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I have a dataset with multiple sampling events that I want to style by season (or "temporada" in my attribute table) and "EP Chinook". Data is from a csv file.

I styled with point displacement to show multiple sampling events.

Then I used categorized render settings to show different seasons, and used size scale fields to show "EP Chinook"

enter image description here

The problem I am running into is the points are way too big. enter image description here

I have tried writing expressions to change the size but there is no geometry for diameter, which is what I want to show. Such as:

$area="EP Chinook"

but this just makes the size of the points stay the same.

I think the reason I am having problems with the points being so big is that the layer still needs to be written so that minimum values remain visible.

I am new to QGIS (about 2 months of self taught) and am coming off of a moderately novice background with ArcMap.

I think I have found another way to style my data that will be appropriate. -different symbols for months -graduated color scale for "EP Chinook" -point displacement for multiple sampling events

However, this is a temporary fix; it would still be nice to label the points using the initial styling I asked about.

  • you have to set the units of your scale-field (mm, px, map units) and probably re-scale your values. (ie: the standard setting is millimeters, so if your values go as high as 1000, qgis draws you a 1.0m circle) – ymirsson Dec 3 '15 at 19:06
  • I changed to map units (I did not see any option for "px") and everything was too small to see without zooming in a ridiculous amount. – S.Confluentus Dec 3 '15 at 20:10
  • you need to rescale your values (new field or OTF) with scale_exp(x, from_min, from_max, to_min, to_max, power) to an appropriate range, depending on your unit setting. – ymirsson Dec 3 '15 at 21:13
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The expression should evaluate to either the area of diameter of the symbols. So instead of:

$area = "EP Chinook"

Try something like:

"EP Chinook"/10.0

Ie, this will scale the value in the field by 1/10th. You'll need to experiment with different factors until it looks right.

Alternatively, if you can upgrade to QGIS >= 2.12 there's a handy "size assistant" option which helps you do this interactively.

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Straightforward process:

1)
The engine needs a unit-system (mm, px, map units) to interpret the values: scr1

Choose one fitting (or almost fitting) to your dataset, projection and map(-goal).

Means:
A dataset [10,35,20,55,2] will be hard to see/distinguish on a px-scale but somewhat right on a mm-scale.
map-units may fit, depending on your projection and map, ie.: 30° is a bigass circle, but an appropriate country-size on a world map. 30m is nothing from a 1:500'000 view, but spans about 1/4 NY-Block.

as example, size = "B" in map units:
EPSG:3857, so everything around 1m scr2

and EPSG:4326, same dataset in deg. scr3

2)
If you have found your unit-system, you can/need/want to refine the domain with:

scale_exp(x, from_min, from_max, to_min, to_max, power)

Find your current min/max and choose a new range and a fitting-curve (ie.: power=1 for linear or higher/smaller than 1 for a slower or more rapid escalating curve).
You can do this either on-the-fly (better option, since scaling may take some try&error): scr4

or - if you can't find it or your version lacks the functionality - via the field calculator: scr5

Hope that helps.

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