If the values to be displayed are large, it may help to use map units
instead of millimeters. This wouldn't be necessary if QGIS allowed more
decimals for 'Size' (option only available with 'Single symbol' style).
Users shouldn't be forced to create new variables, in order to achieve
what they want. Inexperienced users are scared off and files get
polluted this way.
There is another problem with this technique. The user has no choice,
like in old symbology, between scaling the diameter or the area of the
symbol. The latter is most common for thematic maps, especially if the
range of values is large. This could be circumvented by adding
additional fields (using the square root), but in my opinion that
shouldn't be necessary.
Sake Wagenaar
Op 7-3-2012 16:51, M.E.Dodd schreef:
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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