[Qgis-user] how to make marker size related to values in column

[Alister Hood]

Hi,

> Date: Wed, 19 Oct 2011 15:22:02 +0100
> From: M.E.Dodd <m.e.dodd at open.ac.uk>
> Subject: [Qgis-user] how to make marker size related to values in
> 	column
> To: "qgis-user at lists.osgeo.org" <qgis-user at lists.osgeo.org>
> Message-ID:
>
<9932A16F9CE9264FAC3F2E0E008E12210123C337C7 at KIELDERCMS1.open.ac.uk>
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> 
> I would like to make point markers of varying sizes depending on their
value
> i.e. if it has a value of 10 then it would have a larger marker than
if it had
> a value of 1.  This seems to be sort of possible via new
> symbology>advanced>size scale field.  However it only allows the
values
> themselves from the column not any transformation of them e.g. sqrt or
log.
> When producing this kind of map you often want to transform the
numbers as they
> are often of much too large a range to show with a simple linear
scale.

Someone else might like to comment on this.  I'm guessing that rather
than offering a simple choice of using a log or sqrt scale, there will
probably be a full "expression based" feature, similar to the expression
based labelling Nathan has implemented.  Personally I think this will be
better.

> So then tried the attribute table and field calculator to produce some
new
> fields with these transformed values and got caught up in how on earth
to do it
> and it did not seem to have log anyway.  

Log functions were added to trunk recently, so I think they would be
available if you run the QGIS nightly builds.  But they were not added
to the context help at the same time - I filed a patch set yesterday
which includes this. 

> Will simply produce these transformed
> values in spreadsheet outside qgis and reimport them and give it
another go,
> but perhaps something for the wish list is to make it easier to do
these
> different sized markers and have different options for transformations
as you
> often don't know what kind of transformation would be suitable until
you see
> them on the map.

Alister