QGIS Labeling – How to Create Dynamic Leader Lines

Question

I'm trying to create dynamic leader lines by using a PostGIS view in addition to the QGIS „Move Label“ tool.

CREATE VIEW leader_line AS
SELECT
gid,
ST_MakeLine(geom, ST_SetSRID(ST_MakePoint(xcord_label, ycord_label), SRID))::geometry(linestring, SRID) AS geom
FROM point
WHERE xcord_label IS NOT NULL;

This works fine for all labels WHERE ST_X(geom) < xcord_label but creates wrong looking leader lines for labels WHERE ST_X(geom) > xcord_label.

Does anybody know how to get properly placed leader lines for labels WHERE ST_X(geom) > xcord_label? Is there any way to refer to the xmax coordinate of the labels?

Answer 1

You can use QGIS' quadrant placement specifier determined from the line's azimuth to place a better label. The quadrant specifies 8 positions around a point:

[ 0=Above Left | 1=Above | 2=Above Right |
  3=Left       | 4=Over  | 5=Right       |
  6=Below Left | 7=Below | 8=Below Right ]

Here's an example around Null Island, creating a table and two views.

CREATE TABLE points (
  gid serial PRIMARY KEY,
  geom geometry(Point, 4326),
  label_geom geometry(Point, 4326),
  label text
);

INSERT INTO points(geom, label_geom, label)
SELECT origin, pt, round(degrees(ST_Azimuth(origin, pt))) || ' degrees'
FROM (
  SELECT
    ST_SetSRID(ST_MakePoint(0, 0), 4326) AS origin,
    ST_SetSRID(ST_MakePoint(cos(radians(x)), sin(radians(x))), 4326) AS pt
  FROM generate_series(0, 350, 15) AS x
) AS f;

CREATE OR REPLACE VIEW point_labels AS
  SELECT gid, label_geom AS geom,
  CASE
    WHEN ST_Azimuth(geom, label_geom) ISNULL THEN 2 -- default if azimuth cannot be determined
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 22.5 THEN 1 -- Above
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 67.5 THEN 2 -- Above Right
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 112.5 THEN 5 -- Right
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 157.5 THEN 8 -- Below Right
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 202.5 THEN 7 -- Below
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 247.5 THEN 6 -- Below Left
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 292.5 THEN 3 -- Left
    WHEN degrees(ST_Azimuth(geom, label_geom)) < 337.5 THEN 0 -- Above Left
    ELSE 1 -- >= 337.5 Above
  END AS quadrant, label
  FROM points;

CREATE OR REPLACE VIEW leader_line AS
  SELECT gid, ST_MakeLine(geom, label_geom)::geometry(LineString, 4326) AS geom, label
  FROM points;

Then in QGIS, add:

• points – geom
• leader_line – geom – primary key needs to be gid
• point_labels – geom – primary key needs to be gid

Now configure the layer properties for point_labels:

• Change style so the point is not drawn, e.g., change size to 0.0
• Label this layer with label, and change placement to "Offset from point", modifying the "Quadrant" to use the attribute field quadrant

Bingo!

Note that a slightly different approach is required for geography types, since ST_Azimuth behaves differently.

---

Update: When adding new points to the points layer, the geom field is updated as usual, but the label_geom is not. To populate a default value of label_geom with new points, a trigger needs to be created. But if a trigger function is used, the quadrant specifier can be stored in the points table and the point_labels view can be ignored:

For example, let's start again with a slightly different example with one table and one view:

-- DROP TABLE points CASCADE;
CREATE TABLE points (
  gid serial PRIMARY KEY,
  geom geometry(Point, 4326),
  label_geom geometry(Point, 4326),
  quadrant integer,
  label text
);

CREATE FUNCTION label_geom_tg_fn() RETURNS trigger AS
$BODY$
DECLARE
  azimuth float8;
BEGIN
  -- Set a default label_geom
  IF NEW.label_geom ISNULL THEN
    NEW.label_geom := NEW.geom;
  END IF;
  -- Determine quadrant
  azimuth := degrees(ST_Azimuth(NEW.geom, NEW.label_geom));
  NEW.quadrant := CASE
    WHEN azimuth ISNULL THEN 2 -- azimuth cannot be determined, so put Above Right
    WHEN azimuth < 22.5 THEN 1 -- Above
    WHEN azimuth < 67.5 THEN 2 -- Above Right
    WHEN azimuth < 112.5 THEN 5 -- Right
    WHEN azimuth < 157.5 THEN 8 -- Below Right
    WHEN azimuth < 202.5 THEN 7 -- Below
    WHEN azimuth < 247.5 THEN 6 -- Below Left
    WHEN azimuth < 292.5 THEN 3 -- Left
    WHEN azimuth < 337.5 THEN 0 -- Above Left
    ELSE 1 END;-- >= 337.5 Above
  RETURN NEW;
END;$BODY$ LANGUAGE plpgsql;

CREATE TRIGGER label_geom_tg BEFORE INSERT OR UPDATE
   ON points FOR EACH ROW
   EXECUTE PROCEDURE label_geom_tg_fn();

The from the first example, re-do the INSERT INTO points and CREATE OR REPLACE VIEW leader_line statements, as these do not require modification. But ignore the leader_line view.

Then in QGIS, add:

• points – geom
• points – label_geom
• leader_line – geom – primary key needs to be gid

Now configure the layer properties for points with label_geom as the first example did for point_labels. The quadrant specifier will be modified automatically for new and moved points, but you will only notice these changes each time you save your edits.