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I mapping the number of Boko Haram victims in each state of Nigeria for several years. I chose to make a proportional symbol maps using the centroid of each state. For a given year, the area of the circular symbol is directly related to the number of deaths.

Using ArcMap's proportional symbols symbology does the job, but I have a problem with the legend: enter image description here

The default legend offers me 3 categories which are not very convenient (1, 100, 10.000). I would like to be able to change the breaks in the legend to, say, 100, 1.000, 5.000, 10.000.

Is there a way to do this?

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  • Does your data contain values other than those you're interested in showing? More specifically, are all of your data generalized into one of the ranges that you mentioned, or do you have a lot of intermediate values (297, 3530, 17)?
    – JWallace
    Feb 16, 2015 at 21:07
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    I ask because proportional symbols can become pretty tricky to decipher if they are representing a lot of values - you may be better off using graduated symbols if you have just a few ranges that you want to convey. Graduated symbols offers you the ability to define class number, breaks, etc.
    – JWallace
    Feb 16, 2015 at 21:09
  • Thank you for your comment. I only have like 20 observations, so I guess it's a bit too few to go to graduated symbols. Also, I think proportional symbols are more adequate since I don't want to show classes of values (i.e. create groups), but I want to have a taste of the amount of deaths at each point.
    – Damien
    Feb 17, 2015 at 8:25

3 Answers 3

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For those still looking this up:

You need to ungroup the legend first and foremost. The legend wizard will not help with this process for some reason. Select the symbols as they need to be ungrouped a second time. They have some spacing elements attached to them still.

Now, go into the symbology tab and look at the point value that you chose for your smallest data point. Write that down. Also, write down the smallest value that you are symbolizing.

Ex: I used a 10pt circle for my lowest value of 40,683 people.

Now, what is proportional here is the area of the circle that is shown on your map. This means it is not scaled in a linear fashion for larger values.

The relationship goes like this:

(Radius1/Radius2)=[sqrt(Value1)/sqrt(Value2)] => (pt value1/pt value2)=[sqrt(Value1)/sqrt(Value2)]

-I skipped the cancellation of pi and taking the square root of both sides here. Point value works as well as the radius since that is just the diameter-

Now, I know that my symbol for a point representing 40,683 people is a 10pt circle. I would like to set my legend up to show 50k, 100k, 200k, and 400k.

To do that, let's rearrange the equation and solve for our unknown point size of the symbol.

For 50,000:

Radius1 = 10pt Value1 = 40,683

Radius2= [sqrt(50,000)/sqrt(40,683)]*10pt = 11.09pt

For 100,000:

Radius2= [sqrt(100,000)/sqrt(40,683)]*10pt = 15.68pt

For 200,000:

Radius2= [sqrt(200,000)/sqrt(40,683)]*10pt = 22.17pt

and so on...

**This is the formula for circles. If you use squares or random symbols, you probably shouldn't be making maps anyway.

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In the legend wizard, on the last step you will be prompted with spacing options. Recreate your legend and change the "Spacing between patches (Vertically)" until you find a spacing you like.

In terms of creating more breaks, you have to go to the layer properties and select Symbology > Quantities and modify the number of classes.

enter image description here

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  • The screen that you're showing here is for Graduated Symbols. The Proportional Symbols does not offer the Classify options as shown here - that is why I was trying to see if using graduated symbology would work for OP's purposes.
    – JWallace
    Feb 17, 2015 at 16:41
  • Ah, I missed that. It seems as though what OP is trying to achieve can be done using Graduated Symbols, which offers the same class break options as the Graduated Colors option. The OP can do custom breaks for Graduated Colors and then modifying the label, giving it the same visual appearance as the proportional symbol. Feb 17, 2015 at 18:12
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Just convert your legend to graphics, ungroup until it's all separated (you may have to do it several times) and manually edit the circle's size and labels.

Then play a little with the "align" and "distribute" tools to make it look nice and centered.

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