found out that if I zoom in a lot (ratio 85:1) there's a small difference between the two layers. I guess this causes the issue.



enter image description here






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    3















    I have two layers:

    Layer 1 - containing municipalities of country X in the year 2014 (around 600)

    Layer 2 - containing municipalities of country X in the year 2018 (around 500)



    Between 2014-2018 several municipalities have merged. E.g. municipality A, B and C in the 2014 layer no longer exist in 2018 but are now part of municipality D.



    I want to know for each 2014 municipality what its corresponding 2018 municipality is. So for municipality A, B and C the outcome should be D.



    enter image description here



    enter image description here



    I tried to achieve this by performing Join by Location (using intersect). I would expect the joined layer to have 600 features (i.e. the number of 2014 municipalities) with a new attribute containing the corresponding 2018 municipality. However, the joined layer I get has over 2400 features. Municipality A is present 4 times, one entry has municipality D as new attribute (as expected) but the other 3 entries have the neighbouring municipalities of A as new attribute (E, F and I). I.e. QGIS seems to think that municipality A does not only intersect with D but also with all neighbouring municipalities of A.



    Am I doing something wrong here?



    Both layers have the same CRS.



    Edit: Found out that if I zoom in a lot (ratio 85:1) there's a small difference between the two layers. I guess this causes the issue.



    enter image description here






    qgis







    share|improve this question









    New contributor




    tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.























      3












      3








      3








      I have two layers:

      Layer 1 - containing municipalities of country X in the year 2014 (around 600)

      Layer 2 - containing municipalities of country X in the year 2018 (around 500)



      Between 2014-2018 several municipalities have merged. E.g. municipality A, B and C in the 2014 layer no longer exist in 2018 but are now part of municipality D.



      I want to know for each 2014 municipality what its corresponding 2018 municipality is. So for municipality A, B and C the outcome should be D.



      enter image description here



      enter image description here



      I tried to achieve this by performing Join by Location (using intersect). I would expect the joined layer to have 600 features (i.e. the number of 2014 municipalities) with a new attribute containing the corresponding 2018 municipality. However, the joined layer I get has over 2400 features. Municipality A is present 4 times, one entry has municipality D as new attribute (as expected) but the other 3 entries have the neighbouring municipalities of A as new attribute (E, F and I). I.e. QGIS seems to think that municipality A does not only intersect with D but also with all neighbouring municipalities of A.



      Am I doing something wrong here?



      Both layers have the same CRS.



      Edit: Found out that if I zoom in a lot (ratio 85:1) there's a small difference between the two layers. I guess this causes the issue.



      enter image description here






      qgis







      share|improve this question









      New contributor




      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.












      I have two layers:

      Layer 1 - containing municipalities of country X in the year 2014 (around 600)

      Layer 2 - containing municipalities of country X in the year 2018 (around 500)



      Between 2014-2018 several municipalities have merged. E.g. municipality A, B and C in the 2014 layer no longer exist in 2018 but are now part of municipality D.



      I want to know for each 2014 municipality what its corresponding 2018 municipality is. So for municipality A, B and C the outcome should be D.



      enter image description here



      enter image description here



      I tried to achieve this by performing Join by Location (using intersect). I would expect the joined layer to have 600 features (i.e. the number of 2014 municipalities) with a new attribute containing the corresponding 2018 municipality. However, the joined layer I get has over 2400 features. Municipality A is present 4 times, one entry has municipality D as new attribute (as expected) but the other 3 entries have the neighbouring municipalities of A as new attribute (E, F and I). I.e. QGIS seems to think that municipality A does not only intersect with D but also with all neighbouring municipalities of A.



      Am I doing something wrong here?



      Both layers have the same CRS.



      Edit: Found out that if I zoom in a lot (ratio 85:1) there's a small difference between the two layers. I guess this causes the issue.



      enter image description here






      qgis




      qgis






      share|improve this question









      New contributor




      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question









      New contributor




      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question








      edited 12 hours ago







      tilkas













      New contributor




      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      asked 16 hours ago









      tilkastilkas

      184




      184




      New contributor




      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      tilkas is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






















          2 Answers
          2






          active

          oldest

          votes


















          3














          The solution is to do the following:




          • Generate the centroids of the polygons of 2014

          • Spatially join the centroids with the polygons of 2018

          • Join by attribute the polygons of 2014 with their centroids


          I hope this would help.






          share|improve this answer



















          • 2





            To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

            – Ian Turton♦
            15 hours ago



















          2














          Set it up the following:




          • As input layer choose your old municipalities (2014).

