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Los Angeles Ethnicity: Symbolizing standard deviational ellipses
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Symbolizing these ellipses involved choosing colors, creating gradient symbols and assigning display angles.
Symbolizing these ellipses involved choosing color hues, using gradient fills and assigning rotation angles.

What: This map shows overlapping ellipses that represent spatial distributions of various ethnic groups within a southern Los Angeles study area. The ellipses are called standard deviational ellipses and are produced using the Directional Distribution tool. The most important information on this map is the location of each ellipse and the areas where they overlap. We emphasized this on the map through the design of the symbols by carefully considering color hue, adding a gradient to the fills, setting a transparency, and rotating the symbols using an attribute in the data. Labels with the same color hues as the fills for the ellipses provide additional information.

Why: When reading this map, it is important to be able to understand and compare the size, location, orientation, ratio of the major and minor axes, and count of the ellipses. Variations in color hue (red, green, purple, and yellow) distinguish the various ethnic groups. The gradient helps the map reader to see the ellipse orientation. Transparency allows the areas of overlap to be seen. Finally, the symbols were rotated based on the angle the major axis of the ellipse as produced by the Directional Distribution tool. The labels have the same hue as the ellipses but they are darker (have a lower color value) so they are legible.

How: Symbolizing these ellipses involved five steps:

  1. Choosing the color hues for the ellipses,
  2. Creating the gradient fills (which also involves creating color ramps),
  3. Applying the gradient fills,
  4. Setting the angle of the gradient fills, and
  5. Setting the transparency.

Here is how we designed our symbols.

Step I. Choosing the color hues

To choose the color hues for these symbols, we consulted Designing Better Maps, which has appendices with recommended color choice selections. In particular, page 196 has specifications for qualitative color schemes (colors for non-numerical data, like the ethnicity shown by our ellipses). You can find the same information on ColorBrewer.

Step II. Creating the gradient fills

Creating a gradient fill is a two-step process -- first you have to create the color ramp that will be used in the gradient fill and then you have to specify the properties of the gradient fill. We did both these steps using the Style Manager, which is where you usually want to do most of your symbol design. These are the steps we followed:

Creating the color ramps to be use for the gradient fills

  1. Create a new style for the color ramps and gradient fill symbols.
    1. From the Tools menu, choose Styles and then Style Manager.
    2. Click the Styles button at the right, scroll (if necessary) to the bottom and choose Create New to create a new style.
    3. Specify a name and location for the new style and click Save.
  2. Create a new color ramp for one of the gradient fills.
    1. In the left panel of Style Manager, expand the folder tree for your new style.
    2. Click on the Color Ramps folder to select it.
    3. In the right panel, right-click, select New, then select Multi-part Color Ramp. We wanted to create a symbol that could be used to make the center of the ellipses lighter than the outer edges. This requires a multi-part color ramp made of two ramps that are essentially mirror images of one another. The color in the center is the one we selected in Step I (using the appendix in the book or ColorBrewer). The color at the edges are modifications of this.
    4. In the Multi-part Color Ramp Properties dialog click the Add button, then choose Algorithmic Color Ramp. Click Add again to create the second algorithmic color ramp.
    5. Double click the first algorithmic color ramp to open its properties.
      1. Click on the radio button next to Color 2.
      2. Click the color chip that now appears next to Color 2 and choose More Colors. Enter the Red, Green and Blue (RGB) values using the specifications from the appendix or ColorBrewer. Click OK.
      3. Click the color chip next to Color 1 and choose More Colors. This needs to be a darker version of Color 2. To do this more easily, change the color model to Hue, Saturation, Value (click on the arrow at the upper right and select HSV Sliders). Reduced the value by 20% and increase the saturation by 4% to keep the color from becoming too gray. The saturation value will differ based on the hue.
    6. Click OK to get back to the Multi-Part Color Ramp Properties dialog and double click the second color ramp. Repeat Steps 1-3 in Step e. above to set the properties of the second algorithmic color ramp but swap the colors for Color 1 and Color 2.
    7. Click OK to get back to the Style Manager and type in a name for your new color ramp. If the name is not selected, click on the name once and you can now type in a new name.
  3. Create the other color ramps. Once you've created your first color ramp, you can easily copy and paste it to create the other color ramps. This will save you the trouble of repeating a number of the steps above.
    1. Right click on the new color ramp and select Copy. Right click again and select Paste.
    2. Type in a name for the pasted color ramp.
    3. Repeat Steps e and f above with the specifications for the second hue that you want to use.

Creating the gradient fills

Now that you have the color ramps, you can use them, to create the gradient fills. Again, you can do this in Style Manager.

  1. Create the first gradient fill.
    1. While still in Style Manager, click on the Fill Symbols folder to select it.
    2. In the right panel, right-click, select New, then select Fill Symbol.
    3. Click the drop down arrow next to Type and select Gradient Fill Symbol.
    4. Right click on the color ramp and select Graphic View. This allows you to see the names of the color ramps instead of a graphic of them. Scroll down to select one of the color ramps you created.
    5. Change the other gradient fill properties to:
      • Intervals: 100
      • Percentage: 100
      • Angle: The value for this property doesn't matter as we will be setting this later.
      • Style: Linear
      • Outline: Click on this, set the width to Zero and click OK.
  2. Create the other gradient fill symbols. Once you've created your first gradient fill symbol, you can easily copy and paste it to create the other gradient fill symbols. This will save you the trouble of repeating a number of the steps above.
    1. Right click on the new gradient fill symbol and select Copy. Right click again and select Paste.
    2. Type in a name for the pasted gradient fill symbol.
    3. Repeat Steps d and e above using the other color ramps that you created.
  3. Close the Style Manager.

Step III. Applying the gradient fills

  1. Right click on the standard deviational ellipses layer in the Table of Contents and select Layer Properties. Click on the Symbology tab.
  2. Change the symbology Type to Categories, Unique Values.
  3. Set the Value field to be Rotation.
  4. Click the Add All Values button.
  5. Uncheck the <all other values> check box.
  6. Click Apply and then close the Layer Properties dialog.
  7. Double click on one of the symbols. Scroll down and select the gradient fill that you want to use for that feature.
  8. Click Apply to check that the symbol is displayed using the gradient fill that you want.
  9. Double click on each of the other features and set the gradient fills for them as well.

Step IV. Setting the angle of the gradient fills

  1. Note the value for the first symbol on the Symbology tab of the Layer Properties.
  2. Double click the symbol and select Properties.
  3. Use the Rotation value as the angle property for the symbol.
  4. Click OK.
  5. Click Apply to check that the fill symbol is now oriented along the major axis of the ellipse. If it's not, right, subtract 90 from the Rotation value and use that number to set the angle property.
  6. Double click on each of the other features and set the angle properties for the gradient fills for them as well.

Step V. Setting the transparency

  1. While you are in the Layer Properties dialog for your standard deviational ellipses layer, click on the Display tab.
  2. Set the transparency to 40%.
  3. Click OK to close they Layer Properties dialog box.

So you now know how to create symbols for the standard deviational ellipses. Of course, you can use this type of symbology for other features as well, or you can use some of these steps to create other types of symbols. Maybe you only want to know how to create a new color ramp, or how to create a gradient fill. Just use those steps that apply to the type of symbol you are trying to create.

love it posted by Lynn Gionette on Feb 29 2008 7:47AM
I love how this map looks! Thanks for the great ideas!

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