Create contours based on text attribute of point data (isogloss map)

Question

I am trying to make an isogloss map in QGIS to map dialect differences. I have made a point shapefile in which each point represents a place, and the different pronunciations are put in the attribute table. Obviously I have a massive amount of attributes.

As it is not convenient to just constantly change the attribute for the symbology, I wanted to create contours that separate the areas with different pronunciation.

I tried using contour, but it doesn't accept string data.

Imagine the points are plotted as follows, with the colours relating to a specific attribute

The countours I would like to have are something like this (for the same attribute)

I wonder if there is some way to do this easily, so far the only option I found was using voronoi polygons which I then merge based on the characteristics, which is quite a lengthy process, considering I have quite a few attributes to repeat this for. (The other way is changing the symbology and drawing manually)

may be this is what you are looking for: https://gis.stackexchange.com/questions/229955/converting-cluster-of-points-to-polygons-using-qgis or try to use the processing tool: Minimum bounding geometry — eurojam, Aug 12 '20 at 05:56
Maybe this thread can be relevant, please check Displaying multipoint features as polygon using QGIS? — Taras, Aug 12 '20 at 06:02

score 5 · Answer 1 · answered Aug 12 '20 at 06:19

I think the contour idea won't work, your text data is not a continuous phenomena as the elevation.

I suppose you had better to create Voronoi polygons on your point data using Processing toolbox.

I added 10% to the extent of the layer to get polygons outside the extent of the points. You can set another value for you.

This way you get areas around your points (without gaps and overlaps). The Voronoi cells inherit the attributes of the points.

Using dissolve from processing toolbox you can merge polygons with the same string attribute:

Now you can create a thematic map on your text column:

I think there are two more steps can be applied. (3) Apply a Convex hull for all points (4) Make a difference between the result of Dissolving and the result of Step 3. But indeed your answer looks exactly what I wanted to propose, +1 — Taras, Aug 12 '20 at 06:29

score 1 · Answer 2 · answered Aug 12 '20 at 08:29

It may also be worthwhile considering having additional data that can be used to 'bound' your polygons. That is - perhaps topographic features which would 'bound' a certain language or dialect.

eg: A river - people on west of river vs people on east of river. Or perhaps a country/state boundary, road/hwy etc. This additional data may help 'cut' your interpolated polygons to be more fitting to the topographic features which bound the languages.

Create contours based on text attribute of point data (isogloss map)

2 Answers2