          • As join layer choose your new municiplaities (2018).

          • As geometric predicate choose overlaps, within and equals.


          enter image description here



          The result:



          enter image description here



          Of course this will only work if the actual boundaries did not move.






          share|improve this answer
























          • Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

            – tilkas
            12 hours ago






          • 1





            yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

            – MrXsquared
            12 hours ago











          Your Answer








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          2 Answers
          2






          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3














          The solution is to do the following:




          • Generate the centroids of the polygons of 2014

          • Spatially join the centroids with the polygons of 2018

          • Join by attribute the polygons of 2014 with their centroids


          I hope this would help.






          share|improve this answer



















          • 2





            To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

            – Ian Turton♦
            15 hours ago
















          3














          The solution is to do the following:




          • Generate the centroids of the polygons of 2014

          • Spatially join the centroids with the polygons of 2018

          • Join by attribute the polygons of 2014 with their centroids


          I hope this would help.






          share|improve this answer



















          • 2





            To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

            – Ian Turton♦
            15 hours ago














          3












          3








          3







          The solution is to do the following:




          • Generate the centroids of the polygons of 2014

          • Spatially join the centroids with the polygons of 2018

          • Join by attribute the polygons of 2014 with their centroids


          I hope this would help.






          share|improve this answer













          The solution is to do the following:




          • Generate the centroids of the polygons of 2014

          • Spatially join the centroids with the polygons of 2018

          • Join by attribute the polygons of 2014 with their centroids


          I hope this would help.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 15 hours ago









          Mohannad AdhamMohannad Adham

          34415




          34415








          • 2





            To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

            – Ian Turton♦
            15 hours ago














          • 2





            To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

            – Ian Turton♦
            15 hours ago








          2




          2





          To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

          – Ian Turton♦
          15 hours ago





          To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).

          – Ian Turton♦
          15 hours ago













          2














          Set it up the following:




          • As input layer choose your old municipalities (2014).

          • As join layer choose your new municiplaities (2018).

          • As geometric predicate choose overlaps, within and equals.


          enter image description here



          The result:



          enter image description here



          Of course this will only work if the actual boundaries did not move.






          share|improve this answer
























          • Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

            – tilkas
            12 hours ago






          • 1





            yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

            – MrXsquared
            12 hours ago
















          2














          Set it up the following:




          • As input layer choose your old municipalities (2014).

          • As join layer choose your new municiplaities (2018).

          • As geometric predicate choose overlaps, within and equals.


          enter image description here



          The result:



          enter image description here



          Of course this will only work if the actual boundaries did not move.






          share|improve this answer
























          • Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

            – tilkas
            12 hours ago






          • 1





            yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

            – MrXsquared
            12 hours ago














          2












          2








          2







          Set it up the following:




          • As input layer choose your old municipalities (2014).

          • As join layer choose your new municiplaities (2018).

          • As geometric predicate choose overlaps, within and equals.


          enter image description here



          The result:



          enter image description here



          Of course this will only work if the actual boundaries did not move.






          share|improve this answer













          Set it up the following:




          • As input layer choose your old municipalities (2014).

          • As join layer choose your new municiplaities (2018).

          • As geometric predicate choose overlaps, within and equals.


          enter image description here



          The result:



          enter image description here



          Of course this will only work if the actual boundaries did not move.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 14 hours ago









          MrXsquaredMrXsquared

          992216




          992216













          • Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

            – tilkas
            12 hours ago






          • 1





            yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

            – MrXsquared
            12 hours ago



















          • Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

            – tilkas
            12 hours ago






          • 1





            yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

            – MrXsquared
            12 hours ago

















          Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

          – tilkas
          12 hours ago





          Thanks! I tried this but unfortunately still get 2500+ features. When I zoom in a ridiculous amount (85:1!) I can see that there's a small difference between the borders of the two layers. I guess that's the cause of this issue..

          – tilkas
          12 hours ago




          1




          1





          yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

          – MrXsquared
          12 hours ago





          yes, this only works when the border lines stay the same and just the area allocation changed. In your case you will then probably get a better result with centroids (or pointonsurface) as @MohannadAdham suggested.

          – MrXsquared
          12 hours ago










          tilkas is a new contributor. Be nice, and check out our Code of Conduct.










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