How to add Direction and Distance to attribute table?.md

From How to add Direction and Distance to attribute table?

You do not need a plugin. Everything is in the class QgsPoint of PyQGIS

If you examine the contents of a QGIS point class with the Python built-in function dir() in the Python Console.

    dir(point])
    ['__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__'
    , '__getitem__', '__gt__', '__hash__', '__init__', '__le__', '__len__', '__lt__', '__module__', 
    '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', 
    '__str__', '__subclasshook__', '__weakref__', 'azimuth', 
    'multiply', 'set', 'setX', 'setY', 'sqrDist', 'sqrDistToSegment', 'toDegreesMinutesSeconds', 'toString', 'wellKnownText', 'x', 'y']

You can see there are azimuth and sqrDist functions and after a few tries:

- xy[0].azimuth(xy[1]) or xy[1].azimuth(xy[0]) gives the azimuth direction between two points(in degrees, +/- 180°)
- xy[0].sqrDist(xy[1]) give the square distance between two points (in the unit of the project)

The problem

So in the Python console

    def select_all(layer):
         layer.select([])
         layer.setSelectedFeatures([obj.id() for obj in layer])

    myline = qgis.utils.iface.activeLayer()
    select_all(myline)
    for elem in myline.selectedFeatures():
          xy = elem.geometry().asPolyline()

now xy contains all the nodes (points) of the line

    # first point
    print "x=%2d y=%2d" % (xy[0].x(),xy[0].y())
    x=112935 y=117784
    # and others...

Using all node points of the line:

1. azimuth point i to point i + 1 (+/- 180°) (nodes of a line)

    for i in range(len(xy)-1):
         print "x=%2d y=%2d azim=%6.1f azim2=%6.1f" % (xy[i].x(), xy[i].y(), xy[i].azimuth(xy[i+1]), xy[i+1].azimuth(xy[i]))
 
    x=112935 y=117784 azim= 168.4 azim2= -11.6
    x=113032 y=117312 azim=-167.5 azim2=  12.5
    x=112926 y=116835 azim= 177.3 azim2=  -2.7
    x=112943 y=116472 azim= 145.1 azim2= -34.9
    [...]

2. euclidean distance between point i and point i + 1

    for i in range(len(xy)-1):
         print "x=%2d y=%2d dist=%6.1f" % (xy[i].x(), xy[i].y(), xy[i].sqrDist(xy[i+1]))

    x=112935 y=117784 dist=232533.9
    x=113032 y=117311 dist=238243.6
    x=112926 y=116835 dist=131839.8
    x=112943 y=116472 dist=209268.1
    [...]

After, it is not very difficult to add these values to the attribute table.

I use this technique to analyze the lineaments (geology) with matplotlib and the Script Runner plugin