[{_id:"123213",elevation:2300,area:25},{_id:"343221",elevation:1600,area:35},{_id:"545322",elevation:500,area:12},{_id:"234234",elevation:null,area:5}]

db.collection.aggregate([{$bucket:{groupBy:"$elevation",boundaries:[0,1500,3000,5000],default:10000,output:{"count":{$sum:1},"summarizedArea":{$sum:"$area"}}}}])