Shows graduated circles cannot be used at this scale.
Shows 3D symbols depicting the same data.
What: We used 3D symbols to provide some visual pop and they offered us a way to effectively display our data. Why: A practical reason for using 3D symbols on this map is to overcome problems of not having enough space to show all the symbols clearly. This can happen when the range of data values is large (three or more orders of magnitude).
How: Before we created the map on the right, we transformed our data from linear to volumetric. There isn't just one calculation for this transformation; the shape of the 3D symbol determines the volume formula. In this map we used a sphere. The mathematical conversion is needed 4/3Πr3where the "r" is substituted with the values in your data.
Here is how to do it:
- Add a new field to to your data using the Add Field tool. The data type should be double; and use the default settings for the other parameters of this tool. You will ultimately be symbolizing the data using this field.
- Calculate this new field using the Calculate Field tool using the following parameters:
- Input Table: your layer in ArcMap
- Field Name: the field you just added in step 1.
- Expression: (([<field>] / 3.141592) * 0.75)^(1/3). Where the <field> is the field in your layer that contains the raw numeric data you want to map.
- Expression Type: VB
- Code Block: leave this empty
- Symbolize the layer using either proportional symbols or graduated symbols with the new field. This map used proportional symbols and the goal was to pick a symbol for the minimum value that was small, yet able to be seen easily on the map, while not resulting in a symbol for the maximum value that was too large. To see the difference the new field makes, try using the original data field with these settings—see how much larger the maximum value symbol becomes. Do not use the Appearance Compensation (Flannery) option as that is just for 2D circle symbols. Also in making this map, since BTUs are not a geographic measurement, we kept unit option at the default, Unknown Units
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