db={users:[{"_id":ObjectId("61cad304ba727b75f4d5bbcc"),"name":"name","owned_lands":["locationID1","locationID2"]}],lands:[{"_id":"locationID1","location":{"type":"Point","coordinates":[100,100]}},{"_id":"locationID2","location":{"type":"Point","coordinates":[101,101]}}]}

db.users.aggregate([{$lookup:{from:"lands",localField:"owned_lands",foreignField:"_id",pipeline:[{$project:{_id:0,x:{$first:"$location.coordinates"},y:{$last:"$location.coordinates"}}}],as:"detailedLands"}},{$set:{center:{type:"Point",coordinates:[{$avg:"$detailedLands.x"},{$avg:"$detailedLands.y"}]},detailedLands:"$$REMOVE"}},{$merge:{into:"users"}}])