ArcGIS Resource Center

By Mamata Akella, Esri Design Cartographer

On large scale maps, you will often see that the symbols used to represent trees are all one size. As we know from our own real-world experience, tree crowns (i.e., the tops of trees formed by their leaves and branches) are different sizes, so a more realistic representation would be to vary the tree symbol size slightly to account for this natural variation.

Although we know that the height, trunk diameter, and crown diameter all vary depending on the amount of sunlight and water a tree gets, what age it is, and where it is planted (to name a few variables), what if none of this information is available to create variation in your tree symbols? What can you do in ArcMap to at least show cartographically varying crown sizes for your symbolized trees? This blog entry describes a method to symbolize trees using point symbols that give the impression of varied tree crown sizes.

Figure 1: Symbolizing tree data using one symbol size

Figure 2: Symbolizing tree data using symbols of varying size

Recently, I was given detailed data of the Esri campus including the locations of trees and bushes. The data were collected from aerial imagery and had very few attributes. I used these data to design an Esri Campus Basemap (it can be downloaded from the Esri Resource Center’s Map Templates page.)

I wanted to do two things with the tree data: first, I wanted to symbolize each species of tree using a different tree symbol, and second, I wanted to vary slightly the symbol size of each tree within a species to create a more realistic looking representation of the data.

To learn more about how I accomplished the first goal, you can read the recent blog post on Symbolizing trees in ArcGIS: assigning each species a different symbol. This blog entry describes how I accomplished the second goal.

The first step is to add a field in the attribute table named Size with the field type set to Short Integer. Once the field is added, right click on it in the attribute table and select Field Calculator. In the Field Calculator, check the Advanced box and type in the following:

randomize
Dim i as integer
i = Int((3*Rnd))

Then set Size equal to i. A screenshot of the dialog is shown below.

Figure 3: The Field Calculator statement used to generate random numbers from 0-2 for each point feature

What this does is assign a random number (in this case, up to 3, rounded off so that it will equal either 0, 1 or 2) to each point feature that represents the location of a tree (if you want more random numbers generated, simply change the number 3 in the example above to your desired value). The Size field can now be used to symbolize the tree data based on the random number generated (as a surrogate for a crown size attribute) using graduated symbols.

To symbolize your data using graduated symbols, right click on the tree layer in the Table of Contents, click Properties, and click on the Symbology tab. In this dialog, opt to show your data using Quantities > Graduated symbols. Change your Value field to Size, leave normalization set to none, and then click on the Template button on the right side to select or create the tree symbol you want to use. Click the down arrow next to Classes in the classification part of the dialog and set the number of classes to three (for the three random numbers that we generated that include 0, 1, or 2). Finally, set a range for your three symbol sizes (this will vary depending on the scale of the map you are designing).

Figure 4: The symbol properties are assigned using Graduated symbols

When you click Apply, you will see that the symbol sizes now vary slightly in size, giving the trees a more realistic look!