I have a 550 meter by 550 meter square polygon in a shapefile. The QGIS 2.18 project and the shapefile both reference EPGS:32648 WGS84/UTM48N. EPGS:32648 uses meters as the units.
When I use $area to calculate the area of the polygon it populates the field with 75.74. This value is 302,500 square meters in acres.
Why is this field being populated with acres?
Function $area:
Returns the area of the current feature.
The area calculated by this function respects both the current project's ellipsoid setting and area unit settings.
For example, if an ellipsoid has been set for the project then the calculated area will be ellipsoidal, and if no ellipsoid is set then the calculated area will be planimetric.
Therefore, you must specify in the Measurements section of the General tab, in the project Properties, the parameters None / Planimetric for the Ellipsoid, and Square Meters for the Units for area measurement.
And the area will be calculated correctly.
Also, you can use the area($geometry) expression to return the planimetric area in the units of maeasure of the layer CRS.
