6

How can I locate the coordinates of the middle point (midpoint) of a line feature in QGIS?

6

If you bring your line feature into spatialite, you can use the Line_Interpolate_Point() function to find the mid point.

SELECT AsText(Line_Interpolate_Point(your_line.geomemtry, 0.5))

The second parameter 0.5 indicates the fraction of the line length where the point will be located.

6

It is a pure geometry problem that can be solved in the Python console

The problem:

test

Find the midpoint of a segment x1,y1,x2,y2 is easy

x = (x1 + x2)/2
y = (y1 + y2)/2

so in the Python console

def mid(pt1, pt2):
    x = (pt1.x() + pt2.x())/2
    y = (pt1.y() + pt2.y())/2
    return QgsPoint(x,y)

def pair(list):
    '''Iterate over pairs in a list '''
    for i in range(1, len(list)):
        yield list[i-1], list[i]

def create_geometry(point, pr):
    # create geometry record 
    seg = QgsFeature()
    seg.setGeometry(QgsGeometry.fromPoint(point))
    pr.addFeatures( [seg] )

# memory layer
pt_layer = QgsVectorLayer("Point", "midpoint", "memory")
pr = pt_layer.dataProvider()

for elem in mylayer.selectedFeatures():
    line = elem.geometry()
    for seg_start, seg_end in pair(line.asPolyline()):
        line_start = QgsPoint(seg_start)
        line_end = QgsPoint(seg_end)
        # midpoint 
        midpt = mid(line_start, line_end)
        # add midpoint point to layer
        create_geometry(midpt,pr)
        pt_layer.updateExtents()

QgsMapLayerRegistry.instance().addMapLayers([pt_layer])

Result

final

  • 4
    Just a note that this approach finds the mid point of each line segment, not the line itself. – Tim Sutton May 14 '14 at 20:49
  • 1
    @gene I think the following line is missing in your code: mylayer = qgis.utils.iface.activeLayer() – ahmadhanb Apr 6 '17 at 7:48
6

Since this question was asked, an interpolate function has been added to PyQGIS on the QgsGeometry object.

Here is a quick example of how to use this in the Python console inside of QGIS (Plugins -> Python Console).

layer = iface.activeLayer() #layer selected in your layers panel
feature = layer.selectedFeatures()[0] #the first feature of selected features
geom = feature.geometry() #QgsGeometry representing your line
length = geom.length() #length of geometry in the layer CRS. If EPSG:4326 this will be degrees
point = geom.interpolate(length/2.0) #QgsGeometry representing the mid point
x = point.geometry().x() #X coordinate in layer CRS
y = point.geometry().y() #Y coordinate in layer CRS

An even easier option...

In the processing toolbox there is a QGIS function under Vector Geometry that is called Interpolate point on line that is most likely using the interpolate function described above.

For your input layer select the line layer you want to find the midpoints

Warning! Make sure you are using a projected coordinate system appropriate for the area where you are finding midpoints. Geographic coordinate systems like EPSG 4326 (WGS 84) are not accurate for measuring distances, and a world wide projected coordinate system like EPSG 3857 (WGS 84 / Pseudo-Mercator) will not be as accurate as a projected coordinate system for a specific region.

For distance, this is the distance to be interpolated along the line by the given units. If you give a distance of 10 and meters is selected, then a point will be created 10 meters along the line for every line in your layer. What we want here is a calculated value, not a static value like 10. To calculate a value for distance, click on the drop down to the far right (shown circled in red below) and select edit...

Distance input

In the expression box, put in the expression $length / 2.0 This will calculate half the length of the given line feature in the coordinate reference system for that layer (hence the warning above). Select OK.

Run the Interpolate Point on Line function, and it will then create a point layer with a point at the midpoint of every line feature in the selected layer, and each midpoint will have the attributes of the original line feature copied over to it.

  • Rokefellar - Does QGIS 3.8 handle the feature = layer line differently? I get index out of range. – GeorgeC Jul 1 at 13:30
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    @GeorgeC for layer.selectedFeatures()[0] to work, you literally have to have features selected in QGIS. Get the selection tool and select something. The code above is just a very quick and dirty proof of concept. – TJ Rockefeller Jul 1 at 19:09
  • I tried this -what would I need to do to get it to run on the whole dataset? – GeorgeC Jul 1 at 22:40
  • @GeorgeC This is just a proof of concept. What you are asking does not have a trivial answer that can be explained in a comment. You will need to research writing Python scripts in QGIS and possibly ask a more detailed question on this site. You will need to automate looping through all features and figure out what you want your output to be, I'm assuming a point layer? – TJ Rockefeller Jul 2 at 13:31
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    @GeorgeC it had been awhile since I had looked at this question, and looking into it again I found Interpolate Point on Line that I have described above, that I think will do what you are looking for. – TJ Rockefeller Jul 8 at 15:20
0

One way I would address this is to use a 2 step approach with the MMQGIS plugin. The first is to install and use the MMQGIS to find the middle point or center of each line.

Then by creating new columns, I can use the longitude and latitude functions from the field calculator to get the coordinates accordingly.

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