I'm familiar with using QGIS, so this is not a technical question; it's more of a conceptual question. I'm mapping attendance of households at different meetings, and I specifically want to show which meeting or meetings households have attended. I've tried to show this using points, but the map is so large, it makes it look like the meetings haven't had many attendees. (We want to show that the meetings have been well attended.) Instead, I've been using polygons over each house parcel. The issue is that there are households that have been to multiple meetings but the map only shows whatever layer is on the top. I've also tried having the polygons with a line fill, but that didn't work too well. Does anyone have any suggestions for improving this?


A choropleth map would be suitable in this case.

Per Wikipedia:

A choropleth map... is a thematic map in which areas are shaded or patterned in proportion to the measurement of the statistical variable being displayed on the map.

Choropleth maps show areas filled with a colour where the colour varies with the attribute value being displayed.

That said, if your meeting attendance data is stored in different layers, you will probably want to combine them into a single layer first. The combined land parcel polygons would then have some attribute like "total_meetings_attended". This will be much easier to map than multiple layers.

One thing to keep in mind when preparing a choropleth map (or any map) is the importance of selecting a suitable colour palette. White, Slocum, and McDermott refer to a "logical progression of colors". Essentially, you want the colour palette to make sense in the context of the data you are presenting. You have unipolar data (values range from small to large), so a scheme such as the following could be suitable:

  • no meetings = white (i.e., absence of meetings = absence of colour)
  • some meetings = light blue
  • many meetings = dark blue (i.e., most meetings = darkest colour)

It would be different if you wanted to display bipolar data, such as temperature or income relative to median value (values can be low or high positive or high negative). In that case you would want deep colours on both ends of the palette and a neutral or light colour in the centre.

An example from White, Slocum, and McDermott showing palettes used to indicate % home ownership rate: example from White, Slocum, and McDermott showing palettes used to indicate % home ownership rate

On the left they show an example of a logical palette for unipolar data. On the right are three palettes that lack a logical connection to % home ownership data.

Another thing to watch out for with choropleth maps is that large regions can dominate the map and have more visual impact than small regions. I would imagine residential land parcels are similar enough in size that this would not be of concern in your case.

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