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I am working with a shapefile of a forest, and I want the symbol of each of the trees to reflect their exact diameter.

I have tried to sort this out through the Symbology tab in ArcGIS Desktop 10.3 Symbology > Multiple Attributes (I am displaying species, and whether or not the tree is dead or alive) > Symbol Size > Value: DBH (diameter)

However, this menu requires that I make breaks of diameter classes to which it assigns circle symbols that do not correspond in any way to the actual diameter. enter image description here

The best I can do is fiddle with the combination of breaks and ranges to get the symbol to be close to the actual diameter. This is far from perfect, and always seems to favor a certain size class (e.g. 100 cm trees are more accurately represented than 10 cm trees).

I am searching for a means of setting the circle symbol diameter based on a specific attribute parameter (in this case, tree diameter). Is this possible?

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    You could convert them to polygons by buffering based on radius, and then color based on the other attributes. – Tom Mar 4 '16 at 22:21
  • @Tom good idea, you should add that as an answer. – Dan C Mar 8 '16 at 20:12
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You want to use Symbology > Quantities > Proportional Symbols and set to your diameter field.

Unfortunately you won't get an option for Diameter but for Radius, so you may need to add a new field for radius and divide the diameter by two into that field.

enter image description here

  • I just realised you said you wanted Multiple attributes to also display by Species etc... I am unsure if this can be done, but I'll test – Midavalo Mar 4 '16 at 21:17
  • Thank you Midavalo! This does work for just displaying diameter, however as you noticed I am seeking to display multiple attributes, with one of them being proportional and the other two being categorical. – Erika Blomdahl Mar 4 '16 at 21:27
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    @ErikaBlomdahl it sounds like you're going to need to split your features into two layers based on your category before symbolizing each proportionally. Neither Midavalo's answer nor mine allows for symbolizing based on multiple attributes. – Dan C Mar 4 '16 at 21:53
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You can also do this by selecting the Single Symbol option, choosing your point symbol, then click Advanced > Size:

enter image description here

You can then choose a numeric field in your layer to use as the size for your symbols. Their size will be proportional to the value you use, for example a point with a value of 100 will appear twice the size as one with a value of 50. I'm not sure how ArcMap decides how small the smallest point will be.

This way doesn't let you create symbols of an exact real-world size like the Proportional Symbols option does (where you can make a 1-meter tree trunk appear as a 1-meter circle on your map), but I believe that option restricts your symbology to either a plain square or circle, whereas this option lets you use whatever symbol you want.

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  1. Create a new field for radius, and calculate it (simply dbh * 0.5); you could, instead, calculate some factor of radius, depending on the scale at which your map(s) will be displayed.

  2. Buffer your points by the radius field; i.e., click the radio button over Field instead of over Linear Unit, and then select the radius field from the drop-down list. enter image description here

  3. Then, use the polygon feature class instead of your original points. When symbolizing, you only have to worry about your species and status fields. On that note, I would recommend color for species with an x over dead trees; it might even be easier to have two copies of the layer, one with color and one with or without an x or whatever symbol you choose.

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