I would like to set the size of point marker in qgis to be controlled by its data i.e. if the value is larger then the marker itself is larger. This appears to be possible in e.g. new symbology>advanced>size scale field, also possible to get to this other ways. However its not clear how the value in the column affects the size, bigger numbers give bigger markers but no idea how much bigger or what the numbers in the column mean for the size, the particular numbers I had in the column gave huge markers totally unsuitable for the map. So I then tried to alter the numbers by creating extra column in the attributes table using the field calculator to divide the numbers by 100. Firstly this failed because the column title was DBH(mm), it seems the () are not allowed. so I then changed name of column to dbh (in an external editor as I could not do it in the attributes table itself) which then allowed me to divide the numbers by 100. However the new column only showed one figure so e.g. 180/100 = 1 not 1.8 even though that new column was width 10 precision 2, actually it showed number 1 whatever values I set for width and precision. Not sure if any of this is a bug but these potentially very useful functions don't seem to quite work.
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