2018-02-01 13:58:55 8 Comments

I am carrying out an analysis using QGIS 2.14.1 where I am dealing with point data that have been randomly displaced by either up to 2 Km or 5 Km. I have generated a buffer of the appropriate radius around each point and now want to obtain mean values for each polygon from a raster layer.

To do this I have used the Zonal Statistics tool but for about 10% of my points no results have been generated even though there is raster data for that location. All the polygons with missing data have a 2 Km radius. The Zonal Statistics tool worked for some of the polygons with a 2 Km radius, just not all. The raster layer I am dealing with has a resolution of 5 X 5 Km.

When I increase the size of the polygon I am able to obtain zonal statistics results, which suggests to me that the size of the polygon may be too small. However, I cannot alter the polygon size because the point can only be within this radius and therefore enlarging the polygon would make the results inaccurate.

I understand that if polygons overlap this can sometimes cause issues. Whilst some of the polygons do overlap, some of them do not. I also tried doing this for one polygon at a time but still did not get results with the ones I tried. I have included an image below where the smaller green polygon is an example of one of the polygons that has missing Zonal Statistics output.

I am aware of multiple other Q&As addressing this type of issue but nothing I have tried has worked so far. The Q&As I have seen are:

- Missing buffer values when using zonal statistics in ArcGIS Desktop?
- Zonal Statistics as a Table does not calculate statistics for all features in dataset?
- Zonal Statistics As A Table missing Features in Results?
- Polygons too Small for Zonal Statistics
- Zonal statistics results in some NULL values in QGIS?

I do not use Python.

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