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In QGIS 3.20 (or QGIS 3.18) I have a shapefile with point geometry, this shapefile has a field with the value of error in "x" and a field with the value of error in "y". I would like to create error ellipses, then if error of "y" increases in a point, then the symbol only changes the major axis of the ellipse, and the same with "x".

Basically, I need to use two fields to define the size of the symbol in two different axes. Is there any way of doing this?

I have tried Rule-Based symbology but I haven't been able to make it, maybe I just don't know how to use it.

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You could use an ellipse symbol marker with a data-defined override to define the symbol width & height.

As an example, I created some random points and added two fields "x_err" and "y_err". Just for the example I filled these fields with random float values between 2.0 and 6.0.

My attribute table looks like this:

enter image description here

With the error values stored in your attribute table, go to layer properties -> symbology. Click on the simple marker symbol layer and change the symbol layer type to Ellipse marker. You can then set up a data-defined override expression to define symbol width and height by clicking the Data-defined override symbol on the far right and selecting Edit. See Screenshot below:

enter image description here

You can then enter the field names which contain your x and y error values in the expression builder. Depending on what those values are you could multiply or divide them by some constant factor and/ or change the units if required. See below:

enter image description here

You need to do this for both symbol width and symbol height. The resulting symbology is shown below:

enter image description here

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Check "Rectangles, Ovals, Diamonds (Variable)" geoalgorithm in the QGIS's Toolbox

result

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