Polygon area in PostgreSQL differs from QGIS

Question

I downloaded OpenStreetMap data and uploaded to a local PostgreSQL database with osm2pgsql. I created a new table as a result of a ST_Union function with only one record (for now).

CREATE TABLE IF NOT EXISTS m_temp.budapest_merge
(
    id text COLLATE pg_catalog."default" NOT NULL,
    st_union geometry(Polygon,3857),
    ter double precision,
    CONSTRAINT budapest_merge_pkey PRIMARY KEY (id)
)

Then I checked the area of my polygon.

SELECT ST_Area(st_union) FROM m_temp.budapest_merge

The result was 1151156216.758265 sqm which was weird for me, because I was expecting something around 520 sqkm . I opened the PostgreSQL layer in QGIS, calculated the area with the Field Calculator tool's $area function. The result was 526031443.36087507 sqm , which corresponds to my expectation and reality as well.

My question is what can be a problem here? The QGIS project is in the same projection (EPSG:3857).

Also I know that naming a geometry column st_union is not really nice, but that's not the main concern here.

Answer 1

In Qgis, $area is (if defined), the ellipsoidal area, ie the true ground area, while area(geometry) is the planar area, ie highly impacted by the geometry projection.

In Postgis, st_area always use the geometry projection, so it is the equialent of QGIS area(geometry) .

That being said, 3857 highly distorts distances and areas as you move away from the equator. It is wrong to use this projection for computing distances or areas.