QGIS Classification – Creating a Dynamic Category Thematic Map

classificationqgisthematic mapvisualisation

I am working in QGIS 3.8.3-Zanzibar on a shapefile with polygons.

I am zoning by selecting the polygons and giving each group a different number (a zone code).

In order to visualize it, I have a Category Thematic map with the zone number, so I can easily see the different groups I am creating.

But, as I keep on making groups (that is, giving other zone codes to the polygons), it is hard to visualize the result unless I manually update the thematic map.

I would like the map to dynamically change its colours as I change the zone numbers. Until now, I have to re-classify the categories of the thematic map, as it only classifies with the existing numbers (zone codes) but the new ones I give out, the all go to the group of "all other…"

Here I am going to show my work routine with sample data, as suggested:

Initial shapefile with polygons. Visualization is simple symbol.

Visualization with a Category thematic map depending on the zone number (last column of my table).

I select some polygons and change their zone number.

The visualization has change according to the new zone number of the polygons.

I then change other polygons zone number.

The visualization changes again, but with the same colour, because any other zone numbers are classified as “all other”.

I would like to have this visualization without having to manually update the categories of the thematic map.

I don't know how to upload shapefiles, in order to provide the sample data, but here it is a screenshot of my table (it really only matters the last column).

Best Answer

A workaround would be like this:

In Style Panel, type $id for "Value".
Click on "Classify" button. This generates separate symbols for each id.
Change $id to ZoneID.
Now, there is a color for each zone id. I assume that the range of ZoneIDs is the same of the feature ids. You may need to refresh map canvas (F5) to see the result.

Related Solutions

QGIS – Differentiating Overlapping or Isolated Convex Hulls with Unique IDs

To "cluster" several polygons that (partially) overlap into one contiguous polygon with Geometry Generator, first collect all polygons, then buffer them with 0. As this would produce the same result for each feature again and again, just run it once for one of the features.

Above (red): initial polygons with overlaps; below (blue) one multipart polygon, created with Geometry Generator:

Use this expression, where you can replace $geometry with the expression you use to create the single polygons:

case 
when $id = maximum ($id)
then buffer (collect ($geometry), 0)
end

Access each geometry part separately

This creates one multipart geometry/style. If you want to get the different parts individually (e.g. show just one of the clusters), you can convert the geometry parts to a geometry array with geometries_to_array() and use index operator [] (0 for 1st element) to get one of these geometries:

geometries_to_array( 
    case 
    when $id = maximum ($id)
    then buffer (collect ($geometry), 0)
    end
)[2]

This can be helpful if you intend to use each cluster part separately for furter geoprocessing, e.g. creating the centroid for each cluster. To do so, use this expression with array_foreach() to loop through the geometry array:

collect_geometries(
    array_foreach (
        geometries_to_array( 
            case 
            when $id = maximum ($id)
            then buffer (collect ($geometry), 0)
            end
        ),
        centroid (@element)
    )
)

Blue point: centroid for the multipart geomtry (centroid of all 3 blue clusters together); red points: centroid for each cluster individually:

You can now use this centroid to style it as a font marker and for the character use Data Driven override with the variable @geomtry_part_num

Labeling

Labeling the individual buffers with the number of the points is a bit tricky and needs some workarounds as labeling works on a per-feature basis. You can use Rule based labels and than create a separate label rule for each cluster manually. Use the following expression to create the label and change the array index (here: [0]) for each rule to include the other clusters:

sum(
    within(
        $geometry,
        geometries_to_array( 
            buffer (
                collect (
                    buffer ($geometry, 2000)
                ), 
                0
            )
        )[0]
    )
)

To place the label, use Geometry Generator label placement to create the centroids of each cluster (as above):

Creating actual Geometries

By the way: if you want to create actual geometries from this with Geometry by Expression, you must modify the expression as in this context, the aggregate functions collect() and maximum() do not work an neither does the variable @layer that refers to the current layer. You have to use the very layer name (in my case: buffer) and the generic aggregate() function instead with the aggregate type inserted as argument like this:

case 
when $id = aggregate ('buffer','max',$id)
then buffer (aggregate( 'buffer','collect',$geometry), 0)
end

This returns the same number of features as the input layer, but only one feature with a geometry. You can use Remove null geometries to get rid of these. Run Multipart to single parts if you want to have a separate feature for each cluster.

Best Answer

Related Solutions

QGIS – Differentiating Overlapping or Isolated Convex Hulls with Unique IDs

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