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I dissolved UK postcode polygons at the sector level to create the postcode sector polygons I need. However, I seem to have more postcode polygons than I expected, about 800 additional postcodes that aren't in the national statistics data.

I added a binary indicator to all of the postcode sectors I got from the national statistics office in the UK (ONS), and visualised them. I got the following:

enter image description here

And also at a higher zoom, this:

enter image description here

I'm not able to select these small square dots and I don't seem able to find them in the attributes data. For example, when I search for polygons based on area and rank them, the sectors with the smallest areas are too big to be these dots, for example:

Pcd.Sector Area 
M60 8   73
VGU02   129
PH2 1   149
VUB01   616
DL981   807
VSE04   847
M60 7   1101
OL133   1263
W1A 3   1264
EC4N1   1450
EC3A2   1537

I'm really not sure what these are, has anyone seen these before?

marked as duplicate by PolyGeo Sep 28 '16 at 10:20

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  • Joseph, you are right. They are vertical postcodes. Do you want to post this as an answer and I will mark it as correct? Ed – Thirst for Knowledge Sep 26 '16 at 12:20
  • What are you doing with the postcode sectors? Depending on the precision you want you could actually include the vertical streets in your analysis. There is a lookup file which you could join to those vertical streets. I had a look and of the 22000 vertical streets in the data 8000 of them have more than 1 postcode sector to them. You will need to decide if you wan to keep them in or just use one of them (up to you to decide which one). If you do this first then you can dissolve on postcode sector. – DMusketeer Sep 28 '16 at 9:57
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In reference to this post: How to remove small “salt & pepper” polygons from a layer?

After you dissolved your polygons, you seem to have been left with holes which represent vertical streets due to them being to connected to more than one postcode. One method of removing them is to fill them up with the surrounding polygon by running the Fill holes tool from:

Processing Toolbox > QGIS geoalgorithms > Vector geometry tools > Fill holes
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    While I haven't used this tool, I think this will only fix the vertical streets that are completely contained in one polygon. You will still have the ones that are on the boundary and straddle multiple polygons. I guess look at eliminate slivers? – DMusketeer Sep 28 '16 at 8:58

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