Area calculation inconsistencies

[janikmu]

When calculating the area in square meters

openpoiservice/openpoiservice/server/api/views.py

Line 251 in 75791a2

area = transform_geom(geojson_obj, 'epsg:4326', 'epsg:3857').area

the resulting area is sometimes a lot (up to double) bigger than the "real" area.
"Real" area can either mean https://github.com/scisco/area implementation of geojson-area and/or Wikipedia for a given administrative boundary.

Example: Take the administrative boundary of Luzern, CH (https://nominatim.openstreetmap.org/ui/details.html?osmtype=R&osmid=1682891&class=boundary). Wikipedia states 29 square kilometers. The area package (as well as geojson.io) calculates somewhere around 31 square kilometers. openpoiservice, however, calculates 67 square kilometers.

[TimMcCauley]

This is to be expected using 3857... please feel free to raise a PR using a more suitable projection.