QGIS3 - Join by location (falsely) adds neighbouring polygons
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.
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.
qgis
asked Jan 11 at 13:35
tilkastilkas
184
add a comment |
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.
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.
qgis
asked Jan 11 at 13:35
tilkastilkas
184
add a comment |
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.
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.
qgis
asked Jan 11 at 13:35
tilkastilkas
184
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.
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.
qgis
qgis
asked Jan 11 at 13:35
tilkastilkas
184
asked Jan 11 at 13:35
tilkastilkas
184
edited Jan 11 at 17:00
tilkas
asked Jan 11 at 13:35
tilkastilkas
184
asked Jan 11 at 13:35
tilkastilkas
184
asked Jan 11 at 13:35
tilkastilkas
184
184
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
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.
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
add a comment |
Set it up the following:
- As
input layerchoose your old municipalities (2014). - As
join layerchoose your new municiplaities (2018). - As
geometric predicatechoose overlaps, within and equals.
The result:
Of course this will only work if the actual boundaries did not move.
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
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
Jan 11 at 16:55
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
Jan 11 at 17:07
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
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.
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
add a comment |
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.
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
add a comment |
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.
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
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.
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
answered Jan 11 at 13:53
Mohannad AdhamMohannad Adham
37916
37916
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
add a comment |
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
3
3
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
To add to this Intersection includes where the boundaries of a pair of polygons overlap (what you think of as touching).
– Ian Turton♦
Jan 11 at 14:25
add a comment |
Set it up the following:
- As
input layerchoose your old municipalities (2014). - As
join layerchoose your new municiplaities (2018). - As
geometric predicatechoose overlaps, within and equals.
The result:
Of course this will only work if the actual boundaries did not move.
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
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
Jan 11 at 16:55
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
Jan 11 at 17:07
add a comment |
Set it up the following:
- As
input layerchoose your old municipalities (2014). - As
join layerchoose your new municiplaities (2018). - As
geometric predicatechoose overlaps, within and equals.
The result:
Of course this will only work if the actual boundaries did not move.
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
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
Jan 11 at 16:55
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
Jan 11 at 17:07
add a comment |
Set it up the following:
- As
input layerchoose your old municipalities (2014). - As
join layerchoose your new municiplaities (2018). - As
geometric predicatechoose overlaps, within and equals.
The result:
Of course this will only work if the actual boundaries did not move.
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
Set it up the following:
- As
input layerchoose your old municipalities (2014). - As
join layerchoose your new municiplaities (2018). - As
geometric predicatechoose overlaps, within and equals.
The result:
Of course this will only work if the actual boundaries did not move.
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
answered Jan 11 at 14:43
MrXsquaredMrXsquared
1,1731316
1,1731316
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
Jan 11 at 16:55
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
Jan 11 at 17:07
add a comment |
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
Jan 11 at 16:55
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
Jan 11 at 17:07
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
Jan 11 at 16:55
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
Jan 11 at 16:55
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
Jan 11 at 17:07
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
Jan 11 at 17:07
add a comment |
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