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I'm not a professional digital cartographer. I'm a graphic designer who has had to familiarize myself with some processes to fill a real estate broker's needs. I only offer that as an apology for any ignorance you may read in my post. I'm fairly ignorant!

That being said... I'm having some issues rendering population in the form of dot-density in QGIS 3.12.

  • My process begins with generating the population data in ArcGIS Business Analyst Online. (The client relies on ArcGIS for the entirety of their demographic and mapping data.)
  • I share the data out to ArcGIS Online, where I render it as a shapefile for download.
  • After download and extraction, I drag the shapefile into my QGIS project, where it successfully appears in the layers palette.

From here, I'm encountering some issues. I've researched online, and there only appear to be a couple of tutorials for processing the dot-density map that aren't ancient.

  • I select the appropriate layer (pop_density_greater_houston_2010) and, under the "Vector/Research tools" menu, select "Random points within polygons".
  • Select the appropriate input layer
  • Points strategy = point count

enter image description here

  • However, the tool does not give me the option of selecting from which field to pull it's data.
  • I've tried changing symbology to "Categorized" and "Graduated", then classifying based on the density field before running the "Random points..." tool.

Results I've encountered:

  • "Invalid geometry" error messages
  • Endless processing (based on the processing animation at the bottom of the window)
  • single points within each census tract

I cannot seem to accomplish a true dot-density.

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    Which version of QGIS are you using? The dialog for that tool which I see in QGIS 3.4 has an 'Expression' parameter which allows to select a field from the input layer and use its value in an expression e.g. divide by a factor. In your screenshot you have a data defined override button to the right of the 'Point count or density' widget which should work similarly, though you might have to create a new field if you want to scale your values.
    – Ben W
    Apr 3, 2020 at 3:59

1 Answer 1

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Thanks, Ben W. Again, I'm on QGIS 3.12.

That override fly-out menu was the solution! In there, I could either select the "ThematicVa" field as my dataset... enter image description here

or (and this was definitely more useful for my purposes) via the "Edit..." option, I was able to create a simple expression that divided the raw population value by 50 ("ThematicVa"/50) to accomplish a 1 dot=50 people representation.

enter image description here

That was a good pointer. I appreciate the good counsel, @benw!

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