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I have a shapefile for the boundaries of European NUTS regions, and I would like to compute the centroid for each of those regions. When I do this in QGIS (2.6.1 on OSX, using polygon centroids from the fTools menu), I get multiple centroids per region. Sometimes the centroids are nearly overlapping, sometimes they are further apart. How can this be? Am I doing something wrong? To be complete: I am using the NUTS_BN_60M_2006.shp data, available from Eurostat

I've added a screenshot; this part is particularly bad, but other regions have the issue as well.

screenshot

  • Can you provide screenshots of this unusual behaviour please? There's a couple of things you can try: check if there are polygons being overlapped by other polygons; try the Polygon centroids tool from SAGA. – Joseph Feb 3 '15 at 15:44
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    You may have overlapping polygons - try to use the identify tool and see if you get multiple hits. – Simbamangu Feb 3 '15 at 15:44
  • @Joseph I've added a screenshot. I'm on a Mac -- SAGA seems to be available only for Windows and Linux machines. – Matthijs Feb 3 '15 at 21:03
  • @Simbamangu I am very much a neophyte at all things (Q)GIS…where do I find this tool? What does it do? How do I use it? I can find identify features and identify results in the menu, but I don't see how they help to solve my problem. – Matthijs Feb 3 '15 at 21:15
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The reason are multiple overlaps. If you select ones with STAT_LEVL_=0, they are countries, e.g. NUTS_ID=DE is Germany.

They are 34 largest polygons containing smaller levels, i.e. STAT_LEVL_= 1 (115), 2 (317) and 3 (1461).

Select level you'd like to work with, they are overlaps free. However some of them are multipart features, e.g. FR(ance), because it contains islands, thus it is good idea to explode them (see Chris Hogan answer)

  • Thanks to all…had some trouble finding my way around QGIS, but I managed thanks to your answers. – Matthijs Feb 4 '15 at 14:32
  • @Matthijs may I suggest that you give a look into QGIS Documentation? There's a gentle introduction to gis document that may give some insights on the matter. – Alexandre Neto Feb 6 '15 at 9:44
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If the responses by @joseph and @Simbamangu does not point you in the right direction:

Are your polygon multi-part or single-part features?

If you have one NUTS region with multiple separate polygons, QGIS will create one centroid for each. If your data include multi-part polygons you will likely get a single centroid with the centroid in the center of the bounding box for all of the polygons in the single multi-part feature.

QGIS has a tool 'single parts to multipart'

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