Generating lines at the end of each measurement with plugin in QGIS

Question

It would be very useful to have a tool like this when measuring and even more useful to generate lines at the end of the operation.

I use this plugin which is part of a small GIS program (Map Maker 3.5 old version) Map Maker for my needs, but I think it should be an integral part of any GIS program. There are many tools there that are better solved than in QGIS. Video

Answer 1

Coming close to what you want, simply using geometry generator, you can use this expression to create the lines from each point to the cursor position where you click:

make_line (
    $geometry, 
    @canvas_cursor_point 
)

Use the same expression, enclosed in the function length() to create the label.

---

Be aware: projection

Not every projection is good for measurements, due to distortion. You should either use a local projection (like UTM zones or national grid) or calculate ellipsoidal distances.

To automatically convert measurents to a local CRS (even if project and layers are in EPSG:3857 or any other CRS), change the expression accordingly:

length(
    transform(
        make_line ($geometry, @canvas_cursor_point), 
        'EPSG:3857',  -- CRS of your layer
        'EPSG:2056'  -- your local CRS, here: Swiss national grid
    )
)

Answer 2

You would probably need to create a custom map tool. For starters read about using the map canvas and creating map tools at the PyQGIS developer cookbook at https://docs.qgis.org/3.28/en/docs/pyqgis_developer_cookbook/canvas.html# .

The actual line drawing is done using rubber bands ( https://qgis.org/pyqgis/master/gui/QgsRubberBand.html ).

I have implemented a static version of your requirements. Is has hardcoded points for the points the user clicked and the mouse position. It assumes the current map layer is a projected map layer.

The points are in central Germany and the coordinates are in EPSG:3857.

I am not sure of the distance calculation, so please take it only as a demonstration and a hint on how it can be done, not as a final correct calculation, but it corresponds quite well with the QGIS interface length measurement.

My QGIS version is 3.28.3

def createDistanceAnnotation(p1, p2):
    """create an annotation of the distance between two points.
    
    Parameters p1 and p2 are of type QgsPoint 
    
    Adapted from https://gis.stackexchange.com/a/421040/15183 by CodeBard"""
    line = QgsLineString(p1, p2)
    line_geom = QgsGeometry(line)

    # calculate length of line
    d = QgsDistanceArea()
    d.setEllipsoid('WGS84')
    d.setSourceCrs(qgis.utils.iface.activeLayer().crs(), QgsProject.instance().transformContext())
    length = d.measureLength(line_geom)
    # calculate gradient of line to place label at an angle
    gradient = QgsGeometryUtils.gradient(p1, p2)
    # some math to convert gradient to an angle in degrees
    angle = math.degrees(math.atan(gradient))
    
    (x, y) = QgsGeometryUtils.perpendicularOffsetPointAlongSegment( p1.x(), p1.y(), p2.x(), p2.y(), 0.5, 400 )
    # distance Unit enum has changed in 3.30. See https://qgis.org/pyqgis/master/core/Qgis.html#qgis.core.Qgis.DistanceUnit .
    label = QgsAnnotationPointTextItem(d.formatDistance(length, 2,QgsUnitTypes.DistanceUnit.DistanceMeters), QgsPointXY(x, y))
    label.setAngle(angle)
    return label

def resetRB():
    """resets all created rubber bands and the annotation layer"""
    for rb in createdRubberBands:
        rb.reset()
    QgsProject.instance().mainAnnotationLayer().reset()
        

# this stores the points the user has already clicked on
clickedPoints = [
QgsPoint(1069980,6958263), # walsrode
QgsPoint(1053886,6890568), # neustadt
QgsPoint(922838,6936805) # vechta
]

# this stores the current mouse position
mousePosition = QgsPoint(978272,6929652) # middle

# this stores the rubber bands that are created between the clicked points and the mouse position
createdRubberBands = []

# the layer where the lengths will be shown on
annotation_layer = QgsProject.instance().mainAnnotationLayer()

# draw line between each clicked point and the mouse position and draw annotation
for p in clickedPoints:
    rb = QgsRubberBand(iface.mapCanvas() , QgsWkbTypes.LineGeometry)
    rb.setToGeometry(QgsGeometry.fromPolyline([p, mousePosition]), QgsCoordinateReferenceSystem("EPSG:3857"))
    rb.setColor(QColor(255, 0, 0))
    rb.setWidth(2)
    createdRubberBands.append(rb)
    annotation_layer.addItem(createDistanceAnnotation(p, mousePosition))