Adding Direction and Distance into attribute table in QGIS

Question

I just want to add direction (Bearing: i.e N 25 35 E) and Distance (Length: 125 meters) as my new field in polyline/line data.

Is there a Plugin to generate these fields?

I tried to used Export/Add geometry columns in my line data, but only "Length" value was added.

Answer 1

You can calculate bearing in the Field Calculator in QGIS. This works in UTM (metric) coordinates over small distances (hundreds of km), but something more sophisticated would be needed for large distances or for decimal degrees.

Open the attribute table for your line layer, toggle editing, and click the Field Calculator button to open the dialog:

Create a new field as decimal with 1 or 2 precision.

Paste this code into the "Expression" box, and click "OK": (atan((xat(-1)-xat(0))/(yat(-1)-yat(0)))) * 180/3.14159 + (180 *(((yat(-1)-yat(0)) < 0) + (((xat(-1)-xat(0)) < 0 AND (yat(-1) - yat(0)) >0)*2)))

The first part calculates the inverse tangent of the x and y differences and converts it to degrees (180/pi). The second part adds either 180 or 360 to the resulting figure to give a bearing from 0-360°.

Edit - updated formula as per suggestion

CASE
   WHEN ((yat(-1)-yat(0)) = 0 and (xat(-1) - xat(0)) >0) THEN 90
   WHEN ((yat(-1)-yat(0)) = 0 and (xat(-1) - xat(0)) <0) THEN 270
   ELSE (atan((xat(-1)-xat(0))/(yat(-1)-yat(0)))) * 180/pi() + 
       (180 * (((yat(-1)-yat(0)) < 0) + (((xat(-1)-xat(0)) < 0 AND (yat(-1) - yat(0)) > 0)*2)))
END

Answer 2

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

Answer 3

The solution provided by @Simbamangu is pretty effective but not cover all cases. For example,applying the formula with an horizontal displacament will NULL the result, so you must use this formulate in Field Calculator of QGIS

case
   when yat(-1)-yat(0) < 0 or yat(-1)-yat(0) > 0
       then (atan((xat(-1)-xat(0))/(yat(-1)-yat(0)))) * 180/3.14159 + 
       (180 * (((yat(-1)-yat(0)) < 0) + (((xat(-1)-xat(0)) < 0 AND (yat(-1) - yat(0)) > 0)*2)))
   when ((yat(-1)-yat(0)) = 0 and (xat(-1) - xat(0)) >0) then 90
   when ((yat(-1)-yat(0)) = 0 and (xat(-1) - xat(0)) <0) then 270
end