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I have state wide data here that I would like to overlay a grid layer of 1kmx 1km onto. I used the processing toolbox in QGIS 3 (Create Grid) in order to create a grid layer of 1km x 1km lines under the Vector Creation category. After this, I imported the fmod data and changed the symbology so that unique values are mapped to the different colors in the layer. I tried to use the Zonal Statistics tool under the processing toolbox to gather a majority (mode) statistic on each grid cell by using the following settings:

  1. Raster Layer: modified fmod layer
  2. Raster band: band 1
  3. Vector Layer containing zones: a gpkg layer I made via the previously mentioned Create Grid processing tool.
  4. Output Column Prefix: _ (default)
  5. Statistics to calculate: Majority(mode)

However, I believe (looking at the log) the tool only calculates some aggregate stat that is not what I intended on. I'm not very sure how to go about this in QGIS 3 -- currently downloading QGIS 2, but could anyone tell me what I am doing wrong?

I would like to have a grid layer where each cell in the grid contains a value determined by majority vote from the data in another layer (specifically the fmod data I am using from the link above).

  • What kind of data is the "state wide data"? Is it vector or raster? If it's a vector, is it points, lines or polygons? – csk Aug 7 '18 at 20:00
  • I believe it's raster data. Link to the data itself is provided in the description. It is an adf file - I import it as a raster file. – shotes Aug 7 '18 at 22:05
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It sounds to me like you want the Spatial join tool found in: Analysis Tools>Overlay>Spatial Join.

It will take your 1km x 1km grid as the target feature and a point shapefile as the join features.

I believe the default merge rule for the Spatial Join tool is first, so to change it you'll need to add a new column, Type:Double, Merge Rule:Mean. The new column will be empty but it will change the merge rule for all double's to mean rather than first.

Edit: I believe you'll want to change your Match Option to 'completely contains', but you can play around with it.

To turn your raster into a point polygon use the Raster to Point tool found in: Conversion Tools>From Raster> Raster to Point.

I hope this helps.

  • Hi Daniel, I can't seem to find these tools (Spatial Join, and Raster to Point) in QGIS2 / 3. Would greatly appreciate it if you could point me to documentation for this - I'd like to merge using mode (majority win) because the different colors encode different categories. – shotes Aug 7 '18 at 22:04
  • My mistake shotes, I thought you were working in arcmap not QGIS. these two links should help show you how to do the steps I described in QGIS. slideshare.net/shencoop/qgis-raster-to-point and qgistutorials.com/en/docs/points_in_polygon.html . In the second link he just wants a count of the points inside the polygon so he uses points in polygon. You want the mean so use the weighed version, details here - docs.qgis.org/2.18/en/docs/user_manual/processing_algs/qgis/… . If you cannot find the ability to do that with the tool use python – Daniel Mutton Aug 8 '18 at 13:44
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I don't see any issue with your workflow. Just be sure that you create a polygon grid and not a line grid. When running the tool it will add an attribute table field '_majority' to the attribute table containing the mode of the raster cell covered by the grid.

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