[GIS] Proportional Symbols – Custom legend breaks

arcgis-desktoparcmaplegendsymbology

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?

Best Answer

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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