QGIS - Calculate area : different results between script and expression builder

Question

When I run a script to calculate an area with PyQGIS I don't get the same result as when I calculate it with the Expression Builder.

Here is my script:

myLayer.startEditing()
myIndex = myLayer.fields().indexFromName("pyqgis")
for feature in myLayer.getFeatures():
    myLayer.changeAttributeValue(feature.id(), myIndex, round(feature.geometry().area()/10000, 4))        
myLayer.commitChanges()

Here is my expression :

 $area /10000

Both fields are in the same type and have the same precision

But result isn't the same.

I'm uing QGIS 3.10.2 (LTR)

Answer 1

First, make sure you use a projected CRS

There are 2 ways to calculate the area using the field calculator.

1. using area which is always planimetric in the SRS of this geometry. From QGIS help:

This differs from the calculations performed by the $area function, which will perform ellipsoidal calculations based on the project's ellipsoid and area unit settings.

2. using $area

Change your expression to: area($geometry)/10000 (Edit: @Erik)