1

I am using QGIS 3.12 and I would like to plot an error ellipse at the tip of a vector arrow.

I have plotted GNSS velocity vectors in a map and now I would like to plot the error associated to the vector. How could I do it ?

I have tried to add an ellipse in the symbology, but not sure how to locate it at the tip of the vector arrow. The idea is to produce a similar map as the one below. The arrows indicate the displacement direction and velocity (vector length) of Iberia respect a fixed Africa from GNSS measurements and the ellipse the associated error.

The origin of the vector is the location of the GNSS station and the arrow tip location depends on the vector length. Is at that arrow tip that the ellipse would be located.

enter image description here

2
  • 2
    maybe a picture would help ? – Snaileater May 3 '20 at 19:31
  • are you sure you want an ellipse ? i only see circles on your picture ... do u have variables controling major and minor axis of the ellipse ? – Snaileater May 10 '20 at 12:45
1

You can use geometry generator on the symbology tab of the layer properties. There is a make_ellipse function. You need the center of the ellipse, the semi-major and semi-minor axis length and the azimuth of the semi-major axis. For example if you would like to have the ellipse at the endpoint of a polyline you sould enter the following expression:

make_ellipse( make_point($x_at(-1), $y_at(-1)), "major", "minor", "azimut")

$x_at(-1) and $y_at(-1) return the coordinates of the last point of the linestring, make_point creates a point geometry from the two coordinates. Here I supposed the "major", "minor" and "azimut" columns of the attribute table contain the ellipse data.

enter image description here

enter image description here

2
  • Thanks Zoltan! I am plotting the ellipse but its origin is located at the beginning of the arrow. I have also tried $x_at(0) and $y_at(0) but nothing changes. Maybe is because I am plotting the vector arrow using the vector field marker symbol function. I will look for some info to know how vector field marker works. – Hepman May 11 '20 at 14:38
  • Looking for more information about vector field I have found the plugin "Vector field layer manager" that makes exactly what I needed, plots vectors and ellipse errors associated to this vectors. Thanks for your help and time. – Hepman May 11 '20 at 16:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.