The altitude for Airport SVMI is supposed to be 229 Ft above sea level. Running ortho4XP showed an errors for 2 tiles with zero altitude with the result that two tiles 10/67 and 10/66 won't load orthos because of the errors. I remember seeing one log entry showing a -9 ft elevation 
which isn't supposed to happen. It said the tile were incorrectly tagged. I new on openstreetmap but i want to learn how to correct problems like